Ultrafast electron transfer, localization and solvation at ice–metal interfaces: Correlation of structure and dynamics

Progress in Surface Science 78 (2005) 87–100 www.elsevier.com/locate/progsurf

Progress Highlight

Ultrafast electron transfer, localization and solvation at ice–metal interfaces: Correlation of structure and dynamics Uwe Bovensiepen

*

Freie Universita¨t Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin, Germany

Abstract The interaction of an excess electron with a polar molecular environment is well known as electron solvation. This process is characterized by an energetic stabilization and by changes of the electronic spatial extent due to screening of the localized charge through molecular rearrangement. At metal–ice interfaces we photo-inject delocalized electrons from the metal substrate into adsorbed ice layers and analyze the ultrafast dynamics of electron transfer, localization and solvation by femtosecond time- and angle-resolved two-photon photoemission spectroscopy. To acquire further understanding of the individual steps of the complex process we vary the interfacial structure. The substrate is changed between Cu(1 1 1) and Ru(0 0 1) and the electron dynamics in ice islands are compared to closed D2O layers. Contrasting crystalline and amorphous ice we found that electron solvation is mediated through electron localization at favorable structural sites, which occurs very efficiently in amorphous ice, but is less likely in a crystalline layer. Next, we find that in an open ice structure like ice islands the energetic stabilization due to electron solvation proceeds at a rate of 1 eV/ps which is three times faster than in a closed ice layer. We attribute this behavior to differences in the molecular coordination, which determines the molecular mobility and, thus, the transfer rate of electronic energy to solvent modes. The substrateÕs electronic structure, on the other hand, is important to understand the transfer rates from electrons in ice back to the metal. First experiments on trapped electrons in crystalline ice underline the potential to study

*

Tel.: +49 30 838 53340; fax: +49 30 838 56059. E-mail address: [email protected] URL: http://www.physik.fu-berlin.de/~femtoweb

0079-6816/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.progsurf.2005.06.001

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electron solvation not only during the equilibration process, but also in quasi-static conditions, where we find that the stabilization continues, although at much weaker rates. Ó 2005 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4.

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electron dynamics at ice–metal interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Sequence of elementary processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Spatial extent and energetic stabilization of solvated electrons. . . . . . . . 3.3. Influence of the substrateÕs electronic structure on the transfer dynamics 3.4. Molecular structure of ice and the stabilization dynamics . . . . . . . . . . . Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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88 90 90 90 92 95 96 99 99 100

1. Introduction The interaction of excess electrons with interfacial molecular assemblies is not only of fundamental interest to physics, chemistry and biology, but is also of major importance for technologically relevant areas like electrochemistry. Moreover, charge injection and transport dynamics across interfaces are highly relevant for future molecular electronic devices. Investigations on water and ice have always constituted an active area of interdisciplinary research. Because of the rich structure of liquid, amorphous and crystalline states [1,2] and respective interfacial configurations at, e.g., metals [3], the development of microscopic understanding of elementary processes at the ice–metal interface is challenging. Electron localization and solvation represent interactions of the charge with the molecular environment and result from accommodation of the electronic charge density within the surrounding hydrogen bonded network. At ice–metal interfaces these processes in ice compete with the manifold of electronic states in the adjacent metal as outlined in Fig. 1. Excited electrons in a molecular environment experience the competing effects of delocalization and localization. Delocalization results from resonant interaction of overlapping molecular electronic states which leads to formation of electronic bands with the band width determined by the electronic interaction strength. Localization, on the other hand, can lead to energy gain at favorable configurations but requires a localization energy to couple delocalized states within a certain band width interval to form a localized electronic wave packet. According to the uncertainty principle, the ultimate localization limit is given by the available band width [4,5]. Localized electronic states can be formed dynamically by polarizing electronic and molecular

