Vibrational dynamics of adsorbates – Quo vadis?

Progress in Surface Science 86 (2011) 1–40

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Review

Vibrational dynamics of adsorbates – Quo vadis? Heike Arnolds ⇑ Surface Science Research Centre and Department of Chemistry, University of Liverpool, Oxford Street, Liverpool L69 3BX, UK

a r t i c l e

i n f o

Commissioning Editor: M. Bonn Keywords: Surface dynamics Vibrational spectroscopy Ultrafast laser spectroscopy Sum-frequency generation Surface-enhanced Raman scattering Inelastic electron tunneling spectroscopy

a b s t r a c t Vibrational energy is a prime reservoir for activating surface processes such as adsorption, desorption and reaction. On metal surfaces, vibrational energy flow occurs on a femto-to picosecond time scale and competing energy dissipation channels in this time range determine the outcome of chemical reactions at surfaces. Fundamental questions of relaxation time, mode selectivity, importance of intra- versus intermolecular coupling and coupling between electronic and vibrational states can now be tackled for relatively complex adsorbates and surfaces. This review looks at the state-of-the-art of surface vibrational dynamics across a wide range of vibrational spectroscopies and the challenges and exciting prospects that lie ahead to further not only our understanding but also the control of vibrational energy flow in model systems as well as real-world problems. Ó 2010 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Classical vibrational dynamics at surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Excitation of vibrations at surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Decay of vibrations at surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Vibrational dynamics of CO/Cu(1 0 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 State of the art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Dynamics of low-frequency vibrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Coupling between vibrational modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.1. Vibrational energy transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.2. Heat transport across molecules and interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

⇑ Tel.: +44 151 794 3543; fax: +44 151 708 0662. E-mail address: [email protected] 0079-6816/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.progsurf.2010.10.001

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3.3.

4.

Coupling between electronic and nuclear degrees of freedom. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. The effect of hot electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Adsorbate resonances and charge transfer in spectroscopy and photochemistry . . . . . Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19 19 26 30 31 31

1. Introduction Vibrations of molecules at surfaces inform us about the chemical identity of the adsorbate, how and where it bonds to the surface, the strength of the internal bonds, interaction with other molecules and sometimes even the structure of the surface itself. We can extract all of this information from a static vibrational spectrum, simply from line frequencies and intensities. If we include the natural time domain of adsorbate vibrations on metal surfaces, that is the femto- to picosecond time scale, we obtain a far richer picture of surface chemistry. This is the time scale on which energy is transferred between vibrational modes within an adsorbate, to another adsorbate or between substrate and adsorbate, and it is the time scale on which information can be stored in the form of vibrational coherence. This is the regime of adsorbate vibrational dynamics. If all the fundamental vibrational dynamic properties of the adsorbate substrate complex were known – namely lifetime, dephasing time, anharmonic coupling to other vibrational modes and to surface electronic and nuclear degrees of freedom, then we could make the transition from fundamental properties to real-world reaction rates. In a further step, we would like to gain control over all these properties in order to channel energy flow in a particular direction and drive a reaction to a desired outcome. We are still reasonably far from achieving this for the simple reason that many of these fundamental dynamic properties are not known for even some of the most common adsorbates. Over the past 5 years however, progress in this direction has been substantial, though not necessarily in the areas that created the fundamental raft of concepts in surface vibrational dynamics. Some of the fields that have made the largest, recent progress have very different foci and are rarely discussed together, like inelastic electron tunneling, nonlinear optics and surface-enhanced Raman spectroscopy. Surface-enhanced Raman spectroscopy has developed from mainly focusing on enhancement factors to using its full spectroscopic potential. Surface-sensitive nonlinear vibrational spectroscopy is turning from a tool only accessible to ultrafast laser experts into one that can be far more routinely applied, while pushing the boundaries of detection limits to lower frequencies, fewer molecules and more complex interfaces. Inelastic electron tunneling has moved from studying a few select model cases to becoming applicable to a much wider range of molecules and detectable vibrations. In addition, a whole host of new vibrational techniques are being developed and applied to metal surfaces. The time therefore seems ripe for a review that covers a wide range of fields to see how we can make the next step change in our understanding and control of vibrational dynamics at surfaces. I have written this review from the point-of-view of a practitioner of ultrafast nonlinear optical spectroscopy, but despite my undeniable bias in this direction, not all problems in vibrational dynamics can be solved by time-resolved spectroscopy. Surface-sensitive nonlinear optical techniques can often spearhead investigations of interfaces that are more challenging to traditionally vacuum-based surface science techniques, like high pressure studies, amorphous surfaces, nanomaterials or buried interfaces such as in electrochemistry, but to unravel the full complexity of surface reactions normally requires the application of a wide range of techniques. The focus of this review is therefore on how we can draw dynamics information from a variety of sources to learn about energy transfer between electrons, phonons and adsorbate vibrational modes in order to understand chemical reactions from the bottom up. There are many as yet unexplored synergies between communities concerned with vibrational dynamics, and I hope this review will encourage closer collaboration. Since vibrational dynamics underpins a large body of work in diverse areas such as molecular electronics, surface photoswitches, ultrafast dynamics, I have not attempted

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to include all the latest work in these areas, but rather concentrated on research that is pushing the boundaries of current vibrational dynamics and at the same time possesses a large thematic degree of overlap with other areas. The paper is organized as follows. Section 2 is a brief guide to the concepts of surface vibrational dynamics and recounts the main ideas, mostly developed from IR spectroscopy, that have driven this field forward. Section 3 is a look at the state-of-the-art and is divided into the dynamics of external and internal vibrational modes, and the energy transfer between electrons and vibrations. Section 4 is a look into the future and the next big challenges in the field. 2. Classical vibrational dynamics at surfaces 2.1. Excitation of vibrations at surfaces In the majority of studies covered by this review, excitation occurs by interaction of an adsorbate with the electromagnetic field E. The type of interaction depends on the spectroscopy used. In reflection absorption infrared spectroscopy (RAIRS), an electromagnetic field at infrared wavelengths EIR interacts with the dynamic dipole moment, that is the change of dipole moment l with vibrational coordinate Q, which determines the intensity of the transition:

 IIR /

@l @Q

2

jEIR j2 :

ð1Þ

In a vibrational state picture, this corresponds to a transition from the ground vibrational state

t = 0 to the first excited state t = 1, as shown schematically in Fig. 1. In Raman spectroscopy, an incident electric field in the visible range EVIS induces a dipole moment in the molecule and the intensity of light scattered at the incident frequency shifted by the vibrational frequency is determined by the change of polarizability a with vibrational coordinate Q:

IRaman /



@a @Q

2

jEVIS j2 :

ð2Þ

The Raman process corresponds to an excitation to a (virtual or real) excited electronic state and a subsequent emission to a different vibrational state in the electronic ground state. This results in Stokes (t = 0–1) and anti-Stokes (t = 1–0) lines either side of the light scattered at the incident frequency (Rayleigh line). Fig. 1 schematically depicts the Stokes and anti-Stokes Raman processes. The signal is enhanced if the excitation occurs to a real excited electronic state (resonance Raman).

Fig. 1. Comparison of vibrational spectroscopies.

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In both spectroscopies, the local field has to be taken into account. For infrared spectroscopy at metallic surfaces this means that p – polarized (transverse magnetic) light at grazing incidence should be used. In Surface-Enhanced Raman Spectroscopy (SERS) as well as Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS), the local field is strongly enhanced by the presence of a metallic nanostructure, either due to plasmon excitation or lightning rod effects [1]. In SERS, the enhanced local field also affects the radiated power, which can lead to SERS intensity enhancements that typically depend on the fourth power of the electric field amplitude [2]. This electromagnetic enhancement is normally a prerequisite for observing any Raman scattering due to the extremely low Raman crosssections. A coherent combination of an infrared and a Raman transition results in the nonlinear optical technique of vibrational sum-frequency generation (SFG). In a state picture, the molecule first makes an infrared transition to a higher vibrational state and then a Raman transition back to the vibrational ground state with light emitted at the sum frequency of the infrared and visible frequencies (antiStokes transition, see Fig. 1). The intensity of the emitted light now depends both on the dynamic dipole moment and dynamic polarizability:

ISFG /



@l @Q

2 

@a @Q

2

jEIR j2 jEVIS j2 :

ð3Þ

SFG differs from RAIRS and Raman in several aspects. For a vibrational mode to be visible in SFG it has to be infrared as well as Raman active. Another consequence of the second order character of this nonlinear process is that in the dipole approximation only non-centrosymmetric media produce a signal (for example, molecules at an interface). Since SFG is a coherent process, the molecules have to have a net orientation at the interface, otherwise the light emitted from differently oriented adsorbates would interfere destructively. The interaction of a molecule with a (metallic) surface has a number of important consequences. First, the bond to the surface turns the free translations and rotations of the gas phase molecule into frustrated translations and rotations of the adsorbate. These have mostly frequencies below 500 cm1 and play an important role both in the decay of internal vibrational modes and their excitation by electrons or photons. Second, the chemical bond between the adsorbate and the surface modifies both the dynamic dipole moment as well as polarizability of the molecule, changes the selection rules and generates new electronic states, which play a role both in the detection of vibrational modes as well as in the interaction between a vibration and the surface electron reservoir. 2.2. Decay of vibrations at surfaces An excited state can either decay by loss of population (energy relaxation) or by loss of coherence (phase relaxation). The line width Chom (half width half maximum) of a homogeneously broadened transition is then composed of two contributions:

Chom ¼

    1 1 1 1 1 ¼   þ  ; 2p T 2 2p 2T 1 T 2

ð4Þ

where T1 is the lifetime of the transition, T 2 is the dephasing time and T2 the overall relaxation time. In case of inhomogeneous broadening, the overall linewidth includes an additional contribution Cinhom:

Ctotal ¼ Chom þ Cinhom :

ð5Þ

These equations indicate that experimental information about these properties can be obtained either by frequency or time-resolved spectroscopies. The latter have the advantage of being able to separately measure population decay, pure dephasing and inhomogeneous broadening by using a variety of time-delayed pulse sequences [3,4]. Frequency resolved spectroscopies have to resort to lineshape analysis and varying the importance of each contribution through a change of coverage, temperature or isotopic composition. Energy decay occurs by a transfer of vibrational energy into other vibrational modes of adsorbate and surface or through excitation of electron–hole pairs, while phase decay is caused either by elastic

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collisions with surface phonons or anharmonic coupling to other vibrational modes which cause the frequency of the observed mode to fluctuate [5]. The importance of the different decay mechanisms for different vibrational modes is best explained for CO on copper, specifically the (1 0 0) surface, a system that was investigated in great detail in the 1990s with frequency and time-resolved spectroscopies as well as theory. Most of the basic ideas that have driven the subsequent development of surface vibrational dynamics can be illustrated with this example. 2.3. Vibrational dynamics of CO/Cu(1 0 0) CO adsorbs on Cu(1 0 0) in atop sites and forms a c(2  2) structure at half monolayer coverage. It possesses four vibrational modes – the internal C–O stretch vibration at 2078 cm1 (257.8 meV), the perpendicular frustrated translation (metal–molecule stretch) at 345 cm1 (42.8 meV), the frustrated rotation at 288 cm1 (35.3 meV) [6] and the parallel frustrated translation at 32 cm1 (4 meV) [7]. CO has a dynamic dipole moment of 0.1 D in the gas phase, which increases upon adsorption on Cu(1 0 0) to 0.25 D. The increase is caused by bonding to the surface – electrons from the CO 5r orbital are donated to the metal, while electrons from the metal are backdonated into the CO 2p* orbital, forming hybridized states with the metal d-bands. The backdonation into the anti-bonding 2p* orbital weakens the internal C–O bond and causes the C–O frequency to decrease. The three highest frequency modes can all be seen in RAIRS, even though a synchrotron is needed to provide sufficient light in the spectral range below 600 cm1. Even the dipole-forbidden frustrated rotation is detectable: CO adsorbates act as scattering centers to conduction electrons excited by IR light and lead to a broadband reduction in IR reflectivity. At the frequency of the frustrated rotation, the relative motion of the vibrating adsorbate is zero with respect to the light-induced surface current and the surface resistivity is significantly reduced, hence the frustrated rotation appears as an anti-absorption band. The surface resistivity is strongly anisotropic, as shown for CO/Cu(1 1 0) [8], where it is is virtually zero in the [0 0 1] direction, whereas the [1–10] direction displays a similar resistivity to a Cu(1 0 0) surface. The change in reflectivity DR is linked to the width C and density of states q of the 2p* resonance at the Fermi level, and the electronic contribution to the damping s of the frustrated translation of the adsorbate. Persson [9] showed that the two are related by

DR /

1

s

/ CqðEF Þ:

ð6Þ

With a density of states of 0.035 eV1, Persson derives a lifetime of the CO frustrated translation of 70 ps. Adsorbate sites with reduced density of states, such as step sites on a Cu(1 0 0) epitaxial film, contribute far less than terrace sites to CO’s surface resistivity [10]. If surface resistivity is viewed as the creation of electron–hole pairs by incident light, which then scatter inelastically from the adsorbate resulting in a vibrational excitation, the connection to Raman scattering becomes obvious [9,11]. This chemical enhancement mechanism of surface Raman scattering, as schematically depicted in Fig. 2, can be alternatively phrased as charge transfer into the 2p* resonance, which can enhance the signal by two orders of magnitude [12]. Upon electron–hole recombination, a vibrationally excited molecule might be left behind, contributing to the Raman signal, which can become detectable even on single-crystalline surfaces [13–15]. Long lifetimes (narrow adsorbate resonances) of the charge transfer state are detrimental, as the ensuing spatial separation of electrons and holes prevents recombination and subsequent photon emission and thus quenches SERS. The notion of ‘‘SERS-active sites’’ where the hole remains localized, arose very early [16] and ties in with the reduced surface resistivity at defect sites [11]. For CO on copper, a Raman spectrum is only available for a roughened surface, nevertheless all four vibrational modes are seen with frequencies close to the single crystal RAIRS experiments [17]. The 2p* orbital also contributes significantly to the lifetimes of all vibrational modes. When the C–O bond is stretched during vibration, the anti-bonding 2p* level is lowered and charge flows from the metal to the adsorbate (Fig. 2). This charge oscillation equates to an increased dynamic dipole moment but at the same time dampens the resonance (‘‘electronic friction’’). The vibration is too rapid

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Fig. 2. Left: energy decay. Vibration of a bond causes an adsorbate orbital to dip into and out of the Fermi sea, causing nonadiabatic charge flow between surface and adsorbate, which dampens the vibrational resonance. Right: Raman scattering for an adsorbate [303]. The incident laser photon causes charge transfer into the adsorbate resonance (temporary negative ion M), followed by nuclear relaxation and de-excitation of the electron back into the metal together with photon emission. This leaves the adsorbate in an excited vibrational state in the electronic ground state (neutral adsorbate M).

for the electrons to follow the motion adiabatically, causing the generation of electron–hole pairs. The finite lifetime caused by this process is simply a function of the frequency of the oscillation X (in radians per second) and the amount of charge transferred per oscillation (dn) [18]:

1 ¼ 2pXðdnÞ2 : T1

ð7Þ

For CO on Cu(1 0 0), the amount of charge transferred can be estimated as 0.03 e per vibration, and, taking account of orbital degeneracy, this yields a lifetime of 1.8 ps, remarkably close to the experimental value of 2.0 ± 1.0 ps [19]. This process generally dominates the decay of high frequency vibrations on metal surfaces, due to their large energy mismatch with surface phonons. Vibrational lifetimes have been measured in the time domain by infrared absorption as well as sum-frequency generation, as reviewed in [20]. The amount of charge transfer and therefore the lifetime can be controlled in electrochemistry by changing the position of the Fermi level [21–23] and the latest density functional theory can now rationalize the small variations of lifetime with metal substrate [24,25]. The vibrational lifetime and degree of coupling to electron–hole pairs are mode-dependent and have been explained by Head-Gordon and Tully [26] as follows. The efficiency of nonadiabatic vibrational relaxation depends on the availability of LUMOs which spatially overlap with the normal mode derivative of the HOMO. Both the parallel frustrated translation and the frustrated rotation break the symmetry of the CO-surface complex and therefore lead to nonadiabatic transitions between HOMOs and LUMOs of different symmetry. For efficient electron–hole pair generation, the normal mode derivative of the 5r HOMO needs to overlap with the 2p* LUMO and the 2p* HOMO derivative needs to match the 5r LUMO. The 5r HOMO has larger amplitude at the carbon than the oxygen atom and the 2p* HOMO includes the diffused Cu–C bond. Both derivatives are larger for the frustrated rotation than the parallel frustrated translation for the simple reason that the frustrated rotation moves the carbon more than the oxygen. The derivatives of the perpendicular frustrated translation (metal–molecule stretch) as well as the internal stretch are very similar due to their symmetry and both modes therefore decay efficiently via electron–hole pair creation. The damping is more efficient for the internal stretch simply due to a larger charge fluctuation and better overlap with low-lying unoccupied orbitals. The latest theoretical results for CO/Cu(1 0 0) are lifetimes of 3.3 ps for the internal stretch, 3.8 ps for the frustrated rotation, 13.7 ps for the metal–molecule stretch and 19.5 ps for the parallel frustrated translation [24]. While it is generally assumed that coupling to phonons dominates the relaxation of external vibrational

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modes, the mode specificity described earlier for CO on Cu does not carry over to other adsorbates like NO and CN, as shown by Krishna and Tully [24]. Both adsorbates are charged, therefore movement of the molecule as a whole (frustrated translations perpendicular and parallel to the surface) strongly perturb the conduction electrons yielding shorter lifetimes for these modes. This can have important consequences for reaction dynamics, as will be seen in Section 3.3.1. All the frustrated vibrational modes also relax efficiently via decay into surface phonons. In addition, the lifetime of the frustrated translation is strongly coverage dependent as derived from line widths measured by helium atom scattering. The lifetime of the CO frustrated translation on Cu(1 0 0) changes from 8 ps at very low coverage (0.03 ML) to merely 2.3 ps for the c(2  2) structure [7]. This theoretical picture of a vibration that couples to electron–hole pairs has been confirmed by both frequency and time-resolved spectroscopies. The decay of a localized oscillator into a single particle continuum is described in the frequency domain by a Fano line-shape with a characteristic asymmetry [5,27]. The theory was initially applied to CO on Cu(1 0 0), but has been also used for atomic adsorbates [28], large organic molecules [29] and even SERS [30]. The degree of asymmetry of a dipole-allowed vibrational mode as caused by nonadiabatic coupling is determined by the ratio of the vibrational frequency to the width of the relevant adsorbate-induced state [31]. These states are narrower for surfaces with narrow d-bands such as the coinage metals, and narrow even further at defect sites [32]. The most strongly asymmetric bands are therefore seen for CO adsorbed on defect sites on coinage metals [33], but asymmetric line shapes are generally visible for metal films near the percolation threshold [34–36]. Fano resonances have also been calculated to be visible in time-resolved sum frequency spectroscopy of CO on a jellium surface [37]. The time-resolved method for testing to which degree vibrational modes couple to electron–hole pairs is a pump–probe experiment using femtosecond lasers. A sub-picosecond visible or near-IR laser pulse creates a nascent electron distribution, which quickly thermalizes to a hot electron distribution and remains out of equilibrium with phonons for up to a few picoseconds. The probe pulse then detects changes in a vibrational mode which can be related to either electrons or phonons depending on their time scale (Fig. 3). This was done relatively early for the C–O stretch on Cu(1 0 0) using time-resolved IR absorption spectroscopy [38], and a large number of studies mostly using SFG have followed [20]. The relatively weak electron–phonon constant of copper along with its high thermal conductivity results in a much larger temperature difference between electron and phonon baths, such that coupling to each individual bath can be ascertained from the time scale of observed change with a greater certainty. The ensuing red-shift of the C–O stretch frequency was traced to excitation of the frustrated translation, which was seen to couple equally rapidly to electrons (5.1 ps time constant) as well as phonons (4.2 ps), in broad agreement with theoretical results. This initially simple picture turns out to be more complex as other modes can become involved, as will be seen in Section 3.3.1. When the surface is strongly excited such that hot electrons reach temperatures of several thousand Kelvin, desorption of the adsorbate occurs, which is highly nonlinearly dependent on fluence, with CO/Cu(1 0 0) [39] only one of many examples [40]. Desorption is thought to be induced by multiple electronic transitions (DIMET) to an electronically excited state and gradual vibrational ladder climbing in the ground state. The position and width of the anti-bonding 2p* level are crucial to the efficiency of this process [41,42]. Coupling between adsorbate molecules via the dipole–dipole interaction was discussed in great detail for CO/Cu(1 0 0) by Persson and Ryberg [43]. The frequency shift caused by dipole–dipole coupling with increasing coverage and extraction of the chemical shift has been discussed frequently for RAIRS, but only very rarely for SERS [44,45]. The coupling also has a strong effect on the observed line widths and their change with temperature. The increasing line width observed with increasing temperature in both slow and ultrafast heating experiments is due to anharmonic coupling to low frequency modes [46]. The increase however, is much smaller for a delocalized, strongly dipole-coupled CO layer than for a CO minority isotope forming a localized state [47], simply because the dipole coupling masks the increasing linewidth by intensity transfer to higher frequencies. A similar effect has been observed through comparison of C–O and Pt–CO line widths with the line width of their combination band [48] and photon echo measurements of the pure dephasing time T 2 of CO/Ir(1 1 1) [49]. The gradual formation of a delocalized phonon has also been observed in broadband SFG as a merging of

