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Physisorption on a metal surface probed by surface state resonant second harmonic generation
Surface Science 565 (2004) 27–36 www.elsevier.com/locate/susc
Physisorption on a metal surface probed by surface state resonant second harmonic generation Susan M. Dounce *, Minchul Yang, Hai-Lung Dai
*
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 1910-6323, USA Received 26 April 2004; accepted for publication 24 June 2004 Available online 8 July 2004
Abstract Second harmonic generation from a Ag(1 1 0) surface, resonantly enhanced by the surface state transition at 1.74 eV, is found to be greatly affected by submonolayer adsorption. The physisorption of water or methanol causes a monotonic, exponential-like decay of the SH intensity that can be described by a model treating the adsorbate as a delocalized, weak perturbation in the resonantly enhanced SHG. On the other hand weak chemisorption of aniline generates a complex response in the SH intensity that eludes the predictability of the model. Analysis of the SH intensity has determined that water or methanol adsorption causes an upward shift in the minimum energies of the pair of surface states on Ag(1 1 0) and an increase in the transition linewidth. The sensitive response of the surface states to the presence of adsorbates provides a basis for SHG resonantly enhanced by surface state transition as a highly sensitive probe of submonolayer coverage. 2004 Elsevier B.V. All rights reserved. Keywords: Second harmonic generation; Surface electronic phenomena (work function, surface potential, surface states, etc.); Low index single crystal surfaces; Silver; Physical adsorption; Water; Alcohols; Aromatics; Metallic surfaces
1. Introduction Understanding the interaction of adsorbates with metal surface electronic states can have a direct impact on fundamental concepts in surface * Corresponding authors. Tel.: +1 2158987987; fax: +1 2158988296. E-mail addresses: [email protected] (S.M. Dounce), [email protected] (H.-L. Dai).
science that involve surface states such as surface diffusion barriers, adsorbate–substrate and interadsorbate interactions, carrier transport and relaxation, and surface layer growth mechanisms. For example, the presence of occupied surface states (SSÕs) on Pt(1 1 1) terraces leads to clustering of Xe atoms while the unoccupied SSÕs at step sites induce repulsive behavior between Xe atoms [1]. It is therefore important to characterize the effects that adsorbates and SSÕs have on one another.
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.191
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How an adsorbate affects the overall surface electronic structure of a metal and in particular, the surface states, is a complex problem. What is so interesting about adsorbate–SS interactions is that different adsorbates can lead to remarkably different changes in SS properties. Surface state energies depend sensitively on the surface potential which is uniquely altered on a microscopic scale by the presence of different adsorbates. It is known that strong chemisorption or adsorbate-induced surface reconstruction leads to a complete alteration of the electronic structure and therefore the disappearance of the surface states [1]. However, in other cases where the adsorbate–substrate interaction is weaker, the surface states persist upon adsorption with modified energies and lifetimes [2–5]. Many studies using photoemission spectroscopy (PES), inverse photoemission spectroscopy, angle resolved photoemission spectroscopy, and scanning tunneling microscopy (STM) have observed the shift of surface state energies with adsorbate coverage for systems such as Xe on Ag and Cu [2–4], CO on Cu [5], Na on Cu [1,6,7], H on Pd [8], and O on Cu [9]. For the Xe/Ag and CO/Cu systems, an upward shift was observed. In addition, it was suggested that an upward energy shift to above the Fermi level, observed as quenching of surface states, on Ag(1 1 0) is induced by O2 and H2O [10]. On the other hand, a downward linear shift has been measured for the adsorption of Na on Cu, H on Pd, and O on Cu. Although the mechanism for the energy shift is not understood in all cases, a monotonic change in surface state energy with increasing adsorbate coverage up to one monolayer is typical. Second harmonic generation (SHG) is a technique particularly well suited for the study of SSÕs as it is a highly sensitive probe of surfaces and interfaces. Centrosymmetric crystals such as Ag(1 1 0) produce no second harmonic (SH) light from the bulk in the dipole approximation, and consequently the primary contribution to the SH intensity from these crystals is from the surface layers. Within these layers there can be many different sources of SHG. In metals that have a pair of surface states (one of which lies below the Fermi energy, EF), transitions between these states can
enhance SHG when they are in resonance with the fundamental or SH light and be directly probed by resonantly enhanced SHG [11]. Ag(1 1 0) is one example of such a metal. There exists a pair of surface states at Y on Ag(1 1 0), the lower of which lies around 0.1 eV below EF and the other about 1.65 eV above. The surface state enhanced SHG for this system has been characterized previously for the temperature dependence of the bare metal surface [12]. It was demonstrated that a model derived from a quantum mechanical treatment of the second order susceptibility, vijk(2x), and including the linear shift of the occupied surface state, was able to quantitatively describe the exponential decrease in SH intensity with increasing temperature. This analysis was able to extract such quantities as the linewidth of the surface state transition as well as an angleaveraged mass enhancement parameter. We will show that this model forms a basis on which one can account for SH intensity changes as a result of adsorption and coverage dependence. This modeling enables us to define how SSÕs change in the presence of different adsorbates and also allows us to develop the resonantly enhanced SHG technique as a surface coverage probe. In the following, we use the model presented in Ref. [12] and modify it to describe the surface state induced second harmonic intensity as a function of adsorbate coverage for weakly interacting systems. It is observed that the surface state resonance is uniquely sensitive to each different adsorbate. By fitting the model to new SH data for the uptake of water, methanol, and aniline on Ag(1 1 0), we can extract the adsorbate-induced linewidth broadening as a function of coverage. Furthermore, we will show that the superb sensitivity of resonant SHG combined with the traditional benefits of SHG (i.e. real-time, in situ, and nondestructive surface probe) afford us with a new and highly sensitive technique to measure submonolayer surface coverage.