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Fig. 1. Scheme of the two-photon photoemission (2PPE) experiment, the electronic structure at the ice– metal interface and elementary steps of the process excited by an UV femtosecond laser pulse at hm1  4 eV. Successively, as indicated by numbers, (1) electrons are photoinjected into the ice conduction band (CB), (2) localize, (3) are stabilized and solvated by molecular rearrangement screening the localized charge and (4) relax back to the metal. To probe the dynamics electrons are photoemitted by a visible probe pulse hm2  2 eV, time-delayed with respect to hm1. Their kinetic energy and momentum parallel to the interface is determined by a time-of-flight spectrometer (e-TOF). The dashed and the solid curve in the adlayer denote the image charge potential and its modification for a fully localized electron, respectively.

degrees of freedom (polaron formation) on time scales of molecular motions (10
12– 10
15 s) as demonstrated for interfaces by Harris and coworkers [4]. Alternatively, static local variations of the molecular environment (i.e., spatial inhomogeneities) are known to lead to localized electronic states below the conduction band [6]. Such structural perturbations are abundant in amorphous solids, but also crystalline structures may exhibit defects, although less frequently. In general, both localization pathways (dynamic and static) will contribute and the question which one dominates is essential and requires careful investigation. In this respect, ice layers adsorbed on single-crystal metal surfaces are very promising because the structure can be varied by changing the substrate [3] and coverage. Furthermore, the state can be altered from amorphous to crystalline [7]. In addition, ice adds an interesting parameter to the dynamics, namely its permanent electric dipole moment, which makes water a well known solvent for ions and—albeit less relevant in everyday life—for electrons [8]. The corresponding rich dynamics (see Fig. 1) of electron transfer, localization and solvation which have been monitored by femtosecond time-, and momentumresolved two-photon-photoelectron spectroscopy (2PPE) are the focus of the present Progress Highlight. Compared to optical methods, 2PPE offers direct access to the binding energy and the electron momentum parallel to the interface which allows to elucidate details of the localization and solvation dynamics (Fig. 1). In addition, the correlation of the dynamics with the structure of the ice–metal interface, i.e., the electronic structure and the molecular arrangement, will be exploited to further the understanding of underlying mechanisms. Dynamics of solvated electron formation have been studied extensively by femtosecond time-resolved optical spectroscopy [9– 11] and theory [12,13] in liquid water. But only recently, experiments at surfaces and interfaces were successfully carried out by employing 2PPE [14–19]. While Harris and coworkers investigated adsorbed nitriles and alcohols, our laboratory focused

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on H2O and D2O layers. Note that throughout the paper electrons which are localized normal to the surface within the ice layer, but extend over the layer laterally will be referred to as delocalized, while the term localized stands for spatial confinement in all three dimensions.

2. Experimental approach A tunable femtosecond laser system serving as a source of excitation and probe laser pulses has been combined with an ultrahigh vacuum system for sample preparation, characterization and photoelectron detection [20]. H2O or D2O is adsorbed onto metal surfaces at 100 K leading to growth of amorphous ice which is meta stable with respect to crystalline ice Ic. Annealing to 157 K—slightly below water desorption—results in formation of crystalline ice layers [7]. Thermal desorption spectroscopy has been carried out simultaneously with 2PPE spectroscopy to characterize structural properties and the unoccupied electronic structure as a function of temperature and coverage. For amorphous ice we observe a broad energetic continuum referred to as conduction band (CB) which transforms abruptly upon structural ordering into a series of image potential states [21]. Fig. 1 sketches the electronic structure at the ice–metal interface and the 2PPE experiment. Unoccupied states of the metal facilitate optical excitation of hot electron distributions by the UV laser pulse at hm1  4 eV which launches a sequence of processes indicated in Fig. 1. Electrons are photoemitted by a time-delayed second laser pulse hm2  2 eV and are analyzed by a time-of-flight spectrometer according to its kinetic energy Ekin and momentum parallel to the surface hkk. By this method, we measure photoelectron spectra at energies of excited intermediate states E
EF = Ekin + U
hm2 to monitor the temporal evolution of electron transfer and solvation dynamics after excitation. U denotes the work function of the sample and EF the Fermi level.