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Fig. 3. Pump–probe spectroscopy of adsorbate vibrations detects coupling to electronic or lattice degrees of freedom through monitoring the time scale of the vibrational response. The inset shows the development of electron and lattice temperatures with a 800 nm, 150 fs, 12 Jm1 pulse absorbed by an Ir(1 1 1) surface. The main graph shows the changes in frequency and line width of the internal vibrations of adsorbed CO, which couples only to the lattice, and NO, which couples strongly to both electrons and lattice. Source: Adapted with permission from [304]. Copyright (2007) American Chemical Society.

vibrational hot bands [50,51] and can in principle be detected via two-color IR pump–probe spectroscopy, as investigated theoretically for CO/Cu(1 0 0) [52]. The dipole–dipole coupling can have a different dimensionality, depending on the adsorbate structure, which has a profound influence on the extraction of any anharmonicity, as shown by Jakob [53]. Finally, the anharmonicity of a vibrational mode and its anharmonic coupling and energy transfer to other vibrational modes are detectable in the form of vibrational overtones, combination bands and Fermi resonances [54–58], providing a substantial amount of dynamic information from frequency-resolved experiments. In a Fermi resonance, a mixing of different vibrational modes with identical symmetries occurs, essentially an avoided crossing of vibrational levels. This mixing increases the coupling strength between modes and intramolecular energy transfer as well as phase relaxation are enhanced. One of the best known occurrences is associated with the methyl functional group, where the overtone of a C–H bending mode matches the C–H stretching frequency [57]. Another important example is the Fermi resonance between the Ru–CO stretch and the combination band of the frustrated rotation and translation of CO on Ru(0 0 1), which is dipole-active [55]. The RAIRS experiments could show through isotopic dilution that the Ru–CO band contributing to the Fermi resonance is delocalized and that one cannot necessarily think of intra- and intermolecular coupling as separate phenomena. This is a highly condensed and by no means complete account of the knowledge we have gained so far on the vibrational dynamics of CO on Cu(1 0 0) and many other metal surfaces. The corresponding dynamic properties of most other adsorbates are yet unknown, with small pockets of knowledge on NO, cyanide, methyl and alkali adatoms, as will be seen in the following. The investigation of adsorbate vibrational dynamics has moved away from using IR absorption, as in the vast majority of experiments mentioned in this section, to different techniques like nonlinear optical spectroscopy or surface-enhanced Raman scattering, and the most important developments of the last few years will be reviewed in the following.

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3. State of the art 3.1. Dynamics of low-frequency vibrations The interaction between an adsorbate and a surface is embodied in the adsorbate’s external vibrational modes, which directly impact on many surface dynamical processes such as ad- and desorption, diffusion and sliding friction. They also often reflect the type of motion that precedes a reaction. For example, the mode-softening of the frustrated translation of CO on stepped compared to close-packed surfaces goes hand-in-hand with the increased probability of dissociation at step edges [59]. Yet we know very little about the dynamics of these modes, and most of our knowledge has been derived from frequency-resolved experiments. Given how difficult it is to obtain information on these external modes in the first place, the potential of deducing dynamic information from frequency-resolved experiments should not be underestimated, even though time-resolved experiments can derive such information unambiguously. The main experimental methods used have been helium atom scattering (HAS), high resolution electron energy loss spectroscopy (HREELS) and far-IR absorption. Low frequency modes are also frequently seen in inelastic electron tunneling spectroscopy (IETS) or action spectroscopy, which will be discussed in Section 3.3.1., in the context of coupling between electronic and nuclear degrees of freedom. All of these have certain drawbacks – far-IR studies require a synchrotron, HREELS is limited in resolution to around 10 cm1, which is too low in many cases to determine lifetime or inhomogeneity from the lineshape, HAS works well in the low frequency region, but can only really look at frustrated translations and phonon modes up to ca. 250 cm1, not every vibrational mode is visible in IETS. And yet, the adsorbate’s external modes often provide much more information than the internal ones. As an illustrative example, the Pt–CO stretch was investigated by far-IR and IR emission on the unreconstructed Pt(1 1 0)–(1  1) and the reconstructed (1  2) surfaces [60,61]. The Pt–CO frequency is 9 cm1 lower on the reconstructed surface and the band is significantly broader, most likely due to inhomogeneity (CO atop adsorption on ridge atoms and (1 1 1) microfacets). The reconstruction of the surface has important consequences for CO oxidation dynamics as seen in the oscillatory behavior of the reaction [62,63], nevertheless this important difference in CO–Pt bonding is not visible in the C–O stretching frequency, which is identical for both surface structures. Very recently, a beautiful demonstration of how an external mode changes during the formation of a metal–molecule contact was given by Vitali et al. [64]. They recorded IETS spectra at a range of tip-surface distances of CO on Cu(1 1 1) and observed an abrupt mode softening of the frustrated rotation of CO upon contact with the tungsten tip. In the last five years, a range of nonlinear optical techniques have demonstrated that the detection of surface or adsorbate vibrations below 1000 cm1 is possible. Most of these studies were carried out on non-metallic surfaces, but there are no a priori reasons why they cannot be applied to metal surfaces as well. Since these techniques are based on femtosecond and/or picosecond laser pulses, they have the capability to study low frequency vibrational dynamics in the time-domain, though very few of them have done to date. SFG is an established technique for the determination of vibrational life- and dephasing times, inhomogeneity and intra- and intermolecular coupling and has been used to study the dynamics of low-frequency vibrations indirectly through their anharmonic coupling to high frequency internal stretches [20]. The lowest frequency detected to date by SFG is the C–S stretch of thiophenol adsorbed on a silver film at 420 cm1 using a free-electron laser [65]. Using the same setup, Williams et al. [66] did not judge the prospect for observing metal-adsorbate vibrations to be good, as they could not detect the Pt–CO stretch at around 470 cm1. However, lasers as well as detection systems have substantially improved in the last decade and these catalytically important vibrational modes are within reach. For example, recent advances in laser technology have allowed Roke’s group the creation of a table-top laser system that can detect surface vibrations down to 600 cm1, such as skeletal modes of a polymer backbone [67,68]. Shen’s group recorded surface phonons of a-quartz as low as 795 cm1 and analyzed their in-plane symmetry as shown in Fig. 4 [69,70]. This also shows that SFG cannot only obain dynamic information contained in frequencies and linewidths, but access the dynamics of adsorbate motions that change their vibrational symmetries (see below).

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Fig. 4. Sum frequency spectroscopy can detect low-frequency vibrations and reveal their symmetry. Azimuthal distribution of the sum frequency signal of three phonon modes on a-quartz with different polarization combinations of detected sum frequency, visible and IR beams (a) SSS, (b) SPP and (c) PSP. Source: Reprinted with permission from [69]. Copyright (2008) by the American Physical Society.

In addition, a number of proven experimental strategies are now available to overcome the low cross-sections of low-frequency vibrations: electromagnetic field enhancement has been applied with success to infrared as well as Raman spectroscopy (surface enhanced IR absorption, SEIRAS and surface-enhanced Raman scattering, SERS) [1] and in a smaller number of cases to SFG [71–75]. An enhancement also occurs when the Raman process in SFG becomes resonant with an electronic transition in the adsorbate–substrate system (doubly-resonant SFG, see Section 3.3.2.). A much wider range of adsorbate vibrational resonances is accessible in a nonlinear version of Raman scattering – coherent anti-Stokes Raman scattering or CARS. The spectroscopy uses three incoming fields, where the difference frequency between the pump (x1) and Stokes fields (x2 with x2 < x1) is resonant with a vibrational transition in the molecule. The resonance is probed by another x1 photon and light at the anti-Stokes transition 2x1  x2 is detected. This process is not surface sensitive per se as it involves an odd number of incoming photons and is most frequently used for 3D microscopy of biological samples [76]. However, just like SERS, it benefits from electromagnetic field enhancement at metallic nanostructures [77–79] and then becomes surface-sensitive in the same

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sense as SERS. While care has to be taken to remove the unwanted background anti-Stokes emission from the metallic substrate [80], surface-enhanced CARS has been reported to be many orders of magnitude more sensitive than SERS [80], which should translate into high sensitivity for low-frequency vibrational modes. A substantial CARS signal can even be generated by a 26 nm polymer film on an optically smooth gold film [81], so in a UHV environment with control over the number and type of molecules on a surface this could provide a tool to study low frequency dynamics. CARS generated with femtosecond lasers in a multiplex setup similar to SFG can monitor vibrational dynamics in the time domain, although this type of ultrafast Raman spectroscopy has so far only been used to detect the dynamics of bulk vibrations, see for example [82]. As an alternative, femtosecond stimulated Raman spectroscopy can also obtain simultaneous time- and frequency time-resolution [83] and has been applied to look at the structural changes that occur in a coumarin dye attached to TiO2 nanoparticles following electron transfer with a time resolution of around 100 fs [84]. There are indications that stimulated Raman scattering can selectively detect molecules at interfaces under certain enhancement conditions [85]. Inherently surface-sensitive Raman spectroscopy has been developed by Yamaguchi and Tahara with two fourth order nonlinear processes called CARS-type homodyne v(4) Raman and inverseRaman-type heterodyne v(4) Raman [86–88]. In essence, they also start with pump and Stokes fields, but the probe makes a hyper-Raman (2x1) transition. This means an even number of incoming photons, so that like SFG, the overall spectroscopy is only allowed at interfaces or regions without inversion symmetry. The second process has the better signal-to-noise ratio, since the signal is emitted in the same direction as the sum frequency of the x1 and x2 pulses and is therefore effectively enhanced by it. This was used to detect Raman spectra of rhodamine 800 at air/water and fused silica/water interfaces and obtain Raman peaks from 2200 cm1 down to about 900 cm1. The low frequency limit of this technique is determined by the difference frequency between the pump and Stokes fields and much lower frequencies should be detectable given sufficient laser power. So far, frequency-resolved v(4) Raman spectroscopy has not been applied to metallic surfaces or molecules without electronic resonance at the probe frequency, but its inherent surface sensitivity gives it an advantage over CARS-type processes. A different approach to fourth order Raman spectroscopy has been developed by the group of Onishi [89–91]. Visible pulses of around 20 fs length (740 cm1 bandwidth) excite vibrations in an impulsive Raman process [83] and the vibrational coherence is probed by a two-photon hyper-Raman process. This is the time-resolved analogue of the heterodyne v(4) Raman spectroscopy mentioned above. Experimentally, this equates to detecting the second harmonic signal of the probe pulse (the hyper-Raman process) which is modulated in time by the stimulated Raman process created by the pump pulse. The second harmonic response takes the form of an exponential decay with an oscillatory component, which is extracted mathematically and then Fourier-transformed to yield a spectrum. Nomoto and Onishi have demonstrated that TiO2 phonon modes in the 200–900 cm1 range are detectable, as well as a molecular vibration from sub-monolayer p-nitrobenzoic acid [90], as shown in Fig. 5. This fourth order Raman method has been alternatively called time-resolved second harmonic generation (TRSHG) and used with great success in monitoring the dynamics of alkali-metal stretching vibrations, with the latest results uncovering how the formation of a quantum well state for K and Na on Cu(1 1 1) plays a major role in the electronic and nuclear dynamics [92,93]. TRSHG has been reviewed by Matsumoto and Watanabe in 2006 [94] and is an excellent example of what can be learned from such time-resolved dynamics. For example, the dephasing of the coherently excited Cs–Pt stretch was found to be strongly temperature-dependent, which could only be explained by a large dephasing contribution from anharmonic coupling between the alkali-metal stretch and the alkali frustrated translation. The same anharmonic coupling leads to a Cs–Pt dephasing rate that is nonlinearly dependent on pump-fluence, as the hot electrons created by the pump laser strongly couple to the frustrated translation, similar to the behavior of molecular adsorbates as reviewed in Section 3.3. The method also showed a way to control surface phonons – a tailored sequence of laser pulses selectively excited either the Cs–Pt stretching mode or the Rayleigh phonon of the Pt(1 1 1) surface [95]. Such coherent control might be possible for a much wider class of adsorbates on metal surfaces. Yasuike and Nobusada demonstrate theoretically for NO on Pt(1 1 1) that both an impulsive Raman

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Fig. 5. Fourth order Raman spectroscopy of para-nitrobenzoic acid on TiO2(1 1 0). (a) Intensity change of the reflected second harmonic light from pNB/TiO2(1 1 0). (b) Modulated component extracted by subtracting the background. (c) Imaginary and real parts of the Fourier transform of the modulated component. Lines at 825, 439, 363 and 180 cm1 are phonon modes of the substrate, the line at 572 cm1 is a skeletal mode of the adsorbate. Source: Reprinted with permission from [90]. Copyright 2008 Elsevier.

process and photoexcitation of the adsorbate with subsequent ultrafast decay to an electronically excited substrate state can produce coherent adsorbate vibration albeit with a different initial phase [96]. Other current interest in the area of phonon-induced dissipation stems from molecular-scale electronics, in particular the role played by incoherent versus coherent vibrational motion in dissipating energy [97,98] and the strong contribution low-frequency vibrations make to interfacial thermal resistance [99]. As an alternative method for studying very low-frequency vibrations, Nomoto and Onishi employed terahertz pulses to coherently excite the TiO2 transverse optical phonons at 180 cm1 and detected the coherence again through modulation of the second harmonic signal [100]. As a third order process (one far-IR photon and two visible photons in) this method is not surface sensitive, and like the time-resolved fourth order method it is extremely time-consuming with on the order of 105 pulses needed per time delay point. However, the experiment demonstrates that far-IR pulses generated by a table-top laser system can create detectable vibrational coherences. Different nonlinear detection schemes might in future improve the signal size or surface sensitivity.

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Low-frequency motions of adsorbates that lead to a re-orientation of the transition dipole moment, such as rotation within the surface plane or with respect to the surface normal can be detected through a pump–probe scheme with polarization-dependent SFG [101–103]. The first experimental realization came from Eisenthal’s group [101], who studied the re-orientation of a coumarin dye at the air/water interface by SFG of the carbonyl group in the ring of the coumarin molecule following a femtosecond pump pulse at 423 nm. The orientational relaxation time was 220 ps, faster than the 343 ps orientational relaxation time of the permanent dipole of coumarin determined by timeresolved second harmonic generation. An anisotropy in the spatial distribution of surface vibrational dipole moments can also be created by orthogonal IR pump pulses and followed by polarizationresolved SFG, as recently shown by Nienhuys and Bonn theoretically [102] and experimentally [103]. The experiments uncovered that energy transfer through inter- and intramolecular coupling for water and primary alcohols at the air/water interface occurs much faster than the relaxation of orientational anisotropy. While polarization-dependent SFG studies have been mainly carried out for adsorbates on insulating surfaces (see [104,105] for reviews), the same principles can be applied to metal interfaces, as very recently shown by Cecchet et al. [106,107]. Librational motion of water at silver electrode surfaces has been analyzed by SERS with two-dimensional correlation spectroscopy [108], which could form a very promising area for the application of coherent Raman spectroscopy. Such time-resolved studies of the re-orientation of adsorbates at metal surfaces will in future provide an important link between vibrational and reaction dynamics, as surface reactions often proceed by a change in the adsorbate orientation. This would be especially valuable, since not many surface science techniques are sensitive to these motions (see [109–111] for examples) and as electron-based techniques they are more difficult to adapt to time-resolved measurements. A very different approach to detect low-frequency vibrations is inelastic laser-photoemission spectroscopy [112–115]. Here, the third harmonic of a Ti-sapphire oscillator emitting 2 ps pulses excited photoelectrons from a surface with a photon energy only just above the work function of the surface. These very low energy photoelectrons scatter inelastically from vibrational modes, which are seen as a step in the measured Fermi distribution. On copper, the CO frustrated rotation and internal stretching mode were detectable as well as surface phonons and Cu–O in-plane vibration from a Cu(0 0 1)– p p ( 2  2 2)R45°–O layer. Since the photoelectrons are created by relatively short laser pulses, a pump–probe scheme to investigate dynamics is conceivable. The dynamics of frustrated translation modes and diffusion of adsorbates can also be investigated by spin-echo spectroscopy of 3He scattered off solid surfaces, as reviewed last year [116]. This particlebased technique provides correlation measurements on a time scale from pico- to nanoseconds. While it is mainly being used to derive potential energy surfaces for the diffusion of adsorbates, it can determine the friction coefficient that describes dephasing of the frustrated translation mode, for example, for propane/Pt(1 1 1) it is about 1.1 ps1 [117], while for K/Cu(1 0 0) it is 0.2 ps1 [118]. Given the right adsorbate and scattering conditions, this spectroscopy is also sensitive to rotational motion or motion perpendicular to the surface. Studies of the dynamics of low-frequency vibrations could reveal very different types of relaxation mechanisms as we begin to learn more about coherent energy transfer to phonons. Markovian dynamics are usually applied to relaxation of adsorbate vibrations by electron–hole pair excitation, since the electron bath has a femtosecond correlation time which is significantly faster than the picosecond relaxation time of the adsorbate. Leathers et al. pointed out that non-Markovian dynamics will likely play an important role in the relaxation of low-frequency vibrations into substrate phonon modes, which have a much longer correlation time than electrons [119,120]. 3.2. Coupling between vibrational modes 3.2.1. Vibrational energy transfer Vibrational spectroscopy in the frequency [53,121] or time domain [20] can determine the shape of the adsorbate’s potential energy surface through the detection of excited vibrational states or overtones. It can also determine anharmonic coupling between different modes and the degree of localization of a vibrational mode. If all this information is gathered together we can create a fairly

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detailed picture of vibrational energy flow. However, our current understanding is mostly based on RAIRS or SFG studies of CO adsorbed on a small number of different metal surfaces, followed by the cyanide anion on a few electrode surfaces. The only adsorbate with more than one internal bond whose vibrational energy decay has been studied on a metal surface in any detail is methyl thiolate on silver twenty years ago [122]. Vibrational energy transfer in the time domain is substantially better researched for liquid water at a range of non-metallic interfaces (see [20] for an overview and [123–125] for recent work). A discussion of the phenomena caused by hydrogen bonding would lead too far away from adsorbates at metal surfaces that are the focus of this review, but two changes that can be generally expected are immediately obvious – the increased number of intramolecular relaxation channels and the importance of the adsorbate structure for intermolecular relaxation. At metal surfaces, water vibrational dynamics have been mostly explored by tunneling electrons as will be seen in Section 3.3.1. In the absence of time-resolved SFG and IR studies of larger adsorbates on metal surfaces, the occurrence of combination bands or Fermi resonances is a sign for intramolecular anharmonic coupling and vibrational energy transfer. Their observation is not so common because they require very high detection sensitivity. One approach to improve sensitivity and to detect multiquantum vibrational transitions has been developed by Pipino and Michalski [126]. Evanescent-wave cavity ringdown spectroscopy with the near-IR output of a narrowband (0.075 cm1) optical parametric amplifier detected the first and second overtone of the O–H stretch of a HNO3–H2O complex on amorphous silica under ambient conditions with sub-monolayer sensitivity. A clear frequency shift in the second O–H overtone was observed when the ambient conditions were changed (removal of HNO3 from gas phase). The application of near-IR or mid-IR cavity ringdown spectroscopy to adsorbates on solid films is clearly still in its infancy and this type of experiment is not easily adaptable to pulsed laser sources [127]. However, absorbance changes in the region of 107 are detectable [128], which makes this technique two orders of magnitude more sensitive than IR absorption, so it might in the future unlock the dynamic information inherent in overtones and combination bands for surfaces in ambient, liquid or harsh environments. Surface-enhanced Raman scattering has recently made rapid progress in the detection of vibrationally excited states and combinations. The first observation of higher order overtones and combination bands in SER spectra was reported in 1981 by Pettinger for pyridine on a silver electrode [129]. Of a total of 78 modes observed, 18 had a likely origin in overtones and combinations. The study of these modes remained scarce and seems intricately linked to the SERS background continuum, as discussed in the latest report by Rubim and Aroca on a perylene dye, spin-coated onto silver nanostructures [130]. At sub-monolayer coverage, a correlation was found between the intensity of the SERS background emission and the intensities of harmonics and combinations, such that higher order vibrational transitions could show stronger line intensities than fundamentals. Vibrationally hot states were first reported by Kneipp et al. [131,132]. Without vibrational pumping, the anti-Stokes to Stokes intensity ratio should be given by the Boltzmann population ratio of the vibrational ground and first excited states. For crystal violet and rhodamine 6G on silver colloids in aqueous solution and 830 nm excitation, Kneipp and coworkers found significant deviations for modes below 1000 cm1. Moreover, the difference spectrum between high and low-intensity SER spectra showed appreciable frequency shifts on the Stokes side, indicating a contribution from the red-shifted t = 1 ? t = 2 Stokes transition, while the difference spectrum on the anti-Stokes side showed no such changes. This is expected, as it is unlikely that t = 2 is sufficiently populated to see the t = 2 to t = 1 transition. Surface-enhanced Raman scattering as a source for vibrational pumping was initially hotly debated and alternative explanations put forward, such as localized heating by the near-IR Raman laser or a difference in Stokes and anti-Stokes cross-sections due to resonant Raman processes in the adsorbate–substrate complex [133]. The confirmation of vibrational pumping, as recently described in a tutorial review by Maher et al. [134], was achieved by measuring the anti-Stokes to Stokes ratio q as a function of temperature. I will explain the reasoning in detail here, as it provides an important point of overlap between SERS and time-resolved vibrational spectroscopy. The population n1 of the first excited state for the case of weak pumping, where n1 remains small, is given by the following rate equation:

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dn1 rS IL expðhxt =kTÞ n1 þ  : ¼ T1 dt  xL h T1

15

ð8Þ

where the first term is the population per unit time pumped into the level by a laser with photon energy  hxL and intensity IL given a Stokes cross-section rS. The second term is thermal excitation of the vibration with an energy of xt and the third term is the population relaxation with vibrational lifetime T1. In the steady state, the population is given by

n1 ¼

rS I L T 1 þ expðhxt =kTÞ: h  xL

ð9Þ

This equation for the population of the first excited state shows that without pumping, the antiStokes/Stokes ratio decreases exponentially with temperature, but with pumping, q should level off to a constant value below a cross-over temperature. This leveling-off was indeed found and the cross-over temperature as predicted was seen to be mode-dependent, as shown in Fig. 6 for two widely separated modes of rhodamine 6G adsorbed on a silver colloid. One of the important issues in SERS is the determination of Stokes and anti-Stokes cross-sections. For this, one needs to know the statistical distribution of enhancement factors as well as the lifetime of the vibrational mode. The rather broad distribution of enhancement factors has been measured by Dlott’s group, who used a series of laser pulses with increasing power to burn away benzenethiolate molecules from a nanostructured silver surface [135]. In this particular case, the hot spots with enhancement factors above 109 accounted for 63 ppm of the adsorbate but 24% of the total SERS intensity. Chien et al. worked on the same surface, measuring the distribution by Raman confocal microscopy and found that it very sensitively depended on the defect density, which was influenced by the surface treatment [136]. This broad distribution is clearly a problem in extracting quantitative data from SERS. With regards to the vibrational lifetime, Maher et al. point out that this has actually never been measured under SERS conditions, and the derivation of a cross-section will typically involve an order of magnitude estimate of T1 (typically 10 ps) or using the inverse linewidth [134]. In the latter case, this will be heavily influenced by the exact enhancement factor distribution and the distribution of adsorption sites, not to mention any contribution from dephasing or anharmonic coupling to other modes. Time-resolved, nonlinear optical techniques therefore have a major role to play in the future of SERS as an analytical technique.

Fig. 6. Left: anti-Stokes (AS) and Stokes (S) spectra of two widely spaced vibrations of rhodamine 6G (610 and 1650 cm1) adsorbed on silver colloid under 676 nm laser irradiation at 300 and 10 K. The anti-Stokes signals at 10 K are only visible due to vibrational pumping. Right: anti-Stokes/Stokes ratio for the two modes as a function of temperature. The arrows mark the crossover temperature between vibrational pumping and thermal excitation, which increases with vibrational frequency. Source: Reproduced from [134] by permission of The Royal Society of Chemistry.

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Some of these problems have been overcome in SERS by advancing the technique to the state of single-molecule detection [137]. The most successful schemes proving that a spectrum arises from a single molecule involve either adsorbing two isotopes of the same molecule [138] or two different molecules [139] on the substrate (bi-analyte technique). At an average coverage of one molecule per site, single or few molecule spectra can be identified by the number of peaks in the spectra. Etchegoin and Le Ru used this technique to resolve the vibrational lines of individual molecules within an inhomogeneously broadened Raman peak [140], as shown in Fig. 7. By employing low temperatures and a low frequency mode of nile blue adsorbed on a silver colloid attached to poly-L-lysine covered silicon wafers, Etchegoin and Le Ru could establish that the average FWHM of the 590 cm1 mode was 3 cm1, while a single molecule typically displayed a linewidth of only 1 cm1. The frequency wandering causing the broader average peak was mainly attributed to different molecular environments sampled through diffusion, as isotope shifts and instrument response could be excluded. Diffusion of molecules even at low temperatures and site dependence of vibrational spectra is a familiar phenomenon in surface science, but the influence of adsorption site and environment on vibrational spectra is not often discussed in SERS.

Fig. 7. SERS spectra of nile blue adsorbed on a silver colloid at 77 K under 633 nm excitation. (a) Individual single molecule spectra together with the Raman spectrum averaged over 7500 single molecule spectra. (b) Lorentzian fits to the average spectrum (FWHM 3 cm1) and an individual single molecule spectrum (FWHM 1 cm1). Source: Reprinted with permission from [140]. Copyright 2010 The American Chemical Society.

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Vibrational pumping of single molecules was reported by the same group [141]. For nile blue and crystal violet adsorbed on silver colloids, they measured the total, radiative Stokes cross-section and the pump cross-section derived from the degree of vibrational pumping. If each Stokes-shifted photon is scattered and detected in the far-field, that is if the SERS process is radiative, then the pump and Stokes cross-sections should be identical. If however there are non-radiative SERS processes, where the laser excites the molecule to a higher vibrational state, but the Stokes photon is absorbed by the substrate, then the cross-sections can differ. Indeed, these cross-sections differed on average by a factor of 10, but in individual single molecule spectra differed by one or even two orders of magnitude more than the average. Such quenching of radiative processes by metals is well known and Otto has speculated that the chemical contribution to resonant Raman scattering could be related to the lifetime of the electron–hole pair generated [142]. SERS at the single molecule level could particularly benefit from combination with scanning probe techniques to detect electronically excited states or to measure the vibrational lifetime of individual molecules with action spectroscopy (see Section 3.3.1.). Metal tip-enhanced Raman spectroscopy in ultra-high vacuum has imaged single dye molecules on Au(1 1 1) with 15 nm resolution [143]. Given a recent report that single molecule SERS is even possible for non-resonant molecules like adenine [144] there should be a wide enough choice of adsorbates and substrates that are accessible with several techniques to come to a detailed understanding of the vibrational excitation and decay mechanisms of adsorbates at surfaces. 3.2.2. Heat transport across molecules and interfaces Moving from coherent to incoherent excitation of vibrations at surfaces, this section looks at the transport of heat across interfaces and molecules. The heat source in the cases discussed below is an ultrashort laser pulse of either 35 ps or around 100 fs width. Use of the latter leads to a difference between electron and phonon temperatures over a time scale of up to a few picoseconds, but we will mainly be concerned with the effects of hot phonons for now and discuss the effects of hot electrons in the following section. Early experiments with picosecond lasers looked at chemical transformations through ultrafast heating probed by SFG and discovered reactive intermediates like bridged formate on a nickel oxide surface or site hopping of molecules on a range of surfaces, as reviewed in [145]. Using the same method, transport of heat across a saturated CO/Pt(1 1 1) layer into thin and thick layers of crystalline ice was investigated. When these studies were conducted, ice grown on Pt(1 1 1) was thought to exhibit polar order such that several layers collectively contribute to the SFG signal with a decay length of 30 monolayers [146] and data interpretation was based on detecting either all of the thin layer ice or around the first 30 layers of the 140 layer thick ice. The spectral changes were interpreted as melting and recrystallisation, which only occurred for the thinner layer with CO at the interface, and at a significantly slower rate than one would expect from diffusive thermal transport. In the light of recent work on multilayer water adsorption on metal surfaces, reviewed by Hodgson [147], this interpretation has to be reviewed – crystalline ice grows on Pt(1 1 1) by formation of clusters and areas covered by a monolayer of water are still visible up to a thickness of around 50 layers. In addition, adsorbate layers are known to change the growth mode of ice. It is therefore likely that the melting observed for thinner layers was associated with the melting of clusters, whereas potentially both the metal/ice and ice/vacuum interfaces of the 140 layer thick film were probed. Without polar ordering of the region adjacent to the platinum surface, the signal should have arisen from the ice/vacuum interface, in which case the heat transport would have proceeded at the same rate as expected for bulk ice. The ice/CO/Pt(1 1 1) system was recently reinvestigated by near IR pump –SFG probe experiments using femtosecond lasers [148]. Unlike the initial studies, these did not probe the bulk O–D stretching region, instead they looked at the O–D bands from free OD directly interacting with CO at the interface or those non-hydrogen bonded groups at the ice-vacuum interface. For a preparation of 0.5 ML CO and 10 layers of ice, they still detected CO molecules unaffected by D2O, confirming cluster formation. For this layer, the O–D interface stretches both reacted promptly to the pump pulses, such that the CO interface O–D could be described well by the CO temperature, whereas the free O–D behavior could be modeled by diffusive heat transport with a thermal conductivity that was around 15% higher than for bulk ice.

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While the growth of water on metal surfaces probably shows a higher complexity than most other adsorbates, these time-resolved studies illustrate how important it is that the interpretation of spectral changes is supported by other techniques. In addition to ultrafast vibrational spectroscopy, we can now also observe structural changes in the time domain using ultrafast electron diffraction. Yang and Zewail recently applied this technique to study the melting of ice on graphite [149]. Melting was found to occur on a 10 ps time scale and a new phase appeared within 20 ps, whereas the restructuring took up to 400 ps. One should keep in mind though the strong interaction between electrons and ice. For example, hot electrons generated by UV light were found to cause crystallization of amorphous ice layers of graphite by Chakarov [150] and the injection of hot electrons from a metal surface is known to initiate desorption from the ice-vacuum interface [151]. Heat transport across a metal–water interface is also crucial in electrochemistry. The organization of water at electrode surfaces is known to be potential-dependent (for recent results obtained with SFG see [152,153] and [108,154] for SERS). A temperature jump will cause a potential jump at the interface, since the increased temperature randomizes the orientation of water molecules. The size of the potential jump was measured by Yamakata and Osawa following 35 ps green laser pulses with a fast oscilloscope, and was found to be 30 times larger with CO at the platinum interface than without [155]. This was ascribed to the weaker bonding of water to CO compared to platinum, giving interfacial water a greater flexibility. The same group used time-resolved SEIRAS on a thin film Pt electrode to follow the changes in the C–O stretching frequency at a potential of 0.4 V after pump excitation as well as the mid-IR reflectivity at 2000 cm1, away from the CO absorption at 2082 cm1. The IR reflectivity shows a temporal behavior that would be expected from a 35 ps pulse heating the Pt electrode, whereas the maximum of the C–O frequency red-shift does not occur until 200 ps after the pump pulse. Picosecond time-resolved SFG was used by Noguchi et al. to also study the behavior of CO on a polycrystalline Pt electrode at 0 V potential [153]. In this case, the intensity of the CO SFG peak at 2055 cm1 was found to drop immediately, while a new peak transiently appeared at 1980 cm1 and is likely due to a site switching of CO from atop to bridge sites, familiar from picosecond studies conducted in UHV [156,157]. The very different transient behavior of CO in these two studies could possibly be related to the different structure of water at these two interfaces, which has been found to be strongly potential-dependent by SFG, SERS and SEIRAS. The influence of the changing vibrational lifetime of interfacial water, as briefly mentioned above, however has not been discussed or even measured in the area of electrochemistry. A different approach to measure heat transport between metal surfaces and water is to record the time-resolved thermoreflectance of the interface using 500 fs pulses from a Ti:Sapphire oscillator [158], as shown in Fig. 8. This was done for thin gold films rendered hydrophilic and hydrophobic using self-assembled monolayers of 11-mercapto-1-undecanol and 1-octadecanethiol, respectively, and thin Al films that were covered with either hydrophilic PEG-silane or hydrophobic octadecyltrichlorosilane. The interface thermal conductance was larger on the hydrophilic interface compared to the hydrophobic one, with a factor two, respectively, three difference for gold and aluminum. Four processes contribute to this interface conductance – heat has to flow from the metal layer to the bonding groups, then flow along the length of the molecular chain, transfer from the terminal functional group to the interfacial water and from there to bulk water. Thermoreflectance can only look at the overall process, but a number of time-resolved SFG studies have been able to specifically detect some of these processes. For example, interfacial water at lipid membranes is known to be energetically decoupled from the bulk from IR pump, femtosecond SFG probe measurements by Bonn’s group [159]. Heat transport from the metal layer into the attached molecule and then along the molecular chain has been investigated by Dlott’s group who devised the method of ultrafast flash thermal conductance [160]. In their setup, a 500 fs laser pulse heated a supported thin gold film from underneath while femtosecond SFG recorded the response of a particular end group on a range of molecules (Fig. 8). To date, they have studied alkane thiols of varying chain length with methyl or benzene end groups and a range of substituted benzenes [161–164]. Generally, the transport of heat through the molecule causes disorder in the self-assembled layer and thereby reduces the SFG signal. At the same time, thermal excitation of various vibrational modes and anharmonic coupling to the observed modes typically causes a red-shift of these. The time-dependent response could be due to two effects – either it is caused by the rate at which molecules respond to heating or it is due to heat flow into the detected

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Fig. 8. Heat transport across interfaces measured by different methods. Left: time-domain thermoreflectance of 11-mercapto1-undecanol on gold in air (open circles) and under water (solid circles). The intensity of the 500 fs pump pulse train is modulated in time and the plotted ratio of the in-phase and out-of-phase components of the reflected probe pulse intensity can be related to thermal conductance. Short time oscillations are due to longitudinal oscillations of the gold film. Source: Reprinted with permission from [158]. Copyright (2006) by the American Physical Society. Right: vibrational response functions of benzenethiol and hexadecanethiol on gold after flash heating to 1073 K. The vibrational response function is the change in sum frequency intensity of a characteristic head group vibration normalized to the maximum intensity change from a cold surface to a hot, equilibrated Au/SAM layer [162]. Source: Reproduced from [134] by permission of The Royal Society of Chemistry.

group. Molecular dynamics simulations showed however that localized methyl motions responded to heat in less than a picosecond, so the observed transients reflect heat flow into the molecule [135,164]. The intensity reduction only sets in with a certain delay time that depends linearly on chain length, but reaches zero at a finite chain length of 0.8 nm. This indicates that the gold layer transfers its heat into a segment approximately four C–C bonds long rather than into an individual atom. From this segment on, heat is transported ballistically with a dependence on the linking group, with aromatic groups slowing down the heat transfer. The responses of functional groups that are less than about 4 or 5 carbon atoms away from the surface also show an overshoot that is not due to heat but hot electron excitation of low-frequency vibrations that anharmonically couple to the observed mode. While certain molecules react more strongly than others, the overall additional excitation caused by hot electrons was judged to be relatively small, in the region of a 5% increase in the excited state population [163]. This work is of increasing significance as molecular electronic devices are being developed, where the transport of an electron through a nanoscale junction can cause significant heating, which could be detrimental to the performance of such a device or its energy efficiency [165].

3.3. Coupling between electronic and nuclear degrees of freedom 3.3.1. The effect of hot electrons Nonadiabatic coupling between electronic and nuclear degrees of freedom is a pervading phenomenon in chemical reactions at metal surfaces due to the continuum of excited electronic states [166,167]. Wodtke and Auerbach recently reviewed energy transfer processes in molecular beam surface scattering, including the generation of chemicurrents by reacting molecules and de-excitation of highly vibrationally excited gas phase molecules through coupling to surface electrons [168]. How nuclear motion is turned into electronic excitation is now quite well understood theoretically, even for open shell molecules like NO [169], and the use of metal–insulator–metal (MIM) devices permits spectroscopy of the resulting excitations and a more detailed understanding of how surface reactions proceed [170]. In the reverse process, excited electrons cause a surface chemical reaction by transferring energy to vibrational coordinates. This section will concentrate on recent published work that has substantially refined our understanding of this energy transfer and includes hot electrons generated by

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femtosecond lasers and inelastically tunneling electrons from a metal tip. The common theme in both areas is anharmonic coupling between vibrational modes as a means to transfer energy into the reaction coordinate. Nonadiabatic reaction dynamics as triggered and monitored by femtosecond laser pulses was reviewed by Frischkorn and Wolf [40]. The advances made in theory to describe the continuum of electronically excited states were described by Saalfrank for the case of femtosecond laser-induced desorption [171]. The most recent time-resolved SFG experiments in this area have been discussed by Arnolds and Bonn [20]. Given the number of existing reviews, it is not necessary to describe the experiments in detail again, instead I will concentrate on the phenomenological modeling of such experiments that are our current main source of information on the role played by external vibrational modes in the energy transfer. A prerequisite to understanding the dynamics of the interaction of photoexcited electrons with adsorbates is an accurate modeling of the non-equilibrium distribution of electrons generated by a laser pulse and how it evolves as a function of time. Anisimov’s two-temperature model described electrons and lattice as two heat baths that equilibrate through electron–phonon coupling [172], but does not take account of the finite time needed for the nascent electron distribution to develop into a thermalized one, which is strongly dependent on laser fluence as well as the electronic structure of the metal. Carpene included the effects of the initial non-thermal carrier distribution, which relaxes via electron–electron and electron–phonon scattering [173]. Thus, non-thermal electrons provide distinct heat sources for both thermalized electrons and phonons, unlike the classical two-temperature model, where the phonons are only heated via coupling to electrons. The result is a substantially lower electron temperature with a maximum at later times. This model has been implemented for thin gold films and tested in detail using time-resolved reflectivity measurements [174]. Lin et al. [175] recently used first principles electronic structure calculations to demonstrate that with increasing temperature, the electron–phonon coupling parameter is not a constant as usually assumed and that the electron heat capacity is not linear with temperature. Moreover, the changes depend on the d-band structure of the metal. The deviations from the values often used for modeling surface femtochemistry are significant, in particular for desorption and reaction experiments, where the electron temperature often reaches a few thousand Kelvin. These calculations suggest that ultrafast vibrational spectroscopy should always be accompanied by measuring the actual hot electron distribution for example with time-resolved photoemission [176,177], especially when high laser fluences are used. The energy transfer from hot electrons to the adsorbate also needs to be modified from the simple heat transfer equation with constant friction parameters, as this is only valid for harmonic oscillators and a lack of sharp structures in the electronic density of states near the Fermi level. For thermal energies much higher than the vibrational mode energy, this heat transfer should be written as:

dT a ¼ gðT a Þ  ½T e ðtÞ  T a ðtÞ: dt

ð10Þ

where the friction parameter g depends on adsorbate temperature Ta [178]. At really high temperatures, the adsorbate will on average be further away from the surface, so the substrate–adsorbate coupling becomes weaker. How the coupling changes as a function of distance depends of course on the vibrational mode, as shown by Tully [179]. The typical effect on the adsorbate temperature will be to reduce it at short times after the electron temperature maximum, but also to slow down its cooling to the surface temperature, again due to the reduced coupling coefficient. Ueba and Persson considered the heat transfer between different vibrational modes as an indirect path to vibrational excitation in addition to direct excitation by electrons [180,181]. The temperatures of two coupled vibrational modes 1 and 2 with frequencies x1 and x2 then develop as follows:

  dT 1;2 kB T 2;1 ðT e  T 1 Þ: ¼ g1;2  g1;2 dt hx2;1

ð11Þ

The effective friction has the familiar direct contribution g1,2 from the electron heat bath and an indirect term g1;2 that increases linearly with the temperature of the partner mode. Even if there is no direct electron heating of mode 1, its temperature will still increase through indirect heating via mode 2 if this is strongly heated by electrons.