2. Experimental The details of the UHV chamber and optical setup have been described elsewhere [12]. Briefly,
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
the 532 nm output of a 20 Hz pulsed Nd:YAG laser was used to pump a pulsed dye laser which could generate a laser beam in the range of 680– 740 nm as the fundamental for SHG. The primary portion of the beam from the dye laser entered the UHV chamber and was incident on the Ag(1 1 0) single crystal surface along the ½1 1 0 direction. Surface cleanliness was extremely important in these experiments and was maintained by routine cycles of Ar+ bombardment followed by annealing at 700 K. Upon exiting the chamber, the beam passed through a short-pass filter leaving only the SH signal which then passed through a polarizer in order to select the desired polarization of light. The data presented here is for s-polarized incident fundamental (i.e. the optical field is parallel to the surface), and p-polarized SH. The signal was then passed through a monochromator, detected by a photomultiplier tube, and processed, along with a reference signal, by a CAMAC gated detection system and computer. Prior to the experiments, the water (99.9%, Aldrich Co.), methanol (99.8%), and aniline (>99.5%) samples were purified by several freeze-pump-thaw cycles. Molecules were introduced into the UHV chamber (base pressure 1 · 1010 Torr) through a variable leak valve at a pressure of about 2 · 109 Torr while SH intensity was monitored continuously using the optical setup described above. The dosing pressure was measured by an ionization gauge and was adjusted manually to maintain a constant pressure. Sample temperature was controlled through resistive heating and liquid nitrogen cooling and was maintained at 85 (±0.1 K) during deposition. Since it is impossible to exactly reproduce the same surface conditions after each sputtering cycle, the uptake experiments were repeated numerous times to ensure reproducible results.
3. A model for coverage dependence in SHG resonantly enhanced by surface state transition Ref. [12] describes the derivation of a model for the temperature dependence of surface state induced second harmonic generation starting from a quantum mechanical treatment of the second
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order nonlinear susceptibility of a solid (vijk(2x)). Here we extend the model in Ref. [12] to include the adsorbate coverage dependence. It has been shown throughout literature that the weak binding of adsorbates to a metal surface leads to a shift in SS energies that may or may not be linear with coverage [2–5]. This delocalized energy shift is analogous to the temperature-induced shift reported in Refs. [12,13]. We make the assumption that the effective masses of the surface states do not change significantly upon adsorption. It is expected that in the case of chemisorption, the dispersion of the states can be grossly affected by adsorption. Thus the model presented here is applicable only for the case of weak adsorption (physisorption) in which the nature of the states (i.e. dispersion, symmetry etc.) does not change significantly. From Ref. [12] the surface state transition enhanced SH intensity can be expressed as 32 2 Z F ðk ; T Þ a k 7 6 I 2x ðx; T Þ ¼ I 0 ðx; T Þ4 dk k qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi5 2 2 f hx Eba ðk k ; T Þg þ fCðT Þg
ð1Þ
where I0(x,T) is a parameter that accounts for the dispersion effects of the dielectric constant and transition matrices, ki is the wave vector parallel to the surface, Fa(ki,T) is the value of the Fermi distribution function at state jaæ, hx is the incident photon energy, Eba(ki,T) is the energy separation between occupied state jaæ and unoccupied state jbæ, and C is the linewidth of the transition. The adsorbates are assumed to be in a pattern of adsorption which is either random or repulsive without resulting in large island formation. Thus, adsorption is assumed to cause a perturbation to the surface state wave functions in a delocalized way over the entire surface. This leads to a linear shift with coverage in the surface state energies for both states jaæ and jbæ. This behavior has been observed previously by PES studies of CO adsorption on Cu for example [5]. The dispersion of the occupied (jaæ) and unoccupied (jbæ) surface states on the clean surface can be expressed as Ea ¼ Ea1 ðT ; hÞ þ