3. Electron dynamics at ice–metal interfaces 3.1. Sequence of elementary processes As a starting point, experiments on amorphous D2O multilayers wetting a Cu(1 1 1) substrate are discussed since the individual processes can be clearly identified. Fig. 2 presents time-dependent data of 2PPE spectroscopy for 4 BL coverage. In the time- and energy-dependent false color representation of the photoelectron yield and in the extracted 2PPE spectra two different features of (i) an energetically broad continuum at 3–4 eV attributed to the conduction band and (ii) a pronounced peak at lower energy are clearly discerned. The peak maximum of the latter shifts closer to EF with the time delay, i.e., the binding energy with respect to the vacuum level Evac increases (U = EF
Evac = 3.95 eV). From this peak shift a stabilization rate of 270 meV/ps is analyzed. It originates from screening of the electron by rearrangement

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Fig. 2. Left: Time-dependent 2PPE yield as a function of intermediate state energy E
EF in a false color representation at T = 100 K normalized to the maximum. White circles indicate the peak maxima at different delays. The central inset depicts the intensity integrated within the energy intervals of eS and eCB as a function of delay. The right top figure displays 2PPE spectra at different delays which are denoted by the vertical offset. The right bottom panel shows an artistÕs view of the 2PPE experiment. Figure is taken from Ref. [18].

of the surrounding molecular dipoles. This process is well known to occur for electron solvation in liquid water [8,22,23] and is attributed to energy transfer to molecular modes. Note that for e-solvation in ice the level of understanding is considerably less mature [24]. By changing D2O to H2O no isotope effect of the stabilization rate is observed. This is in agreement with studies of the liquid by Wiersma and coworkers [10] who observed an isotope effect in the electron solvation dynamics only for librational modes on time scales below 20 fs being too fast to affect the present stabilization dynamics. This comparison suggests that mainly translational and diffusive modes are engaged in the observed stabilization process. The stabilization rate determined for ice layers is only 2–3 times slower than in the liquid [22,23]. At a first glance this might appear counterintuitive, because the dipolar response times in ice are orders of magnitude slower than in the liquid [25]. However, stabilization by 0.3 eV within 1 ps indicates a highly non-equilibrium situation and energy transfer to the surrounding molecular assembly leading to excitation of the hydrogen-bonded network and molecular modes will certainly enhance the dipolar mobility. Due to the large ice band gap of >7 eV, optical excitation of electron–hole-pairs in ice is unlikely. On the other hand, photo-induced electron transfer from the metal into the ice layer is a highly probable process for sufficiently high photon energy hm1. In this case delocalized hot electrons optically excited in the metal are injected into the ice conduction band as illustrated by Fig. 1, step 1. In the 2PPE spectra these delocalized electrons are observed by the energetic continuum termed eCB which