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Such anharmonic coupling between vibrational modes was first deduced from a number of key femtosecond laser experiments concerned with diffusion. Backus et al. had monitored CO step and terrace populations on a stepped Pt(533) surface with the help of time-resolved SFG spectra after femtosecond excitation and discovered that even though diffusion was mainly driven by the frustrated translation mode, the sub-picosecond response in the SFG spectra had to come from excitation of the frustrated rotation mode [182]. The interpretation was that of a coupled rotation and translation of the CO molecule to overcome the diffusion barrier. Stépan et al. monitored the oxygen step population on a vicinal Pt(1 1 1) surface using SHG after exciting oxygen diffusion with a pump pulse and found that fluence dependent hopping probabilities and a two-pulse correlation curve of the hopping probability could only sensibly be described with a friction parameter that increased with electron temperature [183,184]. They proposed that the increasing coupling strength came from anharmonic coupling between the oxygen–Pt stretch and the frustrated translation. A few years earlier, Komeda et al. had initiated CO hopping on Cu(1 1 0) and Pd(1 1 0) by exciting the C–O stretch with inelastically tunneling electrons from an STM tip [185], and found the hopping probability to be influenced more strongly by the degree of anharmonic coupling between the stretch and the frustrated translation than the diffusion barrier. The indirect heating model has now been employed with good success by Hayashi et al. [186] to both the CO diffusion on vicinal Pt measured by Lawrenz et al. [187] with SHG and the SFG data on stepped platinum by Backus et al. [182]. The effect indirect heating has on the temperature of the frustrated translation is shown in Fig. 9. Even though the indirect mode coupling does not describe both data sets simultaneously without rescaling experimentally determined absorbed fluences, this could be due to temperature-dependent electron–phonon coupling parameters according to Lin et al. [175] or the strongly coverage dependent activation energy for diffusion, recently deduced from 3 He spin echo data by Alexandrowicz et al. [188]. In addition, the latest results by Morgenstern’s group [189] who observed CO diffusion on Cu(1 1 1) with an STM following femtosecond laser excitation reveal an unexpectedly complex behavior with the observation of a transient attraction between CO molecules and formation of tilted CO dimers with sufficient thermal stability to be imaged at 17 K. Such a transient attraction has also been surmised to occur between co-adsorbed CO and NO on Ir(1 1 1) from femtosecond pump – SFG probe studies [190]. One also has to keep in mind that the actual dynamics is multidimensional and that the success of a simple friction model could for example be due to fast intermixing between different degrees of freedom, as shown for the hydrogen associative desorption from Ru(0 0 0 1) [191]. The essence of the phenomenological electronic friction model gives us a picture of an accepting mode (the frustrated rotation in case of CO diffusion) which is strongly heated by electrons and a promoting mode (the frustrated translation) which is only indirectly excited. Different reactions will likely have different accepting and promoting modes, but such indirect excitation should always be suspected if the traditional modeling based on the two-temperature model necessitates friction parameters that depend on electron temperature. If we take such temperature dependence as a pointer, then indirect heating of a promoting via an accepting mode should also be inferred for CO desorption and oxidation on Pd(1 1 1) [192–194] and excitation of the high frequency vibrational modes of NO [195] and co-adsorbed CO and NO on Ir(1 1 1) [190]. The identification of the promoting mode does not have to come from time-resolved vibrational spectroscopy, but can also be made from measuring the nonlinear dependence of the reaction yield on laser fluence. Recent work by Schiøtz’ group investigated the power laws typical for hot-electron induced desorption of CO and NO from transition metal surfaces using a nonadiabatic Newns–Anderson Hamiltonian [196]. The power law exponent was found to be the number of contributing vibrational states with respect to the desorption energy. For example, femtosecond laser-induced desorption of NO from Palladium occurs with a power law exponent of 3.3 and a desorption energy of 1 eV, from which the conclusion can be drawn that hot electrons strongly excite the internal stretch at 210 meV, which then has to couple anharmonically to the desorption coordinate. Recognised early on during studies of femtosecond surface science [197,198], the frustrated rotational mode for upright diatomic adsorbates seems predilected to play an important role in their vibrational dynamics. For CO, NO and CN adsorbed on a range of transition metal surfaces it has the shortest lifetime with a strong contribution from decay into electron–hole pairs [24]. In addition,

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Fig. 9. The effect of indirect heating on the temperature of the frustrated translation of CO/Pt(1 1 1). The black dotted curve shows the electron temperature for two laser pulses incident at 0 ps and 2 ps. The frustrated translation temperature is calculated with mode coupling (in red), without mode coupling (in green) and with an electron-dependent friction model (in blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Source: Reprinted with permission from [186]. Copyright (2009) by the American Physical Society.

the intermode coupling time between the internal stretch and the frustrated rotation is very fast at only 1 ps, while the coupling times to the CO-metal stretch and the frustrated translation are of the order of several picoseconds, as computed for CO adsorbed on Cu(1 1 0) and Ag(1 1 0) [199]. Strong bonding of these adsorbates to the surface also means strong hybridization between molecule and surface states, which implies that the mixing of electronic states of different symmetry by the frustrated rotation is more efficient. The bonding changes with adsorption site, which in turn affects the frequency of the vibrational modes. For example, the frequency of the frustrated rotation of CO is much lower for a bridge than for an atop site, such that this mode now dominates the dephasing of the C–O stretch in temperature-dependent linewidth measurements [200]. The effect of adsorption site on the dynamics was recognised by Camillone’s group [193], when they investigated the efficiency and time-scale of femtosecond laser-induced desorption of CO from Pd(1 1 1), where CO sits in a 3fold hollow site. The consequence of multiple bonding is not only a change in vibrational frequency and lifetime, but it also affects the position and width of the 2p* resonance, which in this case even extends to below EF, making a direct excitation by hot electrons much more likely. Backus et al. [201] similarly observed different dynamics between terrace and step-adsorbed NO and CO on Pt(533), which could be traced back to an increased local density of states at EF by DFT. Although no time-resolved vibrational data exist other than for NO and CO on atop sites, we can nevertheless predict confidently that changing the adsorption site will have a major effect on the dynamics. Given the detailed understanding we have reached, the time is ripe to explore structure–dynamics relationships with femtosecond spectroscopy. While femtosecond laser pulses are excellent for monitoring the temporal evolution of the adsorbate after electronic excitation, it is less straightforward to control the flow of electronic energy into a specific vibrational mode by varying the laser pulse parameters. Visible and near-infrared laser pulses

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mostly interact indirectly with adsorbates on metal surfaces via surface electronic excitation. Infrared laser pulses, due to the high reflectivity of metals in this wavelength region, can affect the adsorbate directly. Control schemes have been devised theoretically, see for example [202,203], but have not actually been carried out yet, as mid-infrared pulse shaping is not yet a common technique and some schemes require fluences up to Joules per cm2, out of reach for table-top laser systems. Mode-selective surface chemistry by infrared excitation has been demonstrated with the high intensities available from a free-electron laser in the desorption of hydrogen from Si(1 1 1) by resonant excitation of the Si–H mode [204]. The control of energy flow from electrons to vibrational modes has been explored recently in great detail by scanning tunneling microscopy and inelastic electron tunneling (IET) into a vibrational energy level. The relative ease with which an electron can be injected or extracted at any desired energy and into a specific location within the molecule has provided a good window onto the reaction dynamics. While this is interesting for molecular vibrational dynamics already from a detection point of view, the developments over the past 3 years or so of using IET to control vibrational excitation and ensuing reaction have been astonishing. Initially only simple reactions like dissociation [205], rotation [206,207] and hopping [185] affecting the external bonds of an adsorbate were investigated, which either involved excitation of the reaction coordinate or a high frequency mode that coupled anharmonically to the reaction coordinate. In the last few years, the degree of complexity of the molecules or reactions that have been manipulated with the STM has gone up radically compared to laser-induced reactions. In the area of water interaction with metal surfaces, a wide range of reactions like diffusion, dissociation and desorption could be induced, depending on the energy of the tunneling electrons and the local adsorbate structure [208,209]. Shin et al. found the longer lifetime of anionic water on an insulating MgO film beneficial to control the dissociation either via vibrational or electronic excitation, yielding different reaction products [210]. The quantum tunneling of hydrogen from vibrationally excited states was found to play an important role in the hydrogen-bond exchange within a water dimer [211,212] and the switching of OH/Cu(1 1 0) between two equivalent tilted orientations [213,214]. Hahn and Ho showed that for a relatively weak interaction with a Ag(1 1 0) surface and the formation of aligned O2–H2O–O complexes, the dissociation rate could be enhanced 100-fold if the O–H stretch vibrations were excited [215]. Therefore even for complex molecules or clusters, vibrational energy can flow efficiently into the reaction coordinate rather than decay into ineffective modes. The concept of action spectroscopy has provided far richer detail than inelastic electron tunneling spectroscopy (IETS), as it can detect vibrational modes that are invisible in IETS [216]. In action spectroscopy, the second derivative of the reaction yield per tunneling electron is recorded as a function of bias voltage and was shown to be proportional to the vibrational density of states for both direct and indirect reactions by Ueba and Persson [217]. The width of the action spectrum therefore gives the vibrational linewidth, including any intrinsic broadening experienced by the molecule. The combination of action spectroscopy with DFT calculations has recently begun to unravel the mechanism of electron–vibration coupling. The mechanism is closely linked to the presence and spatial extent of LUMOs near the Fermi level, as shown by Ohara et al. [218]. For example, the dissociation of (CH3S)2 occurs by excitation of the C–H stretch and a combination of C–H and S–S stretches, because the relevant LUMO near EF encompasses both the S–S and the C–H bond, see Fig. 10. Ohara et al. also find a strong bias dependence of the reaction yield – only electrons injected into the molecule cause dissociation, whereas electrons extracted from it have a negligible influence. This is in line with the asymmetric DOS of this LUMO across EF. The lateral hopping of CH3S on the other hand was found to be independent of bias polarity and the LUMO is homogeneously distributed across EF. The relevant molecular orbital just involves the C–S bond and this is indeed the only bond that is seen in the action spectrum. While the existence of molecular orbitals with density of states (DOS) near the Fermi level is a necessity for some reactions, too high a DOS can be detrimental for STM-induced reactions, due to increased vibrational relaxation by electron–hole pair excitation. For example on Ni(1 1 0), formate adsorbed on long bridge sites has a lower DOS than formate adsorbed on short bridge sites, and can be made to diffuse by excitation of the C–H stretch, unlike the short bridge formate which can efficiently relax and does not diffuse under influence of an STM tip [219]. The requirement for the concerted excitation of two vibrations is a common theme in many other IET induced reactions. For example, the bond angle (adsorption position) of chlorine in the meta and

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Fig. 10. Left: topographic STM images of (CH3S)2 molecules on Cu(1 1 1) before (a) and after (b) injection of tunneling electrons. The corresponding action spectra are shown in (c) for two different isotopes and show peaks at the frequency of the C–H stretch (357.5 mV) and the combination of C–H and S–S stretch (410 mV). Right: partial density of states of an isolated (CH3S)2 molecule on Cu(1 1 1) for HOMO and LUMO and the spatial extent of these orbitals. The green arrows indicate the molecular orbitals on the C–H bond. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Source: Reprinted with permission from [218]. Copyright (2008) by the American Physical Society. Copyright (2008) by the American Physical Society.

ortho position of chloronitrobenzene on Cu(1 1 1) can be modified by exciting a combination mode of a C–C stretch and a C–Cl out-of-plane bend [220]. Constitutional isomerisation (Cl–H exchange) of this molecule and dichlorobenzene on Cu(1 1 1) can be affected by exciting a combination of vibrational modes which involve in-plane bending of hydrogen atoms to bring the chlorine and hydrogen closer together for exchange [221,222]. A similar excitation scheme can also be used to change the chirality of an adsorbate, as shown almost simultaneously by the groups of Morgenstern [223] and Ernst [224]. Morgenstern’s group studied meta-chloronitrobenzene on Au(1 1 1) and showed the molecule changes chirality most likely by flipping around the nitro group, which remains attached to the surface. The chirality change is more efficient when electrons are injected closer to the chlorine atom. The threshold voltage does not match a single vibrational mode, but again the sum of the C–Cl out-of-plane bend and a C–C stretch is the most likely combination of vibrational modes. At a threshold voltage corresponding to a C–H stretch, the molecule simply rotates instead of changing chirality, and the reaction yield depends much less on injection position. Parschau et al. switched the chirality of adsorbed propene on the stepped Cu(211) surface as shown in Fig. 11 [224]. In this system, propene forms two rotamers (methyl group on lower or upper terrace), each of which come in two enantiomeric forms. The rotamers are transformed into one another by either exciting the C@C stretch (methyl group rotates from upper to lower terrace) or the CH3 stretching vibration to induce the backtransformation. If the sample bias is further increased, excitation above the energy of a C(sp2)–H stretching vibration leads to hopping at low tunneling currents and enantiomer interconversion at higher currents. One scenario presented by the authors explains the quadratic dependence of the interconversion rate on tunneling current by a two-electron process – the first electron excites the C(sp2)–H stretch and with it pushes the molecule over the diffusion barrier, the reduced binding to the surface then allows inversion in the molecular plane by excitation of the CH3 stretch. A large number of these STM induced vibrationally mediated reactions are based on exciting electrons into relatively low-lying molecular orbitals. These are all within reach of a 2000 K hot electron bath created by a typical femtosecond laser pulse, but the most complex adsorbate investigated with

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Fig. 11. Interconversion of enantoimeric states of propene on Cu(211). Left: STM images (4.1 nm)2 before (a) and after (b) conversion by tunneling electrons at 500 mV bias and 5 nA current. Right: the reaction order is 1 for hopping of propene, but 2 for conversion. Source: Reproduced with permission from [224]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.

femtosecond time-resolved vibrational spectroscopy to date has been the previously mentioned D2O on a CO covered Pt(1 1 1) surface [148]. There are however better candidates for observing hot-electron induced changes in molecular vibrations. For example, some C–H stretching modes in alkanes and cycloalkanes are softened due to their strong coupling to substrate electronic states [225–227], while dynamic interfacial charge transfer and strong coupling to electron–hole pairs is seen for large p-conjugated molecules like zinc-phthalocyanine [228] and 3,4,9,10-perylene-tetracarboxylic-dianhydride [29,229] on silver surfaces from asymmetric Fano-like line shapes in HREEL spectra. A very promising approach is also the combination of STM with low fluence femtosecond laser excitation, which has revealed rather complex motions like the CO dimer formation mentioned above, but also the diffusion of single water molecules within water clusters on Cu(1 1 1) [189,230,231]. Such a setup allows direct comparison between inelastically tunneling and laser-excited electrons. Besides the obvious benefit of time-resolution for the study of dynamics, a hot electron bath created with higher fluence femtosecond laser irradiation has the advantage that large surface areas are affected equally. The disadvantage is the rather large width of the hot electron energy distribution, which prevents the detection of threshold energies that can be translated into vibrational modes, such as in IET. In a few cases, nascent electrons, with their sharply defined energy distribution, have been shown to be the cause of surface reactions [189,232,233], but the electron thermalisation time is often so short [177] that the effects observed can be entirely explained by a hot electron distribution. One way to create hot electrons with well defined energies over a large spatial area is to use a metal–insulator-metal tunnel junction, first suggested by Gadzuk [234]. However, only a few experimental realizations are known [235–237] and have to date not provided much spectroscopic information about the reactions investigated. A promising, new observation in STM-induced reactions might balance the spatial extent of hot electron excitation with control of excitation energy. Hot electrons injected from an STM tip have been recently shown to cause nonlocal surface chemistry [238,239] or even chain reactions [240]. Nonlocal dissociation of dimethyldisulfide on Au(1 1 1) was seen at distances up to 100 nm from the STM tip [239]. This work by Yates’ group found that the phenomenon occurred for a wide range of surfaces (Au(1 0 0), Cu(1 1 1) and Cu(1 1 0)) and a range of molecules (CH3SH, C6H5SH). In all cases, the energy onset is in the range of 1–2 eV, making dissociative electron attachment the likely cause of the reaction. Lateral hot electron transport via a surface resonance of both Au(1 1 1) and Au(1 0 0) causes a collective reaction in chains of CH3SSCH3 – the reaction involves S–S bond rupture and recombination leaving behind single CH3S species at each end of the chain and CH3SSCH3 molecules in a switched orientation. Up to 10 molecules in a chain have been found to react. Hot electron transport along the surface was also obtained by Chen et al. [238] for a fluorinated copper phthalocyanine on Ag(1 1 1) and Au(1 1 1), which showed defluorination within a 12 nm radius of the tip. Again, dissociative electron attachment, rather than excitation of vibrational modes is the likely explanation.

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The electronic state induced by adsorption lies in the band gaps of the two close-packed surfaces, which favors transport parallel to the surface. The higher quantum yield on Ag compared to Au is explained by better charge transport along the Ag pxy-orbitals compared to the Au dxz and dyz orbitals and a longer lifetime of the anion due to smaller overlap between aromatic carbons and the silver surface. 3.3.2. Adsorbate resonances and charge transfer in spectroscopy and photochemistry Electronic resonances of adsorbates play an important role in surface photochemistry, surface-enhanced Raman scattering and doubly-resonant sum-frequency generation. Vibrational dynamics are not usually directly measured in these experiments, but the nature of the resonance has a strong impact on which bonds are affected in photochemistry and which modes are observed in vibrational spectroscopy, thus forming an integral part of our wider concern of using vibrational spectroscopy to understand energy transfer at surfaces. When a molecule binds to a metal surface, its electronic states shift down in energy and broaden into resonances through interaction with the metal bands. In the d-band model of surface chemistry, the interaction of the molecule with the surface is separated into contributions from coupling to a broad sp-band and the coupling to a much narrower d-band [32]. The sp contribution just yields a broadened and shifted adsorbate resonance, while the interaction with the relatively narrow d-band produces bonding and anti-bonding states below and above the Fermi level. These HOMO and LUMO states shown in Fig. 12 can serve to illustrate the two mechanisms of surface photochemistry, namely substrate-mediated and adsorbate-mediated processes. In the substratemediated mechanism, the incident photon generates a hot electron and a hot hole, which can interact with the adsorbate–metal substrate hybridized states. Hot electron photochemistry is the more commonly invoked mechanism, though reactions triggered by hot holes are known [241,242]. The substrate-mediated mechanism generally dominates the photochemistry on metal surfaces, partly because the surface absorbance in the visible–UV range is typically a factor 1000 higher than the molecular absorbance and partly because the photon energy threshold compared to the gas phase is lowered by several eV into the visible range, when electrons from the Fermi level are excited into an anti-bonding adsorbate resonance. When surface photochemistry is investigated in the UV, adsorbate-mediated processes involving an electronic transition between the bonding and anti-bonding

Fig. 12. Schematic illustration of substrate- and adsorbate-mediated mechanisms in surface photochemistry. In the substratemediated mechanism incident light generates an electron hole pair, where either the hot hole interacts with the molecule’s HOMO (process 1) or the hot electron interacts with the LUMO (process 2). The adsorbate-mediated process 3 is a direct HOMO–LUMO excitation in the molecule.