h2 k 2jj 2m a
ð2aÞ
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Eb ¼ Eb1 ðhÞ þ
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
2 k 2jj h
ð2bÞ
2m b
where, Ea1(T,h), Eb1(h) are the minimum energies for states jaæ, jbæ, and m a , m b are their effective masses [13]. Ea1 is known to depend linearly on temperature and is given by Eq. (3) [13] Ea1 ðT Þ ¼ Ea1 jT ¼0 þ
dEa T: dT
ð3Þ dEa dT
have been The parameters Ea1jT = 0,h = 0 and determined previously to be 106 meV and 0.17 meV/K respectively [13]. In addition, Eb1 was found to be 1.65 eV above the Fermi energy and largely independent of temperature [13]. The adsorbate-induced delocalized shift in surface state energies can be expressed by an additional term to Eq. (3). Thus we have Ea1 ðT ; hÞ ¼ Ea1 jT ¼0 þ
dEa dEa Tþ h; dT dh
ð4aÞ
Eb1 ðT ; hÞ ¼ Eb1 jT ¼0 þ
dEb h: dh
ð4bÞ
From Eq. (1) we see two important factors that affect the SH intensity––the population in the occupied state as described by the Fermi distribution and the resonance condition determined by Eba. If Ea moves above EF and out of the width of the Fermi distribution, the state becomes completely unoccupied and Eq. (1) approaches zero. For the SHG study, the fundamental light was set at 712 nm where hx ¼ Eba ¼ 1:74 eV was the center of the surface state transition at 100 K for Ag(1 1 0) determined in a previous SHG study [11]. A more precise measurement of the center of the transition band by photoemission was reported in Ref. [13], which gave a slightly different value of 1.76 eV. This more precise value is used for the model description of the results here. It is assumed that the non-resonant contribution to I2x(x,T,h) is negligible. We must also consider the effect of the change of the transition linewidth on I2x(x,T,h). The linewidth of the surface state transition represents a convolution of the linewidths of states jaæ and jbæ. The linewidths of the states increase linearly with temperature via electron–phonon coupling [12,14,15]. In addition, it is reasonable to assume
that the linewidth broadens with increasing adsorbate coverage as a result of impurity scattering, and accordingly, the increase is in proportionality to the coverage [16,17]. Previously a small linewidth increase upon adsorption of one monolayer of Xe on Ag(1 1 1) has been observed [2]. Thus, we represent the temperature and coverage dependent linewidth as CðT ; hÞ ¼ C0 þ
dC dC Tþ h: dT dh
ð5Þ
In Ref. [12] the parameters C0 and dC were deterdT mined to be 31 meV and 0.072 meV/K respectively for this system. In this model several parameters remain undedE dE termined: dha1 , dhb1 , dC , and I0. They can be obtained dh from nonlinear least squares fits of the experimentally measured SH intensity at different fundamental wavelengths as a function of adsorbate coverage. The values of these parameters reveal how the adsorbates affect the energies and lifetimes of the surface state electrons and also provide a quantitative understanding of surface state induced SHG.
4. Results and analysis 4.1. Water adsorption on Ag(1 1 0) On the Ag(1 1 0) surface, water adsorbs with a relatively low binding energy of about 42 kJ/mol for all coverages [18] and also a coverage-independent, unity sticking coefficient at temperatures less than the multilayer desorption temperature (Tdes > 135 K) [19]. We have shown through nonresonant SHG that the growth of the first monolayer can be described by a Langmuir adsorption model [20] which relates monolayer coverage to exposure as: h = hs(1exp(kL/hs)) where hs is the saturation coverage, k is the sticking coefficient, and L is the exposure. The saturation coverage was taken from a fitting of the non-resonant SH signal which increases with water coverage until the first monolayer saturates at hs = 1.4 L [20]. Fig. 1 shows the effect of adsorption of water onto the Ag(1 1 0) surface on the surface state enhanced SHG signal for three different incident
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
Fig. 1. SH intensity (Isp) as a function of water exposure at 85 K for three different fundamental wavelengths (712, 722, and 736 nm) within the Ag(1 1 0) surface state transition band. The solid lines through the experimental curves are fittings to Eq. (1).