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originates from a non-equilibrium population of the ice conduction band. In the following the time-dependent 2PPE intensity of both spectral features eS and eCB is compared (Fig. 2, center inset), and their different wave function characters become apparent. The population of eCB decays extremely fast on a time scale <10 fs which indicates an upper limit, because the time evolution of the 2PPE intensity is identical with the temporal profile of the laser pulse (dotted line). The population decay itself is beyond the time resolution of the experiment. For eS the relaxation dynamics can be quantified. Within the first 200 fs a rate equation analysis results in a lifetime si = 110 fs (Fig. 2, solid line, center inset). For delays later than 200 fs the decay slows down as it evolves slower than expected from the solid line. Based on the fact that the transfer probability is determined by the interaction of electrons in the adlayer with substrate states [26], we arrive at the following conclusions: (a) Injection of electrons and their transfer back to the substrate proceeds on different time scales. Electron injection into the conduction band starts the dynamics in ice. Since it evolves on time scales <10 fs, it is based on a more efficient interaction of ice and metal states than the interaction strength responsible for the process of electron back-transfer (step 4, Fig. 1) characterized by considerably longer transfer times P110 fs. Note that this behavior is very different from electron transfer dynamics from bulk to image potential states at bare metal surfaces or at most metal–molecule interfaces where injection and back-transfer proceed at identical rates [27]. (b) The interaction strength of solvated electrons in ice with metal states weakens with time delay as solvation proceeds, because the back-transfer dynamics are not described by a single transfer rate, but the transfer slows down continuously. Since interaction is quantified by wave function overlap, we infer a constriction of the solvated electron wave function increasing with time which lowers the wave function overlap and the back-transfer rate. 3.2. Spatial extent and energetic stabilization of solvated electrons Angle- and time-resolved 2PPE studies provide evidence for such a scenario of coupled stabilization and localization dynamics. Angle-resolved studies facilitate experimental access to the dispersion with electron momentum pk = hkk parallel to the interface. Delocalized electrons present the dispersion of a quasi-free electron within the solid (Bloch state) E = ( hkk)2/(2meff). A localized electron is formed by coupling of wave functions within a momentum band width Dkk, where the spatial extension Dx of the so formed wave packet is proportional to 1/Dkk. However, the wave packet does not experience the periodicity of the solid leading to absence of dispersion. In an earlier study, electron localization dynamics at metal–molecule interfaces have been monitored with time- and angle-resolved 2PPE by analysis of the decay of a dispersing feature and built-up of a non-dispersing one [4]. We applied this method to electron solvation in ice layers on single-crystal metal surfaces. Fig. 3 presents angle-resolved 2PPE spectra at two different delays (a,b), depicts the resulting energy levels for localized and delocalized states (c), and shows the momentum band width of solvated electrons as a function of energy (b) as well as the momentum distribution along kk (d). The localization can be recognized from the

U. Bovensiepen / Progress in Surface Science 78 (2005) 87–100

vac

-1.5

(a)

(eV) -0.9

0.00

-0.6

(c)

= 0 fs

eS

0.05

0.10

0.15 3.0

(eS) 10 fs 50 fs 100 fs

(eCB)

2.9 F

(eV)

α= 0˚ 8˚ 14˚ 18˚

-1.2

93

2.8

= 200 fs

200 fs 300 fs 400 fs

(1/Å)

= 200 fs

1

0.3

0.2 F= 2.8 eV 2.9 eV 3.0 eV

0.1

(b)

(d) 2.4

2.7

3.0 F (eV)

3.3

normalized 2PPE intensity

2PPE intensity

eCB

0 0.00

0.05

0.10 || (1/Å)

0.15

Fig. 3. Angle-dependent 2PPE spectra of 3 BL D2O/Cu(1 1 1) at 0 fs (a) and 200 fs (b) at 100 K; (c) contrasts the dispersion of delocalized electrons eCB and solvated electrons eS; dashed lines guide the eye for datasets recorded at constant time delay, as indicated by open symbols. The observed downward curvature of the latter is a consequence of angle-resolved analysis of the electronic wave packet in k-space, reproduced by model calculations (solid lines in b), and the energy-dependent momentum band width D(kkE) (dash-dotted line in b); see text and Ref. [18] for details. The resulting momentum distribution of solvated electrons according to model calculations (lines at energies indicated) and peak maxima at 2.9 eV (dots) is given in panel (d). Data have been published in Refs. [18,19].

angle-resolved spectra. At Dt = 0 fs (Fig. 3a) the peak maximum eS shifts to higher energy with increasing angle. At 200 fs (Fig. 3b) one observes that the peak occurs at lower energy for larger angle. If the dispersion of the entire photoelectron spectrum is determined (c.f. Fig. 3 in Ref. [16]), a positive dispersion at 0 fs which indicates quasi-free electrons turns within 100 fs into a constant dispersion expected for localized electrons. The observed slightly negative dispersion at later delays is unexpected at a first glance. However, in the following it is pointed out that this behavior is fully consistent with a localized wave function and is a consequence of the momentum distribution of solvated electrons. To achieve this analysis, the spectral contributions eCB and eS which overlap near 3 eV were separated by a two-peak model [18]. The conduction band bottom exhibits a positive dispersion with the band bottom at E
EF = 2.90(5) eV and an effective electron mass of 1.0(2) me (solid line and symbols in Fig. 3c). The spectral contribution of localized/solvated electrons is characterized by a gain in energy with delay as introduced above (Fig. 2). In addition, for all delays an effectively negative dispersion of eS is recognized. We conclude that