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adsorbate–metal hybridized states can become apparent as an extra reaction channel [243]. Substratemediated mechanisms can in principle be distinguished from adsorbate-mediated ones by changing the angle of incidence. If the photoyield has a different angular dependence to the surface absorbance, an adsorbate-mediated process is at play, although this distinction is only possible for transition dipole moments perpendicular to the surface [244]. Otherwise, the mechanism has to be identified by measuring the wavelength dependence of the yield as well as determining the position and width of electronic adsorbate resonances. For UHV studies the latter is often obtained from two-photon photoemission (2PPE), which can also follow photon-induced charge transfer in real time using a pump– probe scheme as reviewed by Lindstrom and Zhu [245]. One of the best researched examples of surface photochemistry is that of NO dimers adsorbed on silver, where recently a new reaction product and a particle size-dependent reaction pathway were revealed [246,247]. The photoreaction of the NO dimer on Ag(1 1 1) is known to occur via hot electron attachment into an orbital about 2 eV above the Fermi level. This resonance orbital is anti-bonding for both the dimer as well as the N–O bond, which leads to a range of reaction products: breaking of the dimer bond leads to NO desorption and breaking of the N–O bond before the dimer bond yields N2O. The detection of N2 was long believed to arise from subsequent dissociation of N2O, but Kim et al. could now show by time-of-flight spectroscopy that concerted N–O bond breaking in the dimer is the actual source of desorbing dinitrogen [246]. A mixture of adsorbate- and substrate-mediated mechanisms is displayed in the photochemistry of N2O on Ag(1 1 1) [243]. At photon energies above 3.5 eV, N2 is desorbed at high velocity into an off-normal direction, while a slower component desorbs in a direction normal to the surface. From angle-of-incidence measurements, Kim et al. could conclude that the fast species derives from direct photoexcitation of a tilted, upright N2O, while the slow N2 stems from a lying down species which is excited indirectly through substrate electrons. Silver nanoparticles deposited on alumina show a size dependence of the reaction pathway in NO dimer dissociation. For a photon energy of 4.7 eV and particles sizes below 2.5 nm, a new reaction channel opens up which instead of a transient negative ion involves formation of a transient positive ion by hole transfer from the Ag d-band [247]. The new mechanism is independent from the general enhancement of the reaction yield by plasmon and thereby hot electron excitation. At this stage, the reason for this change of mechanism is not fully understood, but is no doubt due to a combination of electron confinement within the nanoparticle and changes in the substrate–adsorbate electronic structure with decreasing particle size. Confinement of electrons in thin metal films or nanoparticles, effects of electron excitation in the underlying substrate, spillover and vibrational energy pooling are continuing themes in surface photo- and femtochemistry [248–254], though their effects on vibrational dynamics are almost wholly unknown. A recent example where 2PPE, HREELS and X-ray spectroscopy have been used to identify the intricacies of a substrate-mediated mechanism is the switching of tetra-tert-butyl-azobenzene (TBA) from trans to cis form on Au(1 1 1) [242,255,256]. There is strong interest in photoswitchable molecules with a view to changing macroscopic surface properties such as chirality, wetting and conductance [257,258]. Typically, the switching molecule has to be anchored to the metal surface via a linker group to reduce the electronic interaction between the switching group and the metal surface, in order to avoid rapid quenching of the electronic excitation. In TBA, despite separating the azo-group from the gold surface via four tert-butyl groups, the hybridized gold – TBA HOMO orbital turns out to be crucial for the switching properties of this molecule. The incident photon first creates a hole in the gold d-band, which relaxes to the top of the d-band within a few femtoseconds and transfers to the molecule’s HOMO which results in the isomerisation reaction. Fig. 13 illustrates the mechanism and shows how the effective cross-section of the process possesses two thresholds at 2.2 eV and 4.4 eV, which correspond to single and multiple hot holes generated at the top of the Au d-band [242]. Details of the nuclear dynamics involved in switching could be deduced from homochiral trans-TBA domains, which show chirality-dependent switching behavior [259]. No switching was found for TBA adsorbed on silver because the silver d-band edge is nearly 2 eV lower than on gold, which prevents efficient metal-HOMO hybridization and therefore charge transfer [260]. The photochemical mechanism can be changed from substrate- to adsorbate-mediated by changing the linker group from tetra-tert-butyl to a much larger tripodal thiol linker with adamantane core, which seems to electronically decouple the azo group from the gold surface [256]. A comparison between TBA and a

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Fig. 13. Left: excitation mechanism for the photoinduced trans/cis isomerization of TBA adsorbed on Au(1 1 1) via the creation of a positive ion resonance. Right: effective cross-section for the isomerisation as a function of photon energy, showing thresholds at 2.2 and 4.4 eV corresponding to single and multiple hot hole excitation, respectively. Source: Reprinted with permission [242]. Copyright 2008, American Institute of Physics.

stilbene derivative showed that energetically similar HOMOs are not sufficient to induce isomerisation after transient positive ion formation – the potential energy surface additionally needs a large enough gradient at the Franck–Condon point as well as a low barrier to the isomer to allow switching [261]. Substrate and adsorbate-mediated processes are the photochemical analogues of two chemical enhancement mechanisms in SERS – metal–molecule charge transfer and resonant excitation of the adsorbates’ HOMO to LUMO transition [262,263]. The charge transfer mechanism in SERS distinguishes between metal-to-molecule (‘‘hot electron’’) and molecule-to-metal (‘‘hot hole’’) transfer. The interplay between chemical enhancement and the normally dominant electromagnetic field enhancement through surface plasmon excitation is shown in Fig. 14, which also illustrates how the direction of charge transfer can be deduced in an electrochemical SERS setup by changing the applied voltage. Particularly large enhancements are observed in SERS when two or more enhancement mechanisms contribute at the wavelength of the exciting laser. The existence of a charge transfer enhancement at visible wavelengths is important for transition metals, which in this region possess only strongly damped surface plasmons due to interband transitions and therefore as a rule show less electromagnetic enhancement than the coinage metals. The prevalent enhancement mechanism also strongly influences the vibrational spectrum – when electromagnetic enhancement is dominant, then the main electric field component is perpendicular to the surface and only totally symmetric vibrational modes are strongly enhanced. The charge transfer and molecular resonance mechanisms on the other hand can give rise to non-totally symmetric modes and their presence is a clear sign that mechanisms other than electromagnetic field enhancement play a role. Uetsuki et al. experimentally identified modes enhanced by electromagnetic or chemical enhancement mechanisms by adsorbing 4-aminothiophenol either directly onto gold or silver or onto a 4 nm polyvinylpyridine spacer layer [264]. They showed that all b2-type modes were strongly attenuated in the presence of a spacer layer, whereas a1-type modes only decreased slightly in intensity due to the small increase in distance from the surface. The chemical enhancement was wavelength-dependent with maxima at 530 and 596 nm for Ag and Au, respectively, thus confirming the chemical character of the enhancement. The charge transfer contribution to a SERS spectrum will tend to dominate over the molecular resonance contribution because its transition energies are lower than most adsorbate HOMO–LUMO transitions, which gives it a greater weight in the Lombardi and Birke theory of SERS [262,263,265]. Chenal et al. used electrochemical SERS to quantify the charge transfer mechanism by tuning the electrode potential to bring a certain excitation wavelength into resonance, while keeping the

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Fig. 14. Left: electronic resonances in pyridine adsorbed on a silver nanoparticle: molecular resonances A1, B1, B2, surface plasmon resonance SPR and charge transfer resonance CT. Right: determination of the direction of charge transfer from a plot of potential of maximum SERS intensity versus excitation energy for a range of molecules adsorbed on silver colloids. A positive slope indicates metal to molecule charge transfer, while a negative slope signifies molecule to metal transfer. Source: Reprinted with permission from [262]. Copyright 2009, American Chemical Society.

electromagnetic enhancement constant [265]. By normalizing spectral intensities of certain lines against totally symmetric ones, they studied charge transfer in p-aminothiophenol, piperidine and tetracyanoethylene on gold. The first two molecules are examples for electron and hole transfer (which can be ascertained by plotting the slope of the applied voltage at resonance versus the excitation energy), while tetracyanoethylene allows independent measurement of the degree of charge transfer through the large shift in C–N stretching frequency. The coupling between adsorbate levels and the metal continuum was also very recently confirmed by identifying a Fano profile of line intensity versus applied voltage [30]. The chemical enhancement factor in SERS was controlled by Morton et al. with the help of a molecular photoswitch, where the energetic position of the HOMO with respect to the metal states changes strongly from the open to the closed form [266]. A measurement of the degree of charge transfer of a range of dyes on gold has been made by transient reflecting grating spectroscopy, where the amount of charge transfer was found to increase with decreasing difference between the Fermi and adsorbate levels [267]. Morton and Jensen [268] looked at the dependence of the chemical enhancement on the HOMO–LUMO gap of substituted pyridines on Ag clusters. They found that the enhancement scales as the fourth power of the ratio of the HOMO– LUMO gap in the free molecule over the metal HOMO – molecule LUMO gap in the adsorbate. The lifetime of the charge transfer state is also critical, as Lombardi and Birke found that the charge transfer enhancement scales as C4, where C is the homogeneous linewidth of the charge transfer transition [269]. Again, this is significant for adsorbates on transition metal surfaces because they tend to have wider resonances, shorter lifetimes of excited electronic states and therefore also smaller chemical enhancements. This lifetime also has a strong influence on mode selectivity in femtosecond laser-induced desorption. Schiøtz’ group developed a model of excited-state potential energy surfaces [270] to describe energy transfer from hot electrons to the center-of-mass vibration (the metal–molecule stretch) and the internal vibration for CO and NO on the close-packed surfaces of Pt, Pd, Rh and Ru. This model predicts for widths of the 2p* resonance larger than 0.5 eV, that the calculated desorption rate is entirely governed by the internal mode. For such wide resonances, the lifetime of an electron in the resonance is too short to effectively transfer energy to the center-of-mass mode, which has a vibration period within the range of 60–100 fs. It is known in the SERS field that the presence of a chemical enhancement mechanism often goes hand-in-hand with photochemical decomposition of the molecules under study [271,272], and photo-induced desorption and subsequent readsorption have been identified as a possible source of blinking in few-molecule SERS studies [273]. Overall though there is very little overlap between SERS studies of photochemistry (for some recent examples

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see [274–277]) and photochemistry studied on well defined metal crystal surfaces. The adsorption site, respectively, the structure of the metal surface often has a profound influence on the position of the adsorbate resonances and the ensuing photochemistry. A classic example is the site-dependence in the photochemistry of CO and NO on Pt(1 1 1), where atop molecules desorb, while bridge CO is unreactive and bridge NO dissociates [278,279]. Large differences in the energetic positions of adsorbate LUMOs are also found in molecules more typical for SERS – pyridine on Cu(1 1 1) has the lowest unoccupied state at 3.4 eV [280], while the same state for pyridine on the more open Cu(1 1 0) lies at 2.3 eV for upright molecules and 2.6 eV for tilted ones [281]. There is early work by Campion’s group showing that chemical enhancement is indeed sensitive to the crystal face in a SERS study of pyromellitic dianhydride on Cu(1 1 1) and Cu(1 0 0) [282]. These studies tie the concept of ‘‘SERS-active sites’’ [16,283] to more quantifiable surface properties. Adsorbate resonances also play an important role in doubly-resonant SFG (DR-SFG), and charge transfer [284], surface plasmon [71,285] and molecular resonance [286] enhancements are all known phenomena in SFG. Despite the similarities between DR-SFG and SERS, these techniques are only very rarely used together [75,287–289]. The relationship between intensities of resonance Raman and DR-SFG signals and the selection rules of vibrational modes of chiral molecules has been established theoretically [290] and similar studies could provide the basis for quantitatively comparing surface enhanced spectroscopies. The focus of many doubly-resonant SFG studies could be summarized as using the electronic resonance to learn more about the properties of the adsorbate [291–296]. For example, Maeda et al. used DR-SFG to distinguish the anionic from the neutral form of fluorescin isothyanate on Pt(1 1 1) [297], as the two species have an electronic resonance at different wavelengths. Two conformations of a mutant form of green fluorescent protein were also discriminated on Pt(1 1 1) by different electronic resonances. Wu et al. reported the first observation of a doubly-resonant SFG process, where the vibrational transition occurs in the electronically excited state of the adsorbate – the SFG spectrum of rhodamine 6G at the air/water interface shows an interference between IR-visible and visible-IR processes occurring in the ground and excited electronic states [298]. In the main, the effect of adsorbate resonances on photochemistry and spectroscopic selection rules and intensity is reasonably well understood. What is unknown to date is whether or how these adsorbate resonances influence vibrational dynamics. For example, a recent proof-of-concept of molecular orbital gating in gold – 1,4-benzenedithiol junctions, showed that the dominant transport orbital was strongly coupled to those vibrational modes involving bonds on which that particular orbital is localized [299], reminiscent of the results obtained by Ohara et al. shown in Fig. 11 [218]. Similarly, the propensity rules for electron transport through nanoscale junctions [300] bear great resemblance to the rules for coupling strength between electron–hole pairs and vibrational modes as introduced in Section 2.3. Therefore one could envisage that different vibrational decay channels open as certain adsorbate resonances are excited, offering a way to establish control over vibrational relaxation.

4. Future work The preceding pages have been a collage of recent important developments in vibrational dynamics of adsorbates at metal surfaces. We have arrived at a pivotal stage, where a large number of experimental techniques have reached a maturity and sophistication that allows us to tackle, with confidence, vastly more complex systems than CO/Cu(1 0 0). We can deduce a few general rules of thumb for vibrational energy transfer for adsorbates at a metal/vacuum interface from the work done to date. If we consider transfer from an adsorbate vibrational mode to the surface, then the mismatch to typical phonon energies, lower frequency intramolecular vibrations and the change of the molecular charge density distribution by the vibration will give an indication whether energy transfer occurs to electron–hole pairs, other internal vibrations or phonons. The time scale of energy transfer increases roughly by an order of magnitude between these processes, i.e. 1 ps for electron–hole pairs, 10 ps for other internal vibrations and 100 ps for transfer to phonons. The importance of anharmonic coupling between vibrational modes can be judged in the first instance by establishing the existence of combination modes by RAIRS and SERS. If we are interested in the energy transfer from hot electrons or phonons to the adsorbate to trigger chemical reactions, then the

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accepting vibrational mode will most likely be the one that combines the lowest frequency with the shortest electron–hole pair induced lifetime. The promoting mode can be identified from the vibrational motion needed to push the molecule towards the transition state for the type of reaction induced, e.g. frustrated translation for diffusion, metal–molecule stretch for desorption, internal stretch or frustrated rotation for dissociation. If electrons induce reactions by tunneling or electronic excitation, then a consideration of the molecular orbitals with high density of states at the electron energy and their localization on particular bonds allows an educated guess on which vibrations could promote a particular reaction. An initial evaluation can therefore be made if the partial density of states for a range of molecular orbitals is known. However, this merely provides a rough guide to vibrational energy transfer and there are still many white areas on the map. The big questions we could tackle already today are numerous. What changes on ultrafast time scales if the metal adsorbates do not just interact with the surface or themselves but with a dense gas or a liquid? What changes occur in energy transfer between adsorbate and surface when we look at adsorbates that do not have just one internal vibration but many? What changes in the dynamics when adsorbates cannot interact with an extended surface, but are confined to a nanoparticle? Do surface phonon modes play a significant role in vibrational energy transfer at metal surfaces? The challenges we can tackle tomorrow if we form closer links between different communities are even more exciting. If we look at more complex molecules and interactions, then we are now ready to explore the vibrational dynamics of water at metal surfaces since its structure and the balance between metal–water and hydrogen bonding is now well understood for a wide range of pure and alloy surfaces. The understanding of electron and heat transport through metal/molecule junctions has already benefitted from ultrafast pump–probe studies, but joint endeavors of spatially and time-resolved spectroscopies are still only few and far between [189,301,302]. Furthermore, the direct probe of the dynamics of external adsorbate vibrations is the ‘‘missing link’’ of surface vibrational dynamics, especially for adsorbates on metal surfaces, as Section 3.1. has shown. To achieve this goal, we will have to increase experimental sensitivity, for example by using a wide range of enhancement mechanisms, be that multiple resonances, electric field enhancement, heterodyne detection schemes, make more extensive use of far-IR sources, synchrotron as well as table-top based, and probably develop new detection schemes for vibrational coherences. An understanding of the dynamics of internal as well as external adsorbate vibrations is a prerequisite for achieving mode-selective surface chemistry, which to date has only been accomplished on a semiconductor surface [204]. The control of surface vibrational dynamics with shaped mid-IR pulses has a good chance of success. This area could also make large steps forward by the development of single-molecule dynamics with a joint application of electron- and photon-based techniques. In conclusion, the field of vibrational dynamics has reached a rewarding and exhilarating stage where fundamental understanding can be balanced against real-world questions. Acknowledgements I am grateful to the UK’s Engineering and Physical Sciences Research Council for funding my Advanced Research Fellowship. Special thanks go to Annelie Arnolds-Woycke, Helmut Woycke and Keith Atkins for much needed out-of-the-box discussions. References [1] R.F. Aroca, Surface-Enhanced Vibrational Spectroscopy, John Wiley & Sons, 2006. [2] E.C. Le Ru, P.G. Etchegoin, Rigorous justification of the |E|4 enhancement factor in surface enhanced Raman spectroscopy, Chemical Physics Letters 423 (2006) 63. [3] M. Joffre, in: C. Rulliere (Ed.), Femtosecond Laser Pulses – Principles and Experiments, 2003. [4] S. Mukamel, Principles of Nonlinear Optical Spectroscopy, Oxford University Press, 2005. [5] J.W. Gadzuk, A.C. Luntz, On vibrational lineshapes of adsorbed molecules, Surface Science 144 (1984) 429. [6] C.J. Hirschmugl, Y.J. Chabal, F.M. Hoffmann, G.P. Williams, Low-frequency dynamics of CO/Cu breakdown of Born– Oppenheimer approximation, Journal of Vacuum Science & Technology A – Vacuum Surfaces and Films 12 (1994) 2229. [7] A.P. Graham, The low energy dynamics of adsorbates on metal surfaces investigated with helium atom scattering, Surface Science Reports 49 (2003) 115.