wavelengths. At all three wavelengths the SH intensity decreases monotonically with increasing water coverage. At 712 nm, the decrease appears to be more rapid than at 722 and 728 nm although all curves go to zero at approximately the same exposure. These wavelengths are chosen since at the blue side of the 712 nm surface state transition band there is substantial contribution from the bulk to SHG [11]. Also shown in Fig. 1 is the nonlinear least squares fitting of the experimental data to the model described in Section 3, assuming that only
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the first monolayer affects the surface state resonance condition. It is a reasonable assumption in the case of physisorbed molecules, that multilayers will not perturb the metalÕs surface electronic structure. The values determined for the paramedE dE ters dha1 , dhb1 , dC , and I0 in Eqs. (1)–(5) are summadh rized in Table 1. The fittings reveal that both the occupied and unoccupied surface states shift to higher energy with coverage and the lower state eventually crosses the Fermi energy and becomes completely unoccupied. One monolayer of water increases the energies of jaæ and jbæ by +100 and +150 meV respectively. In addition, the transition linewidth broadens with increasing water coverage. This determination is consistent with the previously reported small increase of linewidth upon Xe adsorption on Ag(1 1 1) [2]. The broadening effect is most pronounced for wavelengths (+200 meV/ML for 712 nm) closer to the resonance condition. 4.2. Methanol adsorption on Ag(1 1 0) Methanol has a similar binding energy, 41 kJ/ mol [21], with the silver surface as compared to water and appears to affect the surface state resonance in a similar way. The results of the wavelength-dependent decay of the resonantly enhanced SH intensity shown in Fig. 2 for methanol adsorption are comparable to those of water––a monotonic decrease with exposure is observed,
Table 1 Fitting results using Eq. (1) for the coverage dependence of SH intensity for different adsorbates and wavelengths Adsorbate
Wavelength (nm)
dEa dh
Water Water Water
712 722 728
0.100 ± 0.0004 0.101 ± 0.0003 0.100 ± 0.0005
0.149 ± 0.011 0.136 ± 0.011 0.157 ± 0.008
0.100 ± 5 · 104
0.147 ± 0.009
0.102 ± 0.0018 0.101 ± 0.0005 0.101 ± 0.0012
0.060 ± 0.018 0.070 ± 0.018 0.072 ± 0.030
0.101 ± 5 · 104
0.067 ± 0.005
Average Methanol Methanol Methanol Average dEa dh
dEb dh
712 722 728
(eV/ML)
dEb dh
(eV/ML)
dC dh
(eV/ML)
I0(x,T)
0.197 ± 0.003 0.077 ± 0.003 0.016 ± 0.006
7.72 · 108 ± 5.0 · 106 6.84 · 108 ± 5.1 · 106 8.22 · 108 ± 1.3 · 107
0.325 ± 0.007 0.178 ± 0.005 0.065 ± 0.009
8.21 · 108 ± 6.0 · 106 6.33 · 108 ± 6.03 · 106 7.52 · 108 ± 1.1 · 107
and are the adsorbate-induced shifts of the minimum energies of states jaæ and jbæ, and dC dh gives the broadening of the transition linewidth with coverage. I0(x,T) is a parameter containing the dielectric constant and transition matrix elements.
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S.M. Dounce et al. / Surface Science 565 (2004) 27–36
Fig. 2. SH intensity (Isp) as a function of methanol exposure at 85 K for three different fundamental wavelengths (712, 722, and 736 nm). The solid lines through the experimental curves are fittings to Eq. (1).
aniline on the SS resonantly enhanced SHG is quite different from the cases of water and methanol. At 712 nm, the initial adsorption causes no decrease in SH intensity until about 0.1 L at which point the signal begins to decay slowly and then more rapidly after 0.2 L. Furthermore, the wavelength dependence is dramatic. With a fundamental wavelength of 722 nm, a more rapid initial decay is observed. The model outlined above fails to predict this behavior and thus no fitting is attempted in Fig. 3. It appears that stronger adsorbates such as aniline change the nature of the surface state (for instance a change in the effective mass), which complicates any quantitative analysis.
5. Discussion with a faster decay for wavelengths near the center of the transition. The data was fit to the model and the results shown in Table 1 indicate that the energies of jaæ and jbæ shift upward by +100 and +67 meV respectively at one monolayer of methanol coverage. The transition linewidth increases with coverage with the effect most pronounced closer to resonance (+320 meV/ML at 712 nm). 4.3. Aniline adsorption on Ag(1 1 0) Aniline is known to weakly chemisorb on Ag(1 1 0) with a binding energy of 73.5 kJ/mol [22]. Fig. 3 shows that the effect of adsorption of
We can account for the decrease in SH intensity at or near the SS resonance in terms of an adsorbate-induced change in minimum energies of the surface states. From the model above it can be seen that there are two factors which determine the intensity of the SH signal––the resonance condition described by the denominator of Eq. (1), and the population of jaæ given by the Fermi distribution in the numerator. The model predicts that SH intensity will decrease if the occupied surface state moves upward toward EF or if the energy separation between jaæ and jbæ moves away from resonance with the photon energy. The exponential-like decrease in SH intensity with increasing water coverage in Fig. 1 shows the adsorbate-induced depopulation/detuning effects and eventual quenching of the surface state transition as state jaæ moves above EF. The dependence on the fundamental wavelength is clear evidence for a delocalized linear shift of the surface state energies, reminiscent of the temperature-induced linear shift deduced in Ref. [12]. 5.1. Surface states minimum energy shifts and linewidth broadening
Fig. 3. SH intensity (Isp) as a function of aniline exposure at 85 K for fundamental wavelengths of 712, and 722 nm.