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localization of the initially delocalized electrons eCB occurs within the first 10 fs through scattering into states eS localized at favorable sites within the amorphous network (c.f. Fig. 1). Coulomb interaction of D2O dipoles with localized electronic charge density leads to an excitation of the hydrogen bonded ice network. Since e.g., librations include periods as short as 15 fs [10], we expect that molecules reorganize already during accumulation of local charge density, which also lowers the electronic energy on the time scale of molecular rearrangement, and localization and stabilization dynamics need to be considered as being coupled. Consequently, electron localization does not follow a purely static or dynamic pathway but combines both, which will be elaborated further in Section 3.4 by comparing amorphous and crystalline ice layers. In case of sufficiently long residence time of electrons in the layer, which is limited to the first picoseconds by interaction with metal states as discussed above (Fig. 2), this process would finally lead to the equilibrated configuration of the solvated electron [28]. The effectively negative dispersion of solvated electrons is a consequence of (i) the large line width of solvated electrons due to inhomogeneous broadening, (ii) the low kinetic energy of photoelectrons of 1 eV and (iii) the spectrum in momentum space of localized electron wave packets. A detailed discussion of this behavior, which has also been observed by two-dimensional simulations [29], is given in Ref. [18]. Here, we concentrate on the basic result. Angle-dependent spectra of eS and the resulting apparently negative dispersion can be explained if the band width of the localized electron in momentum space depends on the intermediate state energy, i.e., Dkk = Dkk(E). This implies that further stabilization leads to more band width and to stronger confinement of the electronic wave function which corroborates the coupling of electron stabilization and localization. As shown in Fig. 3b, model calculations assuming a linear increase in Dkk(E) with decreasing E
EF are in excellent agreement with the experiment. In a next step, the momentum distribution of solvated electrons has been extracted and the variation of Dkk with E
EF is shown in Fig. 3d. At E
EF = 2.9 eV, model calculations (solid lines) agree well with solid points corresponding to peak maxima of the measured spectra in Fig. 3b. For lower (larger) energy than 2.9 eV the momentum band width increases (decreases) as shown by the dashed (dotted) line. The information of energy-dependent changes in Dkk, however, has not been obtained from experimental data alone. It could be unveiled only after taking into account the spectral shape of eS and its variation with electron momentum and time. According to Dkk(E) within E
EF = 2.6–3.0 eV shown by the dash-dotted line in Fig. 3b, we conclude that due to the stabilization process solvated electrons increase their momentum band width 2–3 times. Suitable estimates for the absolute spatial extent of such electrons are demanding, because the determination of the wave function set coupled by formation of the electronic wave packet asks for a fully quantum mechanical two-dimensional simulation as has been carried out for the ice–metal interface [29]. Harris and coworkers on the other hand suggested that separation of the initial and final state wave function into an in-plane and out-of-plane contribution using sets of plane waves presents a useful approximation [14,30]. Employing this strong simplification for ice–metal interfaces, we arrive ˚ at E
EF = 3.0 eV that decreases to 9 A ˚ at 2.65 eV. at an initial value for Dx = 19 A