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[8] A. Otto, P. Lilie, P. Dumas, C. Hirschmugl, M. Pilling, G.P. Williams, Anisotropic electric surface resistance of Cu(1 1 0), New Journal of Physics 9 (2007) 288. [9] B.N.J. Persson, Surface Resistivity And Vibrational Damping In Adsorbed Layers, Physical Review B 44 (1991) 3277. [10] C. Liu, R.G. Tobin, Bonding-site dependence of surface resistivity: CO on epitaxial Cu(1 0 0) films, Journal of Chemical Physics 126 (2007) 124705. [11] M. Hein, P. Dumas, M. Sinther, A. Priebe, P. Lilie, A. Bruckbauer, A. Pucci, A. Otto, Relation between surface resistance, infrared-, surface enhanced infrared- and surface enhanced Raman-spectroscopy’s of C2 H4 and CO on copper, Surface Science 600 (2006) 1017. [12] B.N.J. Persson, On the theory of surface-enhanced Raman-scattering, Chemical Physics Letters 82 (1981) 561. [13] A. Campion, J.K. Brown, V.M. Grizzle, Surface Raman-spectroscopy without enhancement – nitrobenzene on Ni(1 1 1), Surface Science 115 (1982) L153. [14] H.A. Marzouk, E.B. Bradley, K.A. Arunkumar, Normal unenhanced Raman-spectra of CO adsorbed on Ni(1 0 0) at liquidnitrogen temperature and at room-temperature, Surface Science 161 (1985) 462. [15] C.M. Child, J.E. Fieberg, A. Campion, Surface chemistry of polyimide formation on Cu(1 1 1), Surface Science 372 (1997) L254. [16] A. Otto, M. Lust, A. Pucci, G. Meyer, ‘‘SERS active sites’’, facts, and open questions, Canadian Journal of Analytical Sciences and Spectroscopy 52 (2007) 150. [17] W. Akemann, A. Otto, The effect of atomic-scale surface disorder on bonding and activation of adsorbates – vibrational properties of CO and CO2 on copper, Surface Science 287 (1993) 104. [18] B.N.J. Persson, M. Persson, Vibrational lifetime for CO adsorbed on Cu(1 0 0), Solid State Communications 36 (1980) 175. [19] M. Morin, N.J. Levinos, A.L. Harris, Vibrational-energy transfer of CO/Cu(1 0 0) – nonadiabatic vibration electron coupling, Journal of Chemical Physics 96 (1992) 3950. [20] H. Arnolds, M. Bonn, Ultrafast surface vibrational dynamics, Surface Science Reports 65 (2010) 45. [21] M.E. Schmidt, P. GuyotSionnest, Electrochemical tuning of the lifetime of the CO stretching vibration for CO/Pt(1 1 1), Journal of Chemical Physics 104 (1996) 2438. [22] C. Matranga, B.L. Wehrenberg, P. Guyot-Sionnest, Vibrational relaxation of cyanide on copper surfaces: can metal d-bands influence vibrational energy transfer?, Journal of Physical Chemistry B 106 (2002) 8172. [23] A. Peremans, A. Tadjeddine, W.Q. Zheng, A. LeRille, P. GuyotSionnest, P.A. Thiry, Vibrational dynamics of CO at singlecrystal platinum electrodes in aqueous and non-aqueous electrolytes, Surface Science 368 (1996) 384. [24] V. Krishna, J.C. Tully, Vibrational lifetimes of molecular adsorbates on metal surfaces, Journal of Chemical Physics 125 (2006) 054706. [25] M. Forsblom, M. Persson, Vibrational lifetimes of cyanide and carbon monoxide on noble and transition metal surfaces, Journal of Chemical Physics 127 (2007) 154303. [26] M. Head-Gordon, J.C. Tully, Molecular-orbital calculations of the lifetimes of the vibrational-modes of CO on Cu(1 0 0), Physical Review B 46 (1992) 1853. [27] D.C. Langreth, Energy-transfer at surfaces – asymmetric line-shapes and the electron–hole pair mechanism, Physical Review Letters 54 (1985) 126. [28] J. Kroger, Nonadiabatic effects on surfaces: Kohn anomaly, electronic damping of adsorbate vibrations, and local heating of single molecules, Journal of Physics – Condensed Matter 20 (2008) 224015. [29] R. Temirov, S. Soubatch, A. Lassise, F.S. Tautz, Bonding and vibrational dynamics of a large p-conjugated molecule on a metal surface, Journal of Physics-Condensed Matter 20 (2008) 224010. [30] J.R. Lombardi, R.L. Birke, Excitation profiles and the continuum in SERS: identification of Fano line shapes, Journal of Physical Chemistry C 114 (2010) 7812. [31] A.I. Volokitin, B.N.J. Persson, Quantum-theory of infrared-reflection spectroscopy from adsorbate-covered metal-surfaces in the anomalous-skin-effect frequency region, Physical Review B 52 (1995) 2899. [32] B. Hammer, J.K. Norskov, in: Advances in Catalysis, vol. 45, 2000, pp. 71. [33] A.M. Bradshaw, J. Pritchar, Infrared spectra of carbon monoxide chemisorbed on metal films – a comparative study of copper, silver, gold, iron, cobalt and nickel, Proceedings of the Royal Society of London Series A –Mathematical and Physical Sciences 316 (1970) 169. [34] O. Krauth, G. Fahsold, N. Magg, A. Pucci, Anomalous infrared transmission of adsorbates on ultrathin metal films: Fano effect near the percolation threshold, Journal of Chemical Physics 113 (2000) 6330. [35] Z.F. Su, S.G. Sun, C.X. Wu, Z.P. Cai, Study of anomalous infrared properties of nanomaterials through effective medium theory, Journal of Chemical Physics 129 (2008) 044707. [36] D.A. Heaps, P.R. Griffiths, Band shapes in the infrared spectra of thin organic films on metal nanoparticles, Vibrational Spectroscopy 42 (2006) 45. [37] J.D. Lee, M. Nishino, Nonlinear time-resolved study of dynamics of a CO adsorbate on a metal: time-evolving Fano resonance with electron–hole continuum, Physical Review B 72 (2005) 195111. [38] T.A. Germer, J.C. Stephenson, E.J. Heilweil, R.R. Cavanagh, Picosecond time-resolved adsorbate response to substrate heating – spectroscopy and dynamics of CO/Cu(1 0 0), Journal of Chemical Physics 101 (1994) 1704. [39] L.M. Struck, L.J. Richter, S.A. Buntin, R.R. Cavanagh, J.C. Stephenson, Femtosecond laser-induced desorption of CO from Cu(1 0 0): comparison of theory and experiment, Physical Review Letters 77 (1996) 4576. [40] C. Frischkorn, M. Wolf, Femtochemistry at metal surfaces: nonadiabatic reaction dynamics, Chemical Reviews 106 (2006) 4207. [41] M. Brandbyge, P. Hedegard, T.F. Heinz, J.A. Misewich, D.M. Newns, Electronically driven adsorbate excitation mechanism in femtosecond-pulse laser-desorption, Physical Review B 52 (1995) 6042. [42] L. Cai, X.D. Xiao, M.M.T. Loy, Femtosecond desorption of CO from metal surfaces: the role of the 2 pi*-derived states, Surface Science 492 (2001) L688. [43] B.N.J. Persson, R. Ryberg, Vibrational interaction between molecules adsorbed on a metal-surface – the dipole–dipole interaction, Physical Review B 24 (1981) 6954.

H. Arnolds / Progress in Surface Science 86 (2011) 1–40

33

[44] P. Gao, M.J. Weaver, Vibrational coupling effects for cyanide and aromatic adsorbates at gold electrodes – a comparativestudy using surface Raman and infrared spectroscopies, Journal of Physical Chemistry 93 (1989) 6205. [45] P. Zhang, Y.X. Chen, J. Cai, S.Z. Liang, J.F. Li, A. Wang, B. Ren, Z.Q. Tian, An electrochemical in situ surface-enhanced Raman spectroscopic study of carbon monoxide chemisorption at a gold core-platinum shell nanoparticle electrode with a flow cell, Journal of Physical Chemistry C 113 (2009) 17518. [46] B.N.J. Persson, F.M. Hoffmann, R. Ryberg, Influence of exciton motion on the shape of optical-absorption lines – applications to vibrations at surfaces, Physical Review B 34 (1986) 2266. [47] P. Jakob, B.N.J. Persson, Dephasing of localized and delocalized vibrational modes: CO adsorbed on Ru(0 0 1), Physical Review B 56 (1997) 10644. [48] B.N.J. Persson, R. Ryberg, Dipole-coupling-induced line narrowing in adsorbate vibrational spectroscopy, Chemical Physics Letters 174 (1990) 443. [49] I.M. Lane, D.A. King, H. Arnolds, The determination of an inhomogeneous linewidth for a strongly coupled adsorbate system, Journal of Chemical Physics 126 (2007). [50] C. Hess, M. Wolf, M. Bonn, Direct observation of vibrational energy delocalization on surfaces: CO on Ru(0 0 1), Physical Review Letters 85 (2000) 4341. [51] V.L. Zhang, H. Arnolds, D.A. King, Hot band excitation of CO/Ir{1 1 1} studied by broadband sum frequency generation, Surface Science 587 (2004) 102. [52] Y. Nagata, Y. Tanimura, S. Mukamel, Two-dimensional infrared surface spectroscopy for CO on Cu(1 0 0): detection of intermolecular coupling of adsorbates, Journal of Chemical Physics 126 (2007) 204703. [53] P. Jakob, Investigation of one- and two-dimensional vibrational density of states using two-phonon infrared absorption spectroscopy, Journal of Chemical Physics 114 (2001) 3692. [54] P. Jakob, B.N.J. Persson, Infrared spectroscopy of overtones and combination bands, Journal of Chemical Physics 109 (1998) 8641. [55] P. Jakob, Fermi resonance distortion of the Ru–CO stretching mode of CO adsorbed on Ru(0 0 1), Journal of Chemical Physics 108 (1998) 5035. [56] U. Engstrom, R. Ryberg, Freezing out a Fermi resonance. A temperature dependence study of the low-energy modes of CO on Pt(1 1 1), Journal of Chemical Physics 115 (2001) 519. [57] M.P. Andersson, J. Blomquist, P. Uvdal, Surface-induced C–O bond anharmonicity of methoxy adsorbed on Cu(1 0 0): experiments and density–functional theory calculations, Journal of Chemical Physics 123 (2005) 224714. [58] R. Ryberg, Vibrational study of a combination band and the effects of inhomogeneous broadening – CO on Pt(1 1 1), Physical Review B 44 (1991) 13160. [59] G. Witte, Low frequency vibrational modes of adsorbates, Surface Science 502 (2002) 405. [60] C.J. Baily, M. Surman, A.E. Russell, Investigation of the CO induced lifting of the (1 x 2) reconstruction on Pt{1 1 0} using synchrotron far-infrared RAIRS, Surface Science 523 (2003) 111. [61] F.H. Scholes, A. Locatelli, H. Kleine, V.P. Ostanin, D.A. King, Low frequency infrared emission spectroscopy of molecules at single crystal surfaces, Surface Science 502 (2002) 249. [62] S. Jakubith, H.H. Rotermund, W. Engel, A. Vonoertzen, G. Ertl, Spatiotemporal concentration patterns in a surfacereaction – propagating and standing waves, rotating spirals, and turbulence, Physical Review Letters 65 (1990) 3013. [63] M. Gruyters, D.A. King, Effects of restructuring in adsorption and reaction dynamics at metal surfaces, Journal of the Chemical Society-Faraday Transactions 93 (1997) 2947. [64] L. Vitali, R. Ohmann, K. Kern, A. Garcia-Lekue, T. Frederiksen, D. Sanchez-Portal, A. Arnau, Surveying molecular vibrations during the formation of metal–molecule nanocontacts, Nano Letters 10 (2010) 657. [65] R. Braun, B.D. Casson, C.D. Bain, E.W.M. van der Ham, Q.H.F. Vrehen, E.R. Eliel, A.M. Briggs, P.B. Davies, Sum-frequency generation from thiophenol on silver in the mid and far-IR, Journal of Chemical Physics 110 (1999) 4634. [66] C.T. Williams, Y. Yang, C.D. Bain, Prospects for detecting metal-adsorbate vibrations by sum-frequency spectroscopy, Catalysis Letters 61 (1999) 7. [67] A.B. Sugiharto, C.M. Johnson, H.B. De Aguiar, L. Alloatti, S. Roke, Generation and application of high power femtosecond pulses in the vibrational fingerprint region, Applied Physics B – Lasers and Optics 91 (2008) 315. [68] A.B. Sugiharto, C.M. Johnson, I.E. Dunlop, S. Roke, Delocalized surface modes reveal three-dimensional structures of complex biomolecules, Journal of Physical Chemistry C 112 (2008) 7531. [69] W.T. Liu, Y.R. Shen, Sum-frequency phonon spectroscopy on alpha-quartz, Physical Review B 78 (2008) 024302. [70] W.T. Liu, Y.R. Shen, Surface vibrational modes of alpha-quartz(0 0 0 1) probed by sum-frequency spectroscopy, Physical Review Letters 101 (2008) 016101. [71] S. Baldelli, A.S. Eppler, E. Anderson, Y.R. Shen, G.A. Somorjai, Surface enhanced sum frequency generation of carbon monoxide adsorbed on platinum nanoparticle arrays, Journal of Chemical Physics 113 (2000) 5432. [72] A.N. Bordenyuk, C. Weeraman, A.V. Benderskii, in: M.I. Stockman (Ed.), Plasmonics: Metallic Nanostructures and their Optical Properties V, 2007, pp. C6410. [73] Q.F. Li, C.W. Kuo, Z. Yang, P.L. Chen, K.C. Chou, Surface-enhanced IR-visible sum frequency generation vibrational spectroscopy, Physical Chemistry Chemical Physics 11 (2009) 3436. [74] G. Tourillon, L. Dreesen, C. Volcke, Y. Sartenaer, P.A. Thiry, A. Peremans, Total internal reflection sum-frequency generation spectroscopy and dense gold nanoparticles monolayer: a route for probing adsorbed molecules, Nanotechnology 18 (2007) 415301. [75] C. Weeraman, A.K. Yatawara, A.N. Bordenyuk, A.V. Benderskii, Effect of nanoscale geometry on molecular conformation: vibrational sum-frequency generation of alkanethiols on gold nanoparticles, Journal of the American Chemical Society 128 (2006) 14244. [76] J.X. Cheng, X.S. Xie, Coherent anti-stokes Raman scattering microscopy: instrumentation, theory, and applications, Journal of Physical Chemistry B 108 (2004) 827. [77] E.J. Liang, A. Weippert, J.M. Funk, A. Materny, W. Kiefer, Experimental Observation of Surface-Enhanced Coherent antiStokes-Raman Scattering, Chemical Physics Letters 227 (1994) 115.

34

H. Arnolds / Progress in Surface Science 86 (2011) 1–40

[78] C.J. Addison, S.O. Konorov, A.G. Brolo, M.W. Blades, R.F.B. Turner, Tuning gold nanoparticle self-assembly for optimum coherent anti-stokes Raman scattering and second harmonic generation response, Journal of Physical Chemistry C 113 (2009) 3586. [79] N. Hayazawa, T. Ichimura, M. Hashimoto, Y. Inouye, S. Kawata, Amplification of coherent anti-Stokes Raman scattering by a metallic nanostructure for a high resolution vibration microscopy, Journal of Applied Physics 95 (2004) 2676. [80] T.W. Koo, S. Chan, A.A. Berlin, Single-molecule detection of biomolecules by surface-enhanced coherent anti-Stokes Raman scattering, Optics Letters 30 (2005) 1024. [81] Z.D. Schultz, M.C. Gurau, L.J. Richter, Broadband coherent anti-stokes Raman spectroscopy characterization of polymer thin films, Applied Spectroscopy 60 (2006) 1097. [82] B.D. Prince, A. Chakraborty, B.M. Prince, H.U. Stauffer, Development of simultaneous frequency- and time-resolved coherent anti-Stokes Raman scattering for ultrafast detection of molecular Raman spectra, Journal of Chemical Physics 125 (2006) 044502. [83] P. Kukura, D.W. McCamant, R.A. Mathies, Femtosecond stimulated Raman spectroscopy, Annual Review of Physical Chemistry 58 (2007) 461. [84] R.R. Frontiera, J. Dasgupta, R.A. Mathies, Probing interfacial electron transfer in coumarin 343 sensitized TiO2 nanoparticles with femtosecond stimulated Raman, Journal of the American Chemical Society 131 (2009) 15630. [85] H. Yui, H. Kato, Y. Someya, Characteristic wavenumber shifts of the stimulated Raman scattering from interfacial water molecules induced by laser-induced plasma generation at air–water and water–silver interfaces, Journal of Raman Spectroscopy 39 (2008) 1688. [86] S. Yamaguchi, T. Tahara, Interface-specific v(4) coherent Raman spectroscopy in the frequency domain, Journal of Physical Chemistry B 109 (2005) 24211. [87] S. Yamaguchi, T. Tahara, v(4) Raman spectroscopy for buried water interfaces, Angewandte Chemie – International Edition 46 (2007) 7609. [88] S. Yamaguchi, T. Tahara, Novel interface-selective even-order nonlinear spectroscopy, Laser & Photonics Reviews 2 (2008) 74. [89] T. Nomoto, A. Sasahara, H. Onishi, Optically excited near-surface phonons of TiO2 (1 1 0) observed by fourth-order coherent Raman spectroscopy, Journal of Chemical Physics 131 (2009) 084703. [90] T. Nomoto, H. Onishi, Fourth-order Raman spectroscopy of adsorbed organic species on TiO2 surface, Chemical Physics Letters 455 (2008) 343. [91] T. Nomoto, H. Onishi, Fourth-order coherent Raman spectroscopy in a time doma, in: applications to buried interfaces, Physical Chemistry Chemical Physics 9 (2007) 5515. [92] M. Fuyuki, K. Watanabe, D. Ino, H. Petek, Y. Matsumoto, Electron–phonon coupling at an atomically defined interface: Na quantum well on Cu(1 1 1), Physical Review B 76 (2007) 115427. [93] K. Watanabe, K. Inoue, I.F. Nakai, M. Fuyuki, Y. Matsumoto, Ultrafast electron and lattice dynamics at potassium-covered Cu(1 1 1) surfaces, Physical Review B 80 (2009) 075404. [94] Y. Matsumoto, K. Watanabe, Coherent vibrations of adsorbates induced by femtosecond laser excitation, Chemical Reviews 106 (2006) 4234. [95] K. Watanabe, N. Takagi, Y. Matsumoto, Mode-selective excitation of coherent surface phonons on alkali-covered metal surfaces, Physical Chemistry Chemical Physics 7 (2005) 2697. [96] T. Yasuike, K. Nobusada, Photoinduced coherent adsorbate dynamics on a metal surface: nuclear wave-packet simulation with quasi-diabatic potential energy curves using an open-boundary cluster model approach, Physical Review B 80 (2009). [97] R. Jorn, T. Seideman, Dissipation in molecular junctions, Journal of Chemical Physics 129 (2008). [98] R. Jorn, T. Seideman, Competition between current-induced excitation and bath-induced decoherence in molecular junctions, Journal of Chemical Physics 131 (2009). [99] T.F. Luo, J.R. Lloyd, Non-equilibrium molecular dynamics study of thermal energy transport in Au-SAM-Au junctions, International Journal of Heat and Mass Transfer 53 (2010) 1. [100] T. Nomoto, H. Onishi, Time-domain infrared-visible-visible sum-frequency generation for surface vibrational spectroscopy, Journal of Physical Chemistry C 113 (2009) 21467. [101] Y. Rao, D.H. Song, N.J. Turro, K.B. Eisenthal, Orientational motions of vibrational chromophores in molecules at the air/water interface with time-resolved sum frequency generation, Journal of Physical Chemistry B 112 (2008) 13572. [102] H.K. Nienhuys, M. Bonn, Measuring molecular reorientation at liquid surfaces with time-resolved sum-frequency spectroscopy: a theoretical framework, Journal of Physical Chemistry B 113 (2009) 7564. [103] S. Yamamoto, A. Ghosh, H.K. Nienhuys, M. Bonn, Ultrafast inter- and intramolecular vibrational energy transfer between molecules at interfaces studied by time- and polarization-resolved SFG spectroscopy, Physical Chemistry Chemical Physics (2010). doi:10.1039/c0cp00538j. [104] X.Y. Chen, M.L. Clarke, J. Wang, Z. Chen, Sum frequency generation vibrational spectroscopy studies on molecular conformation and orientation of biological molecules at interfaces, International Journal of Modern Physics B 19 (2005) 691. [105] H.F. Wang, W. Gan, R. Lu, Y. Rao, B.H. Wu, Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS), International Reviews in Physical Chemistry 24 (2005) 191. [106] F. Cecchet, D. Lis, J. Guthmuller, B. Champagne, Y. Caudano, C. Silien, A.A. Mani, P.A. Thiry, A. Peremans, Orientational analysis of dodecanethiol and p-nitrothiophenol SAMs on metals with polarisation-dependent SFG spectroscopy, Chemphyschem 11 (2010) 607. [107] F. Cecchet, D. Lis, J. Guthmuller, B. Champagne, G. Fonder, Z. Mekhalif, Y. Caudano, A.A. Mani, P.A. Thiry, A. Peremans, Theoretical calculations and experimental measurements of the vibrational response of p-NTP SAMs: an orientational analysis, Journal of Physical Chemistry C 114 (2010) 4106. [108] R.C. Ambrosio, A.A. Gewirth, Characterization of water structure on silver electrode surfaces by sers with twodimensional correlation spectroscopy, Analytical Chemistry 82 (2010) 1305.