The fitting of the SH intensity coverage dependences to the model has revealed that one monolayer of water induces a +100 meV shift in
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
minimum energy of state jaæ and a +150 meV shift of state jbæ. Likewise, one monolayer of methanol shifts jaæ by +100 meV and jbæ by +67 meV. The magnitudes of these energy shifts are comparable to the Xe and CO induced shifts. The adsorption of one monolayer of Xe on Ag(1 1 1) causes a +119 meV shift in the occupied surface state to above the Fermi level [2]. Similarly, one monolayer of CO adsorbed onto Cu(1 1 1) causes a +250 meV energy shift to above EF [5]. The direction of the water or methanol-induced SS energy shift from our analysis is consistent with the available data for weaker adsorbates and is also consistent with predictions from quenching experiments [10]. Stronger bound adsorbers such as Na, H, and O induce downward SS energy shifts [1,6–9]. These observations combined suggest an apparent trend between the adsorption strength and the direction/magnitude of the shift. Strong bound adsorbers such as Na, where charge transfer occurs upon adsorption, induce a SS shift in the same direction as the work function change. This behavior can be understood in terms of the phase accumulation model discussed elsewhere [1,9,23]. However, this behavior clearly does not hold for weaker bound adsorbates which induce a SS shift in the opposite direction of the work function change. While it is well known that adsorbates often lead to changes in surface state energies, we can only speculate as to the mechanism for the delocalized upward shift observed for weakly bound adsorbates in our experiments. One possible way to explain the upward shift (opposite of work function change) is by the lateral confinement of the surface state electrons––an effect which can be caused, for example, by introducing abrupt barriers in the surface potential in the form of step edges or adsorbate ‘‘corrals’’. On a perfectly flat metal surface, SS wavefunctions are delocalized over the entire surface. When a one-dimensional array of regularly spaced monatomic steps is introduced, however, the SS electrons become confined to terraces by reflection off of these step edges. This effect shifts the SS energy toward EF thus causing a depopulation of the state [24]. The magnitude of the energy shift depends on the number of steps (i.e. size of the terraces). This behavior can be understood in terms
33
of a one-dimensional Kronig–Penney model [24]. A similar effect was observed when surface state electrons were confined within 48-atom Fe ‘‘quantum corrals’’ on Cu(1 1 1) [25]. There the confinement resulted in the shift of the binding energy of the eigenstates toward EF. The weakly bound water and methanol molecules in this study do not introduce sharp or high potential barriers at the surface as compared to step edges or Fe atoms. The interaction may still cause sufficient barriers to induce confinement effects. If this were the case, as coverage increases, the SS wavefunctions would become confined to smaller and smaller areas of the bare surface, causing the SS energies to shift higher and higher. The results we obtain for the delocalized upward energy shift with water or methanol coverage are consistent with surface state electron confinement and suggest that the potential energy barriers created at the adsorption sites are sufficient to produce this effect. We also extract from our model the broadening of the transition linewidth as a function of adsorbate coverage. Adsorbates can essentially act as impurities off which surface state electrons can scatter, thus reducing their lifetimes and increasing the transition width. This results in a contribution (Cimp) to the total transition linewidth which is linearly proportional to the coverage [16,17]. Scattering off the adsorbates imparts the surface state electrons with some finite mean free path. The higher the coverage, the more frequent the scattering, and therefore the smaller the electronÕs mean free path [26]. A reduced mean free path is in accordance with the lateral confinement of the SS electrons. Confinement due to scattering off adsorbates leads to an increased uncertainty in the surface wave vector and thus linewidth broadening [26]. Furthermore, the distribution of confinement sizes may also contribute to inhomogeneous broadening. 5.2. Physisorption versus chemisorption The model outlined above is only applicable to relatively weak adsorption. An assumption made in this treatment is that the adsorbates do not change the fundamental nature of the surface states. If charge transfer between the molecule
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S.M. Dounce et al. / Surface Science 565 (2004) 27–36
and surface is small (as is the case in physisorption) it is reasonable to assume that the dispersion (i.e. effective mass) of the surface states is unchanged. It was found in the case of adsorption of one monolayer of Xe on Ag(1 1 1) the effective mass was unchanged compared to that of the clean surface [2]. On the other hand, the adsorption of C6F6 on Cu(1 1 1), where charge transfer is significant in this weak chemisorption case, led to significant changes in the effective masses of unoccupied electronic states [27]. Strong adsorbate–substrate bonding may significantly change the SS properties though the effect on energy shift may appear to be predictable. For example, PES shows that adsorption of Na on Cu(1 1 0) also leads to a linear shift in surface state energies, but polarization studies suggest that the symmetry of the states is altered by the nearly ionic Na–Cu bond [6]. The case of aniline adsorption on Ag(1 1 0) shown in Fig. 3 further illustrates how a strong binding interaction can affect the nature of surface states. While the adsorption behavior of water and methanol can clearly be described by the model outlined above, anilineÕs effect on the surface state transition eludes the modelÕs predictability. The delayed decrease in SH intensity at a fundamental wavelength of 712 nm during aniline deposition suggests that the adsorbates change some aspect of the surface state resonance other than the minimum energies and transition width. Within the scope of the model presented here, there should be an immediate decrease in SH intensity. A similar delayed decay of surface state resonant SH intensity has been observed for the adsorption of benzene on Cu(1 1 1) [28]. The delay was attributed to the selective adsorption of benzene at step sites since the surface states at step edges are completely unoccupied and thus no change in SH intensity would be observed. However, our results for aniline adsorption show that the initial delayed decay is wavelength-dependent, making the step-adsorption explanation less likely for this system. There are several possible explanations as to why aniline adsorption is not well described by the model set up for physisorption. First, aniline itself is known to produce a SH signal [29]. The SH intensity from aniline itself may compete with the resonant SH decrease brought about by the