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However, from the two-dimensional simulation [29], we conclude that the inference of the spatial extent of localized electron states is clearly non-trivial and depends on the specific system under investigation. Currently it is an open question in how far this system specificity is responsible for the observation of a dependence of Dkk on energy as determined for D2O layers or its absence as reported by Harris and coworkers for nitriles on Ag(1 1 1) [14]. From a certain perspective these experiments extend earlier investigations of the spatial extent of solvated electrons in liquid water by geminate recombination [31] and chemical scavenging yield studies [32]. As shown above, time-resolved photoemission adds the absolute binding energy scale and the electron momentum to the experimentally accessible dimensions. The novel aspect of the present surface science approach is the investigation of a correlation of structure (i.e., static properties) and ultrafast dynamics. As will be shown in the remaining part of the paper, alterations of interface structure comprising the electronic structure of the substrate (Section 3.3) and the molecular arrangement of the ice adsorbate (Section 3.4) influence transfer and solvation dynamics. Through these variations a more detailed understanding of underlying mechanisms and interactions is obtained. 3.3. Influence of the substrateÕs electronic structure on the transfer dynamics As discussed in Section 3.1 and illustrated in Fig. 1, electron injection proceeds through delocalized metal and ice states on a sub 10 fs time scale. The back-transfer of solvated electrons is governed by the interaction with substrate states as well. However unusual compared to typical metal–molecule interfaces, the interaction has been reduced by electron localization after the injection resulting in a smaller wave function overlap. The initial back-transfer times si have been analyzed and will be compared in this section for the two substrates Cu(1 1 1) and Ru(0 0 1) which have been employed for growing amorphous ice films. As shown in Fig. 4 by the timedependent 2PPE intensities, the back-transfer of solvated electrons proceeds during the first 200 fs for amorphous D2O on Ru(0 0 1) three times faster compared to the Cu(1 1 1) case. At later delays the transfer rates on Cu and Ru slow down and approach each other, which suggests that at these later delays the solvated electron wave function dominates the transfer dynamics over the metal contribution to the electron transfer matrix element. Accordingly, at early delays <200 fs the substrate might be responsible for changes in transfer rates. Since the transfer rate is considerably higher on a Ru(0 0 1) substrate than for Cu(1 1 1), the overlap of solvated electrons with Ru states is required to be more effective than for Cu. At a first look this appears reasonable because Cu is a noble metal and Ru a transition metal with unoccupied d-bands. However, a simple density of states (DOS) argument does not explain the observation, since the DOS of both substrates are very comparable at energies of solvated electrons (E
EF = 2.6–3.0 eV) [33]. A clear difference is recognized if the unoccupied momentum-dependent electronic structure of both metal surfaces is considered. They exhibit both an orientation band gap, which starts 1 eV below EF for Cu(1 1 1) and 1.5 eV above EF for Ru(0 0 1). Hence, at E
EF = 2.6–3.0 eV the band

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100

amorphous D2O/Cu(111) 3 BL 2 BL

τi=110 fs

2PPE Intensity

10-1

10-2

120 fs

10-3

34 fs ,

D2O/Ru(001)

-4

10

amorphous 10-5 crystalline, IPS, n=1 <10 fs 10-6

-200

0

200 400 time delay (fs)

600

Fig. 4. Cross-correlation traces of normalized 2PPE intensity in normal emission for the investigated ice– metal interfaces as indicated by symbols. Lines represent fits to the population decay according to a rate equation model. The relaxation times si represent the initial time scale of back-transfer of solvated electrons in ice to metal states. The 2 BL data for D2O/Cu(1 1 1) were taken on an ice film consisting out of islands, while the 3 BL one presents a closed ice layer. For crystalline ice layers the cross-correlation trace represents the delocalized n = 1 image potential state. Data are offset vertically for clarity, published in Ref. [19].

gap extends further into k-space for Cu(1 1 1) than for Ru(0 0 1). The resulting smaller overlap in momentum space of the localized, solvated electron with bulk states for the Cu surface compared to Ru(0 0 1) might accelerate the transfer for the Ru substrate [33], as has been established for Cs on different Cu surfaces [26]. 3.4. Molecular structure of ice and the stabilization dynamics Apart from the influence of the substrate, the molecular structure has been found to influence electron dynamics at the interface. Two examples will be addressed in the following. (a) An increase in the energetic stabilization rate upon lowering the average molecular coordination [34], and (b) severe reduction of the electron localization probability by transforming amorphous to crystalline ice [21]. A first experiment to demonstrate the sensitivity of ultrafast electron solvation dynamics to the solvent structure at the ice–metal interface has been carried out by coverage-dependent studies on amorphous D2O/Cu(1 1 1). As recognized from the right panel in Fig. 5, static 2PPE spectra exhibit in the low coverage regime H < 3 BL two additional contributions which have been identified as the occupied