H. Arnolds / Progress in Surface Science 86 (2011) 1–40

35

[109] S. Titmuss, K. Johnson, Q. Ge, D.A. King, Structure, bonding, and and anharmonic librational motion of CO on Ir{1 0 0}, Journal of Chemical Physics 116 (2002) 8097. [110] M.P. Jigato, W.K. Walter, D.A. King, The temperature-dependent effect of adsorbate hindered vibrations on NEXAFS analyses – NO on Pd(1 1 0), Surface Science 301 (1994) 273. [111] J.T. Yates, J. Ahner, D. Mocuta, Tracking the motion of chemisorbed molecules on their adsorption sites, Proceedings of the National Academy of Sciences of the United States of America 95 (1998) 443. [112] R. Arafune, K. Hayashi, S. Ueda, Y. Uehara, S. Ushioda, Inelastic photoemission due to scattering by surface adsorbate vibrations, Physical Review Letters 95 (2005) 207601. [113] R. Arafune, K. Hayashi, S. Ueda, Y. Uehara, S. Ushioda, Detection of the frustrated rotation mode of CO on Cu(0 0 1) by very low energy photoelectron spectroscopy, Surface Science 600 (2006) 3536. [114] R. Arafune, K. Hayashi, S. Ueda, Y. Uehara, S. Ushioda, Spectroscopic evidence for energy loss of photoelectrons interacting with image charge, Journal of the Physical Society of Japan 76 (2007) 044604. [115] R. Arafune, M.Q. Yamamoto, N. Takagi, M. Kawai, Mechanism of vibrational excitation in inelastic photoemission from solid surfaces, Physical Review B 80 (2009) 073407. [116] A.P. Jardine, H. Hedgeland, G. Alexandrowicz, W. Allison, J. Ellis, Helium-3 spin-echo: principles and application to dynamics at surfaces, Progress in Surface Science 84 (2009) 323. [117] A.P. Jardine, H. Hedgeland, D. Ward, Y. Xiaoqing, W. Allison, J. Ellis, G. Alexandrowicz, Probing molecule–surface interactions through ultra-fast adsorbate dynamics: propane/Pt(1 1 1), New Journal of Physics 10 (2008) 125026. [118] H. Hedgeland, P.R. Kole, H.R. Davies, A.P. Jardine, G. Alexandrowicz, W. Allison, J. Ellis, G. Fratesi, G.P. Brivio, Surface dynamics and friction of K/Cu(0 0 1) characterized by helium-3 spin-echo and density functional theory, Physical Review B 80 (2009) 125426. [119] A.S. Leathers, D.A. Micha, Density matrix treatment of the nonmarkovian dissipative dynamics of adsorbates on metal surfaces, Journal of Physical Chemistry A 110 (2006) 749. [120] A.S. Leathers, D.A. Micha, D.S. Kilin, Density matrix treatment of combined instantaneous and delayed dissipation for an electronically excited adsorbate on a solid surface, Journal of Chemical Physics 131 (2009). p p [121] P. Jakob, Anharmonic coupling effects among adsorbate vibrational modes: the model system Ru(0 0 1)–( 3  3)R30°– CO revisited, Applied Physics A – Materials Science & Processing 75 (2002) 45. [122] A.L. Harris, L. Rothberg, L.H. Dubois, N.J. Levinos, L. Dhar, Molecular vibrational-energy relaxation at a metal-surface – methyl thiolate on Ag(1 1 1), Physical Review Letters 64 (1990) 2086. [123] S. Nihonyanagi, S. Yamaguchi, T. Tahara, Water Hydrogen Bond Structure near Highly Charged Interfaces Is Not Like Ice, Journal of the American Chemical Society 132 (2010) 6867. [124] M. Sovago, R.K. Campen, H.J. Bakker, M. Bonn, Hydrogen bonding strength of interfacial water determined with surface sum-frequency generation, Chemical Physics Letters 470 (2009) 7. [125] C.S. Tian, Y.R. Shen, Sum-frequency vibrational spectroscopic studies of water/vapor interfaces, Chemical Physics Letters 470 (2009) 1. [126] A.C.R. Pipino, M. Michalski, Climbing the vibrational ladder to probe the OH stretch of HNO3 on silica, Journal of Physical Chemistry C 111 (2007) 9442. [127] K. Egashira, A. Terasaki, T. Kondow, Photon-trap spectroscopy applied to molecules adsorbed on a solid surface: probing with a standing wave versus a propagating wave, Applied Optics 49 (2010) 1151. [128] G.A. Marcus, H.A. Schwettman, Cavity ringdown spectroscopy of thin films in the mid-infrared, Applied Optics 41 (2002) 5167. [129] B. Pettinger, Surface enhanced Raman-spectroscopy of pyridine on Ag electrodes – evidence for overtones, Chemical Physics Letters 78 (1981) 404. [130] J.C. Rubim, R.F. Aroca, The observation of high order overtones and combinations in the SERRS spectra of a perylene dye spin coated onto silver island films, Physical Chemistry Chemical Physics 10 (2008) 5412. [131] K. Kneipp, H. Kneipp, R. Manoharan, I. Itzkan, R.R. Dasari, M.S. Feld, Near-infrared surface-enhanced Raman scattering can detect single molecules and observe ‘hot’ vibrational transitions, Journal of Raman Spectroscopy 29 (1998) 743. [132] K. Kneipp, Y. Wang, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Population pumping of excited vibrational states by spontaneous surface-enhanced Raman scattering, Physical Review Letters 76 (1996) 2444. [133] T.L. Haslett, L. Tay, M. Moskovits, Can surface-enhanced Raman scattering serve as a channel for strong optical pumping?, Journal of Chemical Physics 113 (2000) 1641 [134] R.C. Maher, C.M. Galloway, E.C. Le Ru, L.F. Cohena, P.G. Etchegoin, Vibrational pumping in surface enhanced Raman scattering (SERS), Chemical Society Reviews 37 (2008) 965. [135] Y. Fang, N.H. Seong, D.D. Dlott, Measurement of the distribution of site enhancements in surface-enhanced Raman scattering, Science 321 (2008) 388. [136] F.C. Chien, W.Y. Huang, J.Y. Shiu, C.W. Kuo, P.L. Chen, Revealing the spatial distribution of the site enhancement for the surface enhanced Raman scattering on the regular nanoparticle arrays, Optics Express 17 (2009) 13974. [137] B. Pettinger, Single-molecule surface- and tip-enhanced raman spectroscopy, Molecular Physics 108 (2010) 2039. [138] J.A. Dieringer, R.B. Lettan, K.A. Scheidt, R.P. Van Duyne, A frequency domain existence proof of single-molecule surfaceenhanced Raman Spectroscopy, Journal of the American Chemical Society 129 (2007) 16249. [139] E.C. Le Ru, M. Meyer, P.G. Etchegoin, Proof of single-molecule sensitivity in surface enhanced Raman scattering (SERS) by means of a two-analyte technique, Journal of Physical Chemistry B 110 (2006) 1944. [140] P.G. Etchegoin, E.C. Le Ru, Resolving Single Molecules in Surface-Enhanced Raman Scattering within the Inhomogeneous Broadening of Raman Peaks, Analytical Chemistry 82 (2010) 2888. [141] C.M. Galloway, E.C. Le Ru, P.G. Etchegoin, Single-molecule vibrational pumping in SERS, Physical Chemistry Chemical Physics 11 (2009) 7372. [142] A. Otto, The ‘chemical’ (electronic) contribution to surface-enhanced Raman scattering, Journal of Raman Spectroscopy 36 (2005) 497. [143] J. Steidtner, B. Pettinger, Tip-enhanced Raman spectroscopy and microscopy on single dye molecules with 15 nm resolution, Physical Review Letters 100 (2008).

36

H. Arnolds / Progress in Surface Science 86 (2011) 1–40

[144] E.J. Blackie, E.C. Le Ru, P.G. Etchegoin, Single-molecule surface-enhanced Raman spectroscopy of nonresonant molecules, Journal of the American Chemical Society 131 (2009) 14466. [145] J. Kubota, K. Domen, Study of the dynamics of surface molecules by time-resolved sum-frequency generation spectroscopy, Analytical and Bioanalytical Chemistry 388 (2007) 17. [146] X.C. Su, L. Lianos, Y.R. Shen, G.A. Somorjai, Surface-induced ferroelectric ice on Pt(1 1 1), Physical Review Letters 80 (1998) 1533. [147] A. Hodgson, S. Haq, Water adsorption and the wetting of metal surfaces, Surface Science Reports 64 (2009) 381. [148] M. Nagao, K. Watanabe, Y. Matsumoto, Ultrafast vibrational energy transfer in the layers of D2O and CO on Pt(1 1 1) studied with time-resolved sum-frequency-generation spectroscopy, Journal of Physical Chemistry C 113 (2009) 11712. [149] D.S. Yang, A.H. Zewail, Ordered water structure at hydrophobic graphite interfaces observed by 4D, ultrafast electron crystallography, Proceedings of the National Academy of Sciences of the United States of America 106 (2009) 4122. [150] D. Chakarov, B. Kasemo, Photoinduced crystallization of amorphous ice films on graphite, Physical Review Letters 81 (1998) 5181. [151] E.H.G. Backus, M.L. Grecea, A.W. Kleyn, M. Bonn, Ultrafast electron-induced desorption of water from nanometer amorphous solid water films, Journal of Physical Chemistry B 111 (2007) 6141. [152] S.K. Shaw, A.A. Gewirth, Potential dependence of the structure of water at the hydrophobic liquid interface, Journal of Electroanalytical Chemistry 609 (2007) 94. [153] H. Noguchi, T. Okada, K. Uosaki, SFG study on potential-dependent structure of water at Pt electrode/electrolyte solution interface, Electrochimica Acta 53 (2008) 6841. [154] J.F. Li, Y.F. Huang, S. Duan, R. Pang, D.Y. Wu, B. Ren, X. Xu, Z.Q. Tian, SERS and DFT study of water on metal cathodes of silver, gold and platinum nanoparticles, Physical Chemistry Chemical Physics 12 (2010) 2493. [155] A. Yamakata, M. Osawa, Dynamics of double-layer restructuring on a platinum electrode covered by CO: laser-induced potential transient measurement, Journal of Physical Chemistry C 112 (2008) 11427. [156] S. Katano, S. Dobashi, J. Kubota, K. Onda, A. Wada, S.S. Kano, K. Domen, Structural change of CO adsorbed on Pt(1 1 1) by laser heating: time-resolved sum-frequency generation study, Chemical Physics Letters 377 (2003) 601. [157] J. Kubota, E. Yoda, N. Ishizawa, A. Wada, K. Domen, S.S. Kano, Site-hopping of adsorbed CO in c(4  2)–CO/Ni(1 1 1) by laser-induced temperature jump: time-resolved sum-frequency generation observation, Journal of Physical Chemistry B 107 (2003) 10329. [158] Z.B. Ge, D.G. Cahill, P.V. Braun, Thermal conductance of hydrophilic and hydrophobic interfaces, Physical Review Letters 96 (2006) 186101. [159] A. Ghosh, M. Smits, J. Bredenbeck, M. Bonn, Membrane-bound water is energetically decoupled from nearby bulk water: an ultrafast surface-specific investigation, Journal of the American Chemical Society 129 (2007) 9608. [160] Z.H. Wang, J.A. Carter, A. Lagutchev, Y.K. Koh, N.H. Seong, D.G. Cahill, D.D. Dlott, Ultrafast flash thermal conductance of molecular chains, Science 317 (2007) 787. [161] J.A. Carter, Z.H. Wang, D.D. Dlott, Spatially resolved vibrational energy transfer in molecular monolayers, Journal of Physical Chemistry A 112 (2008) 3523. [162] J.A. Carter, Z.H. Wang, D.D. Dlott, Ultrafast nonlinear coherent vibrational sum-frequency spectroscopy methods to study thermal conductance of molecules at interfaces, Accounts of Chemical Research 42 (2009) 1343. [163] J.A. Carter, Z.H. Wang, H. Fujiwara, D.D. Dlott, Ultrafast excitation of molecular adsorbates on flash-heated gold surfaces, Journal of Physical Chemistry A 113 (2009) 12105. [164] Z.H. Wang, D.G. Cahill, J.A. Carter, Y.K. Koh, A. Lagutchev, N.H. Seong, D.D. Dlott, Ultrafast dynamics of heat flow across molecules, Chemical Physics 350 (2008) 31. [165] E. Pop, Energy Dissipation and Transport in Nanoscale Devices, Nano Research 3 (2010) 147. [166] E. Hasselbrink, How non-adiabatic are surface dynamical processes?, Current Opinion in Solid State & Materials Science 10 (2006) 192 [167] A.M. Wodtke, J.C. Tully, D.J. Auerbach, Electronically non-adiabatic interactions of molecules at metal surfaces: can we trust the Born–Oppenheimer approximation for surface chemistry?, International Reviews in Physical Chemistry 23 (2004) 513 [168] A.M. Wodtke, D. Matsiev, D.J. Auerbach, Energy transfer and chemical dynamics at solid surfaces: the special role of charge transfer, Progress in Surface Science 83 (2008) 167. [169] N. Shenvi, S. Roy, J.C. Tully, Dynamical Steering and Electronic Excitation in NO Scattering from a Gold Surface, Science 326 (2009) 829. [170] E. Hasselbrink, Non-adiabaticity in surface chemical reactions, Surface Science 603 (2009) 1564. [171] P. Saalfrank, Quantum dynamical approach to ultrafast molecular desorption from surfaces, Chemical Reviews 106 (2006) 4116. [172] S.I. Anisimov, Bl Kapeliov, T.L. Perelman, Electron-emission from surface of metals induced by ultrashort laser pulses, Zhurnal Eksperimentalnoi I Teoreticheskoi Fiziki 66 (1974) 776. [173] E. Carpene, Ultrafast laser irradiation of metals: beyond the two-temperature model, Physical Review B 74 (2006) 024301. [174] J. Garduno-Mejia, M.P. Higlett, S.R. Meech, Modelling the influence of nonthermal electron dynamics in thin and ultrathin gold films, Chemical Physics 341 (2007) 276. [175] Z. Lin, L.V. Zhigilei, V. Celli, Electron–phonon coupling and electron heat capacity of metals under conditions of strong electron–phonon nonequilibrium, Physical Review B 77 (2008) 075133. [176] W.S. Fann, R. Storz, H.W.K. Tom, J. Bokor, Direct measurement of nonequilibrium electron-energy distributions in subpicosecond laser-heated gold-films, Physical Review Letters 68 (1992) 2834. [177] M. Lisowski, P.A. Loukakos, U. Bovensiepen, J. Stahler, C. Gahl, M. Wolf, Ultra-fast dynamics of electron thermalization, cooling and transport effects in Ru(0 0 1), Applied Physics a-Materials Science & Processing 78 (2004) 165. [178] B.N.J. Persson, H. Ueba, Heat transfer at surfaces exposed to short-pulsed laser fields, Physical Review B 76 (2007) 125401. [179] J.C. Tully, Chemical dynamics at metal surfaces, Annual Review of Physical Chemistry 51 (2000) 153.

H. Arnolds / Progress in Surface Science 86 (2011) 1–40

37

[180] H. Ueba, B.N.J. Persson, Heat transfer between adsorbate and laser-heated hot electrons, Journal of Physics – Condensed Matter 20 (2008) 224016. [181] H. Ueba, B.N.J. Persson, Heating of adsorbate by vibrational-mode coupling, Physical Review B 77 (2008) 035413. [182] E.H.G. Backus, A. Eichler, A.W. Kleyn, M. Bonn, Real-time observation of molecular motion on a surface, Science 310 (2005) 1790. [183] K. Stepan, M. Durr, J. Güdde, U. Höfer, Laser-induced diffusion of oxygen on a stepped Pt(1 1 1) surface, Surface Science 593 (2005) 54. [184] K. Stepan, J. Güdde, U. Höfer, Time-resolved measurement of surface diffusion induced by femtosecond laser pulses, Physical Review Letters 94 (2005) 236103. [185] T. Komeda, Y. Kim, M. Kawai, B.N.J. Persson, H. Ueba, Lateral hopping of molecules induced by excitation of internal vibration mode, Science 295 (2002) 2055. [186] M. Hayashi, Y. Ootsuka, M. Paulsson, B.N.J. Persson, H. Ueba, Lateral hopping of CO molecules on Pt(1 1 1) surface by femtosecond laser pulses, Physical Review B 80 (2009) 245409. [187] M. Lawrenz, K. Stepan, J. Güdde, U. Höfer, Time-domain investigation of laser-induced diffusion of CO on a vicinal Pt(1 1 1) surface, Physical Review B 80 (2009) 075429. [188] G. Alexandrowicz, P.R. Kole, E.Y.M. Lee, H. Hedgeland, R. Ferrando, A.P. Jardine, W. Allison, J. Ellis, Observation of uncorrelated microscopic motion in a strongly interacting adsorbate system, Journal of the American Chemical Society 130 (2008) 6789. [189] M. Mehlhorn, H. Gawronski, K. Morgenstern, Diffusion and dimer formation of CO molecules induced by femtosecond laser pulses, Physical Review Letters 104 (2010) 076101. [190] H. Arnolds, D.A. King, I.M. Lane, Inducing non-adiabatic effects through coadsorption: CO + NO on iridium, Chemical Physics 350 (2008) 94. [191] A.C. Luntz, M. Persson, S. Wagner, C. Frischkorn, M. Wolf, Femtosecond laser induced associative desorption of H2 from Ru(0001): comparison of ‘‘first principles’’ theory with experiment, Journal of Chemical Physics 124 (2006) 244702. [192] P. Szymanski, A.L. Harris, N. Camillone, Temperature-dependent electron-mediated coupling in subpicosecond photoinduced desorption, Surface Science 601 (2007) 3335. [193] P. Szymanski, A.L. Harris, N. Camillone, Temperature-dependent femtosecond photoinduced desorption in CO/Pd(1 1 1), Journal of Physical Chemistry A 111 (2007) 12524. [194] P. Szymanski, A.L. Harris, N. Camillone, Efficient subpicosecond photoinduced surface chemistry: the ultrafast photooxidation of CO on palladium, Journal of Physical Chemistry C 112 (2008) 15802. [195] I.M. Lane, D.A. King, Z.P. Liu, H. Arnolds, Real-time observation of nonadiabatic surface dynamics: the first picosecond in the dissociation of NO on iridium, Physical Review Letters 97 (2006) 186105. [196] T. Olsen, J. Schiøtz, Origin of Power Laws for Reactions at Metal Surfaces Mediated by Hot Electrons, Physical Review Letters 103 (2009) 238301. [197] C. Springer, M. HeadGordon, Simulations of the femtosecond laser-induced desorption of CO from Cu(1 0 0) at 0.5 ML coverage, Chemical Physics 205 (1996) 73. [198] C. Springer, M. Headgordon, J.C. Tully, Simulations of femtosecond laser-induced desorption of CO from Cu(1 0 0), Surface Science 320 (1994) L57. [199] N. Lorente, H. Ueba, CO dynamics induced by tunneling electrons: differences on Cu(1 1 0) and Ag(1 1 0), European Physical Journal D 35 (2005) 341. [200] B.N.J. Persson, R. Ryberg, Vibrational phase relaxation at surfaces – CO on Ni(1 1 1), Physical Review Letters 54 (1985) 2119. [201] E.H.G. Backus, M. Forsblom, M. Persson, M. Bonn, Highly efficient ultrafast energy transfer into molecules at surface step sites, Journal of Physical Chemistry C 111 (2007) 6149. [202] J.C. Tremblay, S. Beyvers, P. Saalfrank, Selective excitation of coupled CO vibrations on a dissipative Cu(1 0 0) surface by shaped infrared laser pulses, Journal of Chemical Physics 128 (2008) 194709. [203] J.C. Tremblay, P. Saalfrank, Selective subsurface absorption of hydrogen in palladium using laser distillation, Journal of Chemical Physics 131 (2009) 084716. [204] Z.H. Liu, L.C. Feldman, N.H. Tolk, Z.Y. Zhang, P.I. Cohen, Desorption of H from Si(1 1 1) by resonant excitation of the Si–H vibrational stretch mode, Science 312 (2006) 1024. [205] B.C. Stipe, M.A. Rezaei, W. Ho, S. Gao, M. Persson, B.I. Lundqvist, Single-molecule dissociation by tunneling electrons, Physical Review Letters 78 (1997) 4410. [206] B.C. Stipe, M.A. Rezaei, W. Ho, Coupling of vibrational excitation to the rotational motion of a single adsorbed molecule, Physical Review Letters 81 (1998) 1263. [207] B.C. Stipe, M.A. Rezaei, W. Ho, Inducing and viewing the rotational motion of a single molecule, Science 279 (1998) 1907. [208] E. Fomin, M. Tatarkhanov, T. Mitsui, M. Rose, D.F. Ogletree, M. Salmeron, Vibrationally assisted diffusion of H2O and D2O on Pd(1 1 1), Surface Science 600 (2006) 542. [209] A. Mugarza, T.K. Shimizu, D.F. Ogletree, M. Salmeron, Chemical reactions of water molecules on Ru(0001) induced by selective excitation of vibrational modes, Surface Science 603 (2009) 2030. [210] H.J. Shin, J. Jung, K. Motobayashi, S. Yanagisawa, Y. Morikawa, Y. Kim, M. Kawai, State-selective dissociation of a single water molecule on an ultrathin MgO film, Nature Materials 9 (2010) 442. [211] T. Kumagai, M. Kaizu, S. Hatta, H. Okuyama, T. Aruga, I. Hamada, Y. Morikawa, Direct observation of hydrogen-bond exchange within a single water dimer, Physical Review Letters 100 (2008). [212] S.G. Tikhodeev, H. Ueba, How vibrationally assisted tunneling with STM affects the motions and reactions of single adsorbates, Physical Review Letters 102 (2009) 246101. [213] E.R.M. Davidson, A. Alavi, A. Michaelides, Dynamics of quantum tunneling: effects on the rate and transition path of OH on Cu(1 1 0), Physical Review B 81 (2010). [214] T. Kumagai, M. Kaizu, H. Okuyama, S. Hatta, T. Aruga, I. Hamada, Y. Morikawa, Tunneling dynamics of a hydroxyl group adsorbed on Cu(1 1 0), Physical Review B 79 (2009).