quenching of the surface state transition. It is more likely that stronger adsorbate–metal charge transfer in the case of aniline–Ag(1 1 0) leads to a fundamental change in the nature of the surface states––for example in the effective mass. In principle such an adsorbate-induced modification of the surface states, though complex, could be included in the model presented above. 5.3. Surface state resonant SHG as a surface coverage probe The response of surface states to the presence of adsorbates is unique for each adsorbate. If one knows a priori how surface states respond to different atoms or molecules, the measured change can in turn provide information as to the type of adsorbates present on a surface. For example, this fact has become most useful to us in the identification of the existence of impurities in our water sample. As the adsorption of water molecules leads to quenching of the surface state transition, it is expected that the resonant SH signal should be recovered upon desorption of water from the Ag(1 1 0) surface, if the water sample contains no impurity. On the other hand, a contaminated water sample would produce anomalous behavior in the SH response as the contaminant would have a different desorption temperature and may interact with the surface state differently. Surface state resonant SHG has proven to be a highly sensitive tool for measuring submonolayer surface coverage. The resonant signal is many orders of magnitude larger than any non-resonant SH signal and does not require that the adsorbates themselves produce SHG. The quantitative relationship between the SH intensity and the coverage can be rigorously established for physisorbed molecules and serves as a basis for measurement of submonolayer coverage. Significantly, the surface state enhanced SH intensity can be modeled to relate to submonolayer coverage. The SH intensity changes as the occupied surface state energy shifts in response to the presence of adsorbates, thus offering a unique probe of the covered or uncovered surface area. This is different from most other techniques for monitoring the amount of the adsorbate on the surface. This Ônot-coveredÕ sur-
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
face area is in some cases a significant, but hard to measure, quantity. Furthermore, as our experiments have shown, different adsorbates affect the surface states in unique ways that can be drastically different depending on the nature of the adsorbate–surface interaction. This observation affords many possibilities from careful studies of molecule–surface interactions to the use of surface states as chemical sensors. Thus, this highly sensitive and chemically discriminative technique could, for example, monitor a surface reaction in situ as the reactants are converted to products which themselves alter the surface states in a different way.
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highly sensitive probe of submonolayer adsorbate coverage. Acknowledgment This work is supported by a grant from the Air Force Office of Scientific Research. S.M.D. acknowledges the receipt of a Graduate Assistance In Areas Of National Need Fellowship from the US Department of Education. The equipment for this research was supported by a grant from the National Science Foundation, MRSEC program, No. DMR00-79909. References
6. Conclusion The effect of adsorption of water, methanol and aniline to second harmonic generation from a Ag(1 1 0) surface, resonantly enhanced by the surface state transition, has been examined. The physisorption of water and methanol appears to induce a straightforward, exponential-like decay of the SH intensity as a function of surface coverage while the weak chemisorption of aniline results in a complex change in the SH intensity. A model based on quantum mechanical description of the nonlinear optical susceptibility in resonance with a surface state transition and empirical treatment of the adsorption as a delocalized effect has been developed to describe the adsorbate coverage dependence in the second harmonic intensity. Fitting to the experimental data is able to extract the surface state energy shifts and transition linewidth broadening as a result of water or methanol adsorption. It has been determined that water and methanol molecules cause an upward shift in the minimum energies of the pair of surface states on Ag(1 1 0) and also cause the transition width to increase. These effects could be understood in terms of adsorbate-induced lateral confinement of the surface state electrons. The model presented here can in principle be applied to any weak adsorption on a metal with a pair of surface states. The surface statesÕ sensitive reaction to the presence of adsorbates provides a basis for SHG resonantly enhanced by surface state transition as a
[1] N. Memmel, Surface Science Reports 32 (1998) 93. [2] H. Hovel, B. Grimm, B. Reihl, Surface Science 477 (2001) 43. [3] R. Paniago, R. Matzdorf, G. Meister, A. Goldmann, Surface Science 325 (1995) 336. [4] J.Y. Park, U.D. Ham, S.J. Kahng, Y. Kuk, K. Miyake, K. Hata, H. Shigekawa, Physical Review B 62 (2000) R16341. [5] S.A. Lindgren, J. Paul, L. Wallden, Surface Science 117 (1982) 426. [6] C. Schwab, G. Meister, A. Goldmann, E. Bertel, Surface Science 469 (2000) 93. [7] E. Bertel, Surface Science 333 (1995) 1136. [8] Z. Lenac, M. Sunjic, H. Conrad, M.E. Kordesch, Physical Review B 36 (1987) 9500. [9] C.T. Chen, N.V. Smith, Physical Review B 40 (1989) 7487. [10] B. Reihl, R.R. Schlittler, H. Neff, Physical Review Letters 52 (1984) 1826. [11] L.E. Urbach, K.L. Percival, J.M. Hicks, E.W. Plummer, H.L. Dai, Physical Review B 45 (1992) 3769. [12] S.M. Dounce, M. Yang, H.L. Dai, Physical Review B 67 (2003) 205410/1. [13] A. Gerlach, G. Meister, R. Matzdorf, A. Goldmann, Surface Science 443 (1999) 221. [14] B.A. McDougall, T. Balasubramanian, E. Jensen, Physical Review B 51 (1995) 13891. [15] K. Takahashi, A. Tanaka, H. Sasaki, W. Gondo, S. Suzuki, S. Sato, Physical Review B 60 (1999) 8748. [16] T. Valla, A.V. Fedorov, P.D. Johnson, S.L. Hulbert, Physical Review Letters 83 (1999) 2085. [17] F. Theilmann, R. Matzdorf, G. Meister, A. Goldmann, Physical Review B 56 (1997) 3632. [18] K. Bange, T.E. Madey, J.K. Sass, E.M. Stuve, Surface Science 183 (1987) 334. [19] M.A. Henderson, Surface Science Reports 46 (2002) 5. [20] S.M. Dounce, S.-H. Jen, M. Yang, H.-L. Dai, to be submitted.