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Fig. 5. The left panel depicts the time-dependent energy of solvated electrons for various amorphous coverages of D2O/Cu(1 1 1) and for several multilayer coverages on Ru(0 0 1). In the right panel 2PPE spectra for the mass equivalent coverage of 1 BL and 4 BL D2O/Cu(1 1 1) are shown. Photon energies used for closed ice layers (ice islands) are hm1 = 3.90 eV (4.20 eV) and hm2 = 1.95 eV (2.10 eV). Note that the surface state (SS) and the first image potential state (IPS) of the bare Cu(1 1 1) appear for an effective coverage of 1 BL, which indicates that in this coverage regime the ice forms islands rather than a closed layer. As soon as the ice wets the Cu surface, SS and IPS disappear. The illustrations at right suggest bulk and boundary configurations of solvated electrons. Data are taken from Refs. [19,34].

surface state and the first image potential state of bare Cu(1 1 1). Hence, within this coverage interval ice covered and bare Cu(1 1 1) prevail in parallel, i.e., ice islands are formed which percolate at a coverage of 3 BL. These structures have been imaged recently by low temperature scanning tunneling microscopy performed by K. Morgenstern and coworkers [35]. The transition at 3 BL is accompanied by a change in electron stabilization dynamics. In the left panel of Fig. 5 the temporal evolution of the eS peak maxima are plotted for different coverage. Two regimes are evident for D2O/Cu(1 1 1). Up to 1.8 BL (open symbols) the electron solvation proceeds at a rate of 1 eV/ps which is four times faster than for the closed adlayer (solid symbols) where the stabilization occurs at 270 meV/ps. Note that the population dynamics do not alter significantly upon island percolation, because the crosscorrelation traces for the island and the closed layer arrangement are very similar (Fig. 4). Therefore, we consider that the stabilization rate is determined by the solvent mobility, or in other words, by the energy transfer rate to solvent modes. The faster stabilization for islands therefore asks for an increase in molecular mobility. As a first approximation, we consider the boundaries of islands to offer favorable sites for electron solvation which present on average a lower molecular coordination than their bulk counterparts. Hence, these molecules are more mobile and might stabilize localized electrons more rapidly. It is not conceivable that these faster stabilized electrons reside in the internal part of the cluster, because adsorption of further layers on top of the first 3 BL does not alter the stabilization rate, once the islands percolated. This scenario is depicted by the illustration in Fig. 5 where potential bulk and boundary sites are sketched.