38

H. Arnolds / Progress in Surface Science 86 (2011) 1–40

[215] J.R. Hahn, W. Ho, Vibrational mode specific bond dissociation in a single molecule, Journal of Chemical Physics 131 (2009) 044706. [216] Y. Sainoo, Y. Kim, T. Okawa, T. Komeda, H. Shigekawa, M. Kawai, Excitation of molecular vibrational modes with inelastic scanning tunneling microscopy processes: examination through action spectra of cis-2-butene on Pd(1 1 0), Physical Review Letters 95 (2005) 246102. [217] H. Ueba, B.N.J. Persson, Action spectroscopy for single-molecule motion induced by vibrational excitation with a scanning tunneling microscope, Physical Review B 75 (2007) 041403. [218] M. Ohara, Y. Kim, S. Yanagisawa, Y. Morikawa, M. Kawai, Role of molecular orbitals near the Fermi level in the excitation of vibrational modes of a single molecule at a scanning tunneling microscope junction, Physical Review Letters 100 (2008) 136104. [219] S. Katano, Y. Kim, Y. Kagata, M. Kawai, Single-molecule vibrational spectroscopy and inelastic-tunneling-electroninduced diffusion of formate adsorbed on Ni(1 1 0), Journal of Physical Chemistry C 114 (2010) 3003. [220] V. Simic-Milosevic, K. Morgenstern, Bending a bond within an individual adsorbed molecule, Journal of the American Chemical Society 131 (2009) 416. [221] V. Simic-Milosevic, M. Mehlhorn, K.H. Rieder, J. Meyer, K. Morgenstern, Electron induced ortho–meta isomerization of single molecules, Physical Review Letters 98 (2007) 116102. [222] V. Simic-Milosevic, J. Meyer, K. Morgenstern, Electron-induced isomerisation of dichlorobenzene on Cu(1 1 1) and Ag(1 1 1), Physical Chemistry Chemical Physics 10 (2008) 1916. [223] V. Simic-Milosevic, J. Meyer, K. Morgenstern, Chirality change of chloronitrobenzene on Au(1 1 1) induced by inelastic electron tunneling, Angewandte Chemie – International Edition 48 (2009) 4061. [224] M. Parschau, D. Passerone, K.H. Rieder, H.J. Hug, K.H. Ernst, Switching the chirality of single adsorbate complexes, Angewandte Chemie – International Edition 48 (2009) 4065. [225] K.A. Fosser, R.G. Nuzzo, P.S. Bagus, C. Woll, The origin of soft vibrational modes of alkanes adsorbed on Cu: an experimental and theoretical investigation, Journal of Chemical Physics 118 (2003) 5115. [226] T. Koitaya, A. Beniya, K. Mukai, S. Yoshimoto, J. Yoshinobu, Low-temperature observation of the softened C–H stretching vibrations of cyclohexane on Rh(1 1 1), Physical Review B 80 (2009) 193409. [227] I. Razmute-Razme, Z. Kuodis, O. Eicher-Lorka, G. Niaura, SERS observation of soft C–H vibrational mode of bifunctional alkanethiol molecules adsorbed at Au and Ag electrodes, Physical Chemistry Chemical Physics 12 (2010) 4564. [228] P. Amsalem, L. Giovanelli, J.M. Themlin, T. Angot, Electronic and vibrational properties at the ZnPc/Ag(1 1 0) interface, Physical Review B 79 (2009) 235426. [229] F.S. Tautz, Structure and bonding of large aromatic molecules on noble metal surfaces: the example of PTCDA, Progress in Surface Science 82 (2007) 479. [230] M. Mehlhorn, H. Gawronski, L. Nedelmann, A. Grujic, K. Morgenstern, An instrument to investigate femtochemistry on metal surfaces in real space, Review in Science Instrumentation 78 (2007) 033905. [231] M. Mehlhorn, J. Carrasco, A. Michaelides, K. Morgenstern, Local Investigation of Femtosecond Laser Induced Dynamics of Water Nanoclusters on Cu(1 1 1), Physical Review Letters 103 (2009) 026101. [232] T.H. Her, R.J. Finlay, C. Wu, E. Mazur, Surface femtochemistry of CO/O2/Pt(1 1 1): the importance of nonthermalized substrate electrons, Journal of Chemical Physics 108 (1998) 8595. [233] C. Bauer, J.P. Abid, D. Fermin, H.H. Girault, Ultrafast chemical interface scattering as an additional decay channel for nascent nonthermal electrons in small metal nanoparticles, Journal of Chemical Physics 120 (2004) 9302. [234] J.W. Gadzuk, Resonance-assisted hot electron femtochemistry at surfaces, Physical Review Letters 76 (1996) 4234. [235] T. Wadayama, M. Yokawa, K. Murakami, Y. Baba, in: L.S.O. Johansson, J.N. Andersen, M. Gothelid, U. Helmersson, L. Montelius, M. Rubel, J. Setina, L.E. Wernersson (Eds.), Proceedings of the 17th International Vacuum Congress/13th International Conference on Surface Science/International Conference on Nanoscience and Technology, 2008, p. 072034. [236] R.G. Sharpe, S. Dixonwarren, P.J. Durston, R.E. Palmer, The electronic catalyst – dissociation of chlorinated hydrocarbons by metal-insulator-metal electron emitters, Chemical Physics Letters 234 (1995) 354. [237] M.P. Ray, R.E. Lake, L.B. Thomsen, G. Nielson, O. Hansen, I. Chorkendorff, C.E. Sosolik, Towards hot electron mediated charge exchange in hyperthermal energy ion-surface interactions, Journal of Physics –Condensed Matter 22 (2010) 084010. [238] L. Chen, H. Li, A.T.S. Wee, Nonlocal chemical reactivity at organic–metal interfaces, Acs Nano 3 (2009) 3684. [239] P. Maksymovych, D.B. Dougherty, X.Y. Zhu, J.T. Yates, Nonlocal dissociative chemistry of adsorbed molecules induced by localized electron injection into metal surfaces, Physical Review Letters 99 (2007) 016101. [240] P. Maksymovych, D.C. Sorescu, K.D. Jordan, J.T. Yates, Collective reactivity of molecular chains self-assembled on a surface, Science 322 (2008) 1664. [241] D. Yamaguchi, T. Matsumoto, K. Watanabe, N. Takagi, Y. Matsumoto, Photochemistry of cyclohexane on Cu(1 1 1), Physical Chemistry Chemical Physics 8 (2006) 179. [242] S. Hagen, P. Kate, F. Leyssner, D. Nandi, M. Wolf, P. Tegeder, Excitation mechanism in the photoisomerization of a surfacebound azobenzene derivative: role of the metallic substrate, Journal of Chemical Physics 129 (2008) 164102. [243] K.H. Kim, K. Watanabe, D. Menzel, H.J. Freund, UV photo-dissociation and photodesorption of N2 O on Ag(1 1 1), Journal of Physics – Condensed Matter 22 (2010) 084012. [244] L.J. Richter, S.A. Buntin, D.S. King, R.R. Cavanagh, Constraints on the use of polarization and angle-of-incidence to characterize surface photoreactions, Chemical Physics Letters 186 (1991) 423. [245] C.D. Lindstrom, X.Y. Zhu, Photoinduced electron transfer at molecule–metal interfaces, Chemical Reviews 106 (2006) 4281. [246] K.H. Kim, K. Watanabe, D. Menzel, H.J. Freund, Photoinduced abstraction reactions within NO dimers on Ag(1 1 1), Journal of the American Chemical Society 131 (2009) 1660. [247] D. Mulugeta, K.H. Kim, K. Watanabe, D. Menzel, H.J. Freund, Size effects in thermal and photochemistry of (NO)2 on Ag nanoparticles, Physical Review Letters 101 (2008) 146103. [248] M. Nest, P. Saalfrank, Enhancement of femtosecond-laser-induced molecular desorption by thin metal films, Physical Review B 69 (2004) 235405.

H. Arnolds / Progress in Surface Science 86 (2011) 1–40

39

[249] K. Watanabe, D. Menzel, N. Nilius, H.J. Freund, Photochemistry on metal nanoparticles, Chemical Reviews 106 (2006) 4301. [250] V.P. Zhdanov, B. Kasemo, Substrate-mediated photoinduced chemical reactions on ultrathin metal films, Surface Science 432 (1999) L599. [251] V.P. Zhdanov, B. Kasemo, Specifics of substrate-mediated photo-induced chemical processes on supported nm-sized metal particles, Journal of Physics – Condensed Matter 16 (2004) 7131. [252] K. Al-Shamery, A. Al-Shemmary, R. Buchwald, D. Hoogestraat, M. Kampling, P. Nickut, A. Wille, Elementary processes at nanoparticulate photocatalysts, European Physical Journal B 75 (2010) 107. [253] A. Al-Shemmary, R. Buchwald, K. Al-Shamery, Surface photochemistry of CO adsorbed at alumina supported nanoparticulate platinum, Journal of Physics – Condensed Matter 22 (2010) 084011. [254] E. Hasselbrink, Photochemistry on ultrathin metal films, Surface Science 602 (2008) 3184. [255] R. Schmidt, S. Hagen, D. Brete, R. Carley, C. Gahl, J. Dokic, P. Saalfrank, S. Hecht, P. Tegeder, M. Weinelt, On the electronic and geometrical structure of the trans- and cis-isomer of tetra-tert-butyl-azobenzene on Au(1 1 1), Physical Chemistry Chemical Physics 12 (2010) 4488. [256] S. Wagner, F. Leyssner, C. Kordel, S. Zarwell, R. Schmidt, M. Weinelt, K. Ruck-Braun, M. Wolf, P. Tegeder, Reversible photoisomerization of an azobenzene-functionalized self-assembled monolayer probed by sum-frequency generation vibrational spectroscopy, Physical Chemistry Chemical Physics 11 (2009) 6242. [257] N. Katsonis, M. Lubomska, M.M. Pollard, B.L. Feringa, P. Rudolf, Synthetic light-activated molecular switches and motors on surfaces, Progress in Surface Science 82 (2007) 407. [258] W.R. Browne, B.L. Feringa, Light switching of molecules on surfaces, Annual Review of Physical Chemistry 60 (2009) 407. [259] M.J. Comstock, D.A. Strubbe, L. Berbil-Bautista, N. Levy, J. Cho, D. Poulsen, J.M.J. Frechet, S.G. Louie, M.F. Crommie, Determination of photoswitching dynamics through chiral mapping of single molecules using a scanning tunneling microscope, Physical Review Letters 104 (2010) 178301. [260] P. Tegeder, S. Hagen, F. Leyssner, M.V. Peters, S. Hecht, T. Klamroth, P. Saalfrank, M. Wolf, Electronic structure of the molecular switch tetra-tert-butyl-azobenzene adsorbed on Ag(1 1 1), Applied Physics A-Materials Science & Processing 88 (2007) 465. [261] F. Leyssner, S. Hagen, L. Ovari, J. Dokic, P. Saalfrank, M.V. Peters, S. Hecht, T. Klamroth, P. Tegeder, Photoisomerization Ability of Molecular Switches Adsorbed on Au(1 1 1): comparison between azobenzene and stilbene derivatives, Journal of Physical Chemistry C 114 (2010) 1231. [262] J.R. Lombardi, R.L. Birke, A unified view of surface-enhanced Raman scattering, Accounts of Chemical Research 42 (2009) 734. [263] J.R. Lombardi, R.L. Birke, A unified approach to surface-enhanced Raman spectroscopy, Journal of Physical Chemistry C 112 (2008) 5605. [264] K. Uetsuki, P. Verma, T. Yano, Y. Saito, T. Ichimura, S. Kawata, Experimental identification of chemical effects in surface enhanced Raman scattering of 4-aminothiophenol, Journal of Physical Chemistry C 114 (2010) 7515. [265] C. Chenal, R.L. Birke, J.R. Lombardi, Determination of the degree of charge-transfer contributions to surface-enhanced Raman spectroscopy, Chemphyschem 9 (2008) 1617. [266] S.M. Morton, E. Ewusi-Annan, L. Jensen, Controlling the non-resonant chemical mechanism of SERS using a molecular photoswitch, Physical Chemistry Chemical Physics 11 (2009) 7424. [267] K. Shibamoto, K. Katayama, T. Sawada, Ultrafast charge transfer in surface-enhanced Raman scattering (SERS) processes using transient reflecting grating (TRG) spectroscopy, Chemical Physics Letters 433 (2007) 385. [268] S.M. Morton, L. Jensen, Understanding the molecule–surface chemical coupling in SERS, Journal of the American Chemical Society 131 (2009) 4090. [269] J.R. Lombardi, R.L. Birke, Time-dependent picture of the charge-transfer contributions to surface enhanced Raman spectroscopy, Journal of Chemical Physics 126 (2007) 244709. [270] T. Olsen, J. Gavnholt, J. Schiøtz, Hot-electron-mediated desorption rates calculated from excited-state potential energy surfaces, Physical Review B 79 (2009) 035403. [271] L. Brus, Noble metal nanocrystals: plasmon electron transfer photochemistry and single-molecule Raman spectroscopy, Accounts of Chemical Research 41 (2008) 1742. [272] K.F. Domke, D. Zhang, B. Pettinger, Enhanced Raman spectroscopy: single molecules or carbon?, Journal of Physical Chemistry C 111 (2007) 8611 [273] A. Weiss, G. Haran, Time-dependent single-molecule Raman scattering as a probe of surface dynamics, Journal of Physical Chemistry B 105 (2001) 12348. [274] E.J. Bjerneld, F. Svedberg, P. Johansson, M. Kall, Direct observation of heterogeneous photochemistry on aggregated Ag nanocrystals using Raman spectroscopy: the case of photoinduced degradation of aromatic amino acids, Journal of Physical Chemistry A 108 (2004) 4187. [275] D.H. Jeong, J.S. Suh, M. Moskovits, Enhanced photochemistry of 2-aminopyridine adsorbed on silver colloid surfaces, Journal of Raman Spectroscopy 32 (2001) 1026. [276] K. Kim, Y.M. Lee, H.B. Lee, Y. Park, T.Y. Bae, Y.M. Jung, C.H. Choi, K.S. Shin, Visible laser-induced photoreduction of silver 4nitrobenzenethiolate revealed by Raman scattering spectroscopy, Journal of Raman Spectroscopy 41 (2010) 187. [277] K.S. Shin, H.S. Lee, S.W. Joo, K. Kim, Surface-induced photoreduction of 4-nitrobenzenethiol on Cu revealed by surfaceenhanced Raman scattering Spectroscopy, Journal of Physical Chemistry C 111 (2007) 15223. [278] K. Fukutani, Y. Murata, R. Schwarzwald, T.J. Chuang, UV-laser-induced desorption of NO from Pt(1 1 1), Surface Science 311 (1994) 247. [279] K. Fukutani, M.B. Song, Y. Murata, Photodesorption of CO and CO+ from Pt(1 1 1) – mechanism and site-specificity, Journal of Chemical Physics 103 (1995) 2221. [280] Q. Zhong, C. Gahl, M. Wolf, Two-photon photoemission spectroscopy of pyridine adsorbed on Cu(1 1 1), Surface Science 496 (2002) 21. [281] D.B. Dougherty, J. Lee, J.T. Yates, Role of conformation in the electronic properties of chemisorbed pyridine on Cu(1 1 0): an STM/STS study, Journal of Physical Chemistry B 110 (2006) 11991.

40

H. Arnolds / Progress in Surface Science 86 (2011) 1–40

[282] P. Kambhampati, C.M. Child, M.C. Foster, A. Campion, On the chemical mechanism of surface enhanced Raman scattering: experiment and theory, Journal of Chemical Physics 108 (1998) 5013. [283] A. Otto, M. Futamata, in: Surface-Enhanced Raman Scattering: Physics and Applications, 2006, pp. 147. [284] K.C. Chou, S. Westerberg, Y.R. Shen, P.N. Ross, G.A. Somorjai, Probing the charge-transfer state of CO on Pt(1 1 1) by twodimensional infrared-visible sum frequency generation spectroscopy, Physical Review B 69 (2004) 153413. [285] O. Pluchery, C. Humbert, M. Valamanesh, E. Lacaze, B. Busson, Enhanced detection of thiophenol adsorbed on gold nanoparticles by SFG and DFG nonlinear optical spectroscopy, Physical Chemistry Chemical Physics 11 (2009) 7729. [286] T. Nagahara, T. Suemasu, M. Aida, T. Ishibashi, Self-assembled monolayers of double-chain disulfides of adenine on Au: an IR-UV sum-frequency generation spectroscopic study, Langmuir 26 (2010) 389. [287] B. Bozzini, B. Busson, G.P. De Gaudenzi, L. D’Urzo, C. Mele, A. Tadjeddine, An in situ SFG and SERS investigation into the  electrodeposition of Au from AuðCNÞ 2 and AuðCNÞ4 solutions, Journal of Electroanalytical Chemistry 602 (2007) 61. [288] Z.D. Schultz, Z.V. Feng, M.E. Biggin, A.A. Gewirth, Vibrational spectroscopic and mass spectrometric studies of the interaction of bis(3-sulfopropyl)-disulfide with Cu surfaces, Journal of the Electrochemical Society 153 (2006) C97. [289] A.N. Bordenyuk, C. Weeraman, A. Yatawara, H.D. Jayathilake, I. Stiopkin, Y. Liu, A.V. Benderskii, Vibrational sum frequency generation spectroscopy of dodecanethiol on metal nanoparticles, Journal of Physical Chemistry C 111 (2007) 8925. [290] H.L. Wang, M. He, D.M. Chen, T.J. He, F.C. Liu, Intensity relation between IR-UV doubly resonant sum-frequency spectra and antisymmetric resonant vibrational Raman scattering of chiral solutions, Chemical Physics Letters 452 (2008) 215. [291] B. Bozzini, L. D’Urzo, C. Mele, B. Busson, C. Humbert, A. Tadjeddine, Doubly resonant sum frequency generation spectroscopy of adsorbates at an electrochemical interface, Journal of Physical Chemistry C 112 (2008) 11791. [292] C. Humbert, Y. Caudano, L. Dreesen, Y. Sartenaer, A.A. Mani, C. Silien, J.J. Lemaire, P.A. Thiry, A. Peremans, Self-assembled organic and fullerene monolayers characterisation by two-colour SFG spectroscopy: a pathway to meet doubly resonant SFG process, Applied Surface Science 237 (2004) 462. [293] C. Humbert, L. Dreesen, S. Nihonyanagi, T. Masuda, T. Kondo, A.A. Mani, K. Uosaki, P.A. Thiry, A. Peremans, Probing a molecular electronic transition by two-colour sum-frequency generation spectroscopy, Applied Surface Science 212 (2003) 797. [294] Y. Caudano, C. Silien, C. Humbert, L. Dreesen, A.A. Mani, A. Peremans, P.A. Thiry, Electron–phonon couplings at C-60 interfaces: a case study by two-color, infrared-visible sum-frequency generation spectroscopy,, Journal of Electron Spectroscopy and Related Phenomena 129 (2003) 139. [295] C. Humbert, B. Busson, C. Six, A. Gayral, M. Gruselle, F. Villain, A. Tadjeddine, Sum-frequency generation as a vibrational and electronic probe of the electrochemical interface and thin films, Journal of Electroanalytical Chemistry 621 (2008) 314. [296] T. Miyamae, Y. Miyata, H. Kataura, Two-color sum-frequency generation study of single-walled carbon nanotubes on silver, Journal of Physical Chemistry C 113 (2009) 15314. [297] T. Maeda, T. Nagahara, M. Aida, T. Ishibashi, Identification of chemical species of fluorescein isothiocyanate isomer-I (FITC) monolayers on platinum by doubly resonant sum-frequency generation spectroscopy, Journal of Raman Spectroscopy 39 (2008) 1694. [298] D. Wu, G.H. Deng, Y. Guo, H.F. Wang, Observation of the interference between the intramolecular IR-visible and visible-IR processes in the doubly resonant sum frequency generation vibrational spectroscopy of rhodamine 6G adsorbed at the air/water interface, Journal of Physical Chemistry A 113 (2009) 6058. [299] H. Song, Y. Kim, Y.H. Jang, H. Jeong, M.A. Reed, T. Lee, Observation of molecular orbital gating, Nature 462 (2009) 1039. [300] M. Paulsson, T. Frederiksen, H. Ueba, N. Lorente, M. Brandbyge, Unified description of inelastic propensity rules for electron transport through nanoscale junctions, Physical Review Letters 100 (2008). [301] L. Bartels, F. Wang, D. Moller, E. Knoesel, T.F. Heinz, Real-space observation of molecular motion induced by femtosecond laser pulses, Science 305 (2004) 648. [302] H. Gawronski, M. Mehlhorn, K. Morgenstern, Real-Space Investigation of Non-adiabatic CO2 Synthesis, Angewandte Chemie – International Edition 49 (2010) 5913. [303] H. Ueba, Vibrational lineshapes of adsorbates on solid surfaces, Progress in Surface Science 22 (1986) 181. [304] I.M. Lane, Z.P. Liu, D.A. King, H. Arnolds, Ultrafast vibrational dynamics of NO and CO adsorbed on an iridium surface, Journal of Physical Chemistry C 111 (2007) 14198.