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[21] R.M. Zhang, A.J. Gellman, Journal of Physical Chemistry 95 (1991) 7433. [22] M.C. Yang, T.J. Rockey, D. Pursell, H.L. Dai, Journal of Physical Chemistry B 105 (2001) 11945. [23] N.V. Smith, Physical Review B 32 (1985) 3549. [24] O. Sanchez, J.M. Garcia, P. Segovia, J. Alvarez, A.L.V. Deparga, J.E. Ortega, M. Prietsch, R. Miranda, Physical Review B 52 (1995) 7894.
[25] M.F. Crommie, C.P. Lutz, D.M. Eigler, Science 262 (1993) 218. [26] R. Matzdorf, Surface Science Reports 30 (1998) 153. [27] C. Gahl, K. Ishioka, Q. Zhong, A. Hotzel, M. Wolf, Faraday Discussions (2000) 191. [28] H. Ishida, R. Mizoguchi, K. Onda, C. Hirose, S.S. Kano, A. Wada, Surface Science 526 (2003) 201. [29] M. Yang, S.M. Dounce, H.-L. Dai, to be submitted.
Physisorption on a metal surface probed by surface state resonant second harmonic generation Susan M. Dounce *, Minchul Yang, Hai-Lung Dai
*
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 1910-6323, USA Received 26 April 2004; accepted for publication 24 June 2004 Available online 8 July 2004
Abstract Second harmonic generation from a Ag(1 1 0) surface, resonantly enhanced by the surface state transition at 1.74 eV, is found to be greatly affected by submonolayer adsorption. The physisorption of water or methanol causes a monotonic, exponential-like decay of the SH intensity that can be described by a model treating the adsorbate as a delocalized, weak perturbation in the resonantly enhanced SHG. On the other hand weak chemisorption of aniline generates a complex response in the SH intensity that eludes the predictability of the model. Analysis of the SH intensity has determined that water or methanol adsorption causes an upward shift in the minimum energies of the pair of surface states on Ag(1 1 0) and an increase in the transition linewidth. The sensitive response of the surface states to the presence of adsorbates provides a basis for SHG resonantly enhanced by surface state transition as a highly sensitive probe of submonolayer coverage. 2004 Elsevier B.V. All rights reserved. Keywords: Second harmonic generation; Surface electronic phenomena (work function, surface potential, surface states, etc.); Low index single crystal surfaces; Silver; Physical adsorption; Water; Alcohols; Aromatics; Metallic surfaces
1. Introduction Understanding the interaction of adsorbates with metal surface electronic states can have a direct impact on fundamental concepts in surface * Corresponding authors. Tel.: +1 2158987987; fax: +1 2158988296. E-mail addresses: [email protected] (S.M. Dounce), [email protected] (H.-L. Dai).
science that involve surface states such as surface diffusion barriers, adsorbate–substrate and interadsorbate interactions, carrier transport and relaxation, and surface layer growth mechanisms. For example, the presence of occupied surface states (SSÕs) on Pt(1 1 1) terraces leads to clustering of Xe atoms while the unoccupied SSÕs at step sites induce repulsive behavior between Xe atoms [1]. It is therefore important to characterize the effects that adsorbates and SSÕs have on one another.