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Furthermore, the left panel of Fig. 5 contains data on the time-dependent stabilization for 4 BL amorphous D2O/Ru(0 0 1), which present a significantly higher stabilization rate than at comparable coverage on Cu(1 1 1). At first these data underline that electron solvation at ice–metal interfaces is a general phenomenon independent on the substrate. However, quantitative differences in the dynamics contain information about underlying mechanisms and interactions. To explain the increase in the stabilization rate, we consider two contributions [33]. One is related to the more efficient electron back-transfer on Ru(0 0 1), as discussed in Section 3.3. The second one relates to the more complex D2O structure on Ru(0 0 1) [3,36], which is currently under debate and details are beyond the scope of this contribution. Briefly, pffiffiffi due pffiffiffi to a larger D2O–Ru interaction than on, e.g., noble metals, a well ordered ð 3  3Þ first bilayer consists most probably of a mixed structure with the D of water molecules pointing in various directions. Since this layer serves as a seed layer for further adsorption, one might speculate that an effectively lower coordination in layers adjacent to the first bilayer is responsible for the faster stabilization of solvated electrons than for multilayers D2O/Cu(1 1 1). Such a lower H-bond coordination is reasonable, because D2O/Ru(0 0 1) exhibits very similar stabilization rates as have been determined for the low coverage regime of D2O/Cu(1 1 1). On the other hand, contributions of the faster back-transfer to the Ru substrate contribute to the observed stabilization dynamics. A separation of contributions from electron transfer and energetic stabilization to the observed peak shift with time delay will be published in the future [33]. Now we turn to a different example of correlation between structure and dynamics. It has been pointed out in the beginning that ice offers the possibility to compare the ultrafast dynamics of amorphous and crystalline structures. In our investigation we obtained such systematic data for D2O/Ru(0 0 1), however, they are not shown here, but in Refs. [21,33]. We find that upon crystallization the peak eS in the 2PPE spectrum which has been attributed to solvated electrons disappears. This observation confirms that electron solvation is mediated by initial charge accumulation at favorable sites which prevail in the amorphous environment and are in a crystalline layer much less abundant and eventually reduced below sensitivity. In parallel the energetically broad continuum of the conduction band is transformed into a series of sharp image potential states with a well defined dispersion relation of delocalized electrons in ice which is a consequence of periodicity in the crystalline layer. The n = 1, 2 states are found at E
EF = 3.2 eV and 3.8 eV, respectively [21]. In Fig. 4 a cross-correlation trace of this delocalized state is given. The observed transfer is extremely efficient and only an upper limit of <10 fs has been determined. This compares well with the transfer dynamics of eCB in amorphous ice (Fig. 2), which favors a similar interaction strength of delocalized states in the amorphous and crystalline state. This differs fundamentally from the localization process. Here, we concluded that the onset of periodicity strongly reduces the density of favorable sites for electron localization. Note that very recently we observed a minority species of long-living localized electrons in crystalline ice layers which are attributed to static defect sites. Thus, through a comparison of amorphous and crystalline ice layers we have shown that localized

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electrons acts as precursors to electron solvation, which emphasizes the potential of structural variation as a route to investigate elementary mechanisms in ultrafast dynamics.

4. Conclusions and outlook By employing established surface science techniques and ultrafast photoelectron spectroscopy, non-equilibrium electron dynamics initiated by excess electrons at ice–metal interfaces have been investigated. The rich dynamics observed experimentally originate from a sequence of individual elementary steps of photo-induced electron transfer into ice and back to the metal, electron localization and solvation. The assignment of these elementary steps benefited from the time-resolved detection and well established advantages of angle-resolved photoemission spectroscopy, i.e., access to the electron momentum parallel to the interface and the binding energy. Unlike earlier studies of electron solvation, the present surface science approach allowed to look into the dependence of ultrafast dynamics on molecular and electronic structure. Therefore, several different interfaces like amorphous D2O and H2O on Cu(1 1 1) and amorphous and crystalline D2O/Ru(0 0 1) have been explored. First examples demonstrate how structural variation contributes to the understanding of underlying mechanisms in such complex processes. We expect a considerable step forward towards a fundamental understanding of electron–molecule interactions and their dynamical properties which are relevant for future applications, e.g., in molecular electronics. Moreover, we would like to stimulate theoretical investigations of structure and dynamics which are demanding, but would be very valuable to further a deeper insight. Note that recently we extended our investigation to electrons trapped in defects of crystalline ice layers and observed that stabilization dynamics continues up to ls time scales. We expect that such electrons enable us to focus on the quasi-equilibrated solvated electron rather than being restricted to electronic species en route to equilibration. On the other side, such extremely long lifetimes and the high local charge density turns these trapped electrons in crystalline ice into promising candidates to investigate surface and atmospheric chemistry.

Acknowledgements It is a pleasure to thank all coworkers who made this work possible, especially C. Gahl, J. Sta¨hler and P. Loukakos who carried out the experiments in the group of M. Wolf to whom I am very grateful for his active support during recent years. Fruitful discussions with I. Bezel, C.B. Harris, T. Klamroth, M. Mehlhorn, K. Morgenstern, P. Saalfrank, and A.N. Unterreiner are gratefully acknowledged. The project was generously supported by G. Ertl and the Deutsche Forschungsgemeinschaft through SPP 1093.

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