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.191
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S.M. Dounce et al. / Surface Science 565 (2004) 27–36
How an adsorbate affects the overall surface electronic structure of a metal and in particular, the surface states, is a complex problem. What is so interesting about adsorbate–SS interactions is that different adsorbates can lead to remarkably different changes in SS properties. Surface state energies depend sensitively on the surface potential which is uniquely altered on a microscopic scale by the presence of different adsorbates. It is known that strong chemisorption or adsorbate-induced surface reconstruction leads to a complete alteration of the electronic structure and therefore the disappearance of the surface states [1]. However, in other cases where the adsorbate–substrate interaction is weaker, the surface states persist upon adsorption with modified energies and lifetimes [2–5]. Many studies using photoemission spectroscopy (PES), inverse photoemission spectroscopy, angle resolved photoemission spectroscopy, and scanning tunneling microscopy (STM) have observed the shift of surface state energies with adsorbate coverage for systems such as Xe on Ag and Cu [2–4], CO on Cu [5], Na on Cu [1,6,7], H on Pd [8], and O on Cu [9]. For the Xe/Ag and CO/Cu systems, an upward shift was observed. In addition, it was suggested that an upward energy shift to above the Fermi level, observed as quenching of surface states, on Ag(1 1 0) is induced by O2 and H2O [10]. On the other hand, a downward linear shift has been measured for the adsorption of Na on Cu, H on Pd, and O on Cu. Although the mechanism for the energy shift is not understood in all cases, a monotonic change in surface state energy with increasing adsorbate coverage up to one monolayer is typical. Second harmonic generation (SHG) is a technique particularly well suited for the study of SSÕs as it is a highly sensitive probe of surfaces and interfaces. Centrosymmetric crystals such as Ag(1 1 0) produce no second harmonic (SH) light from the bulk in the dipole approximation, and consequently the primary contribution to the SH intensity from these crystals is from the surface layers. Within these layers there can be many different sources of SHG. In metals that have a pair of surface states (one of which lies below the Fermi energy, EF), transitions between these states can
enhance SHG when they are in resonance with the fundamental or SH light and be directly probed by resonantly enhanced SHG [11]. Ag(1 1 0) is one example of such a metal. There exists a pair of surface states at Y on Ag(1 1 0), the lower of which lies around 0.1 eV below EF and the other about 1.65 eV above. The surface state enhanced SHG for this system has been characterized previously for the temperature dependence of the bare metal surface [12]. It was demonstrated that a model derived from a quantum mechanical treatment of the second order susceptibility, vijk(2x), and including the linear shift of the occupied surface state, was able to quantitatively describe the exponential decrease in SH intensity with increasing temperature. This analysis was able to extract such quantities as the linewidth of the surface state transition as well as an angleaveraged mass enhancement parameter. We will show that this model forms a basis on which one can account for SH intensity changes as a result of adsorption and coverage dependence. This modeling enables us to define how SSÕs change in the presence of different adsorbates and also allows us to develop the resonantly enhanced SHG technique as a surface coverage probe. In the following, we use the model presented in Ref. [12] and modify it to describe the surface state induced second harmonic intensity as a function of adsorbate coverage for weakly interacting systems. It is observed that the surface state resonance is uniquely sensitive to each different adsorbate. By fitting the model to new SH data for the uptake of water, methanol, and aniline on Ag(1 1 0), we can extract the adsorbate-induced linewidth broadening as a function of coverage. Furthermore, we will show that the superb sensitivity of resonant SHG combined with the traditional benefits of SHG (i.e. real-time, in situ, and nondestructive surface probe) afford us with a new and highly sensitive technique to measure submonolayer surface coverage.
2. Experimental The details of the UHV chamber and optical setup have been described elsewhere [12]. Briefly,
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
the 532 nm output of a 20 Hz pulsed Nd:YAG laser was used to pump a pulsed dye laser which could generate a laser beam in the range of 680– 740 nm as the fundamental for SHG. The primary portion of the beam from the dye laser entered the UHV chamber and was incident on the Ag(1 1 0) single crystal surface along the ½1 1 0 direction. Surface cleanliness was extremely important in these experiments and was maintained by routine cycles of Ar+ bombardment followed by annealing at 700 K. Upon exiting the chamber, the beam passed through a short-pass filter leaving only the SH signal which then passed through a polarizer in order to select the desired polarization of light. The data presented here is for s-polarized incident fundamental (i.e. the optical field is parallel to the surface), and p-polarized SH. The signal was then passed through a monochromator, detected by a photomultiplier tube, and processed, along with a reference signal, by a CAMAC gated detection system and computer. Prior to the experiments, the water (99.9%, Aldrich Co.), methanol (99.8%), and aniline (>99.5%) samples were purified by several freeze-pump-thaw cycles. Molecules were introduced into the UHV chamber (base pressure 1 · 1010 Torr) through a variable leak valve at a pressure of about 2 · 109 Torr while SH intensity was monitored continuously using the optical setup described above. The dosing pressure was measured by an ionization gauge and was adjusted manually to maintain a constant pressure. Sample temperature was controlled through resistive heating and liquid nitrogen cooling and was maintained at 85 (±0.1 K) during deposition. Since it is impossible to exactly reproduce the same surface conditions after each sputtering cycle, the uptake experiments were repeated numerous times to ensure reproducible results.
3. A model for coverage dependence in SHG resonantly enhanced by surface state transition Ref. [12] describes the derivation of a model for the temperature dependence of surface state induced second harmonic generation starting from a quantum mechanical treatment of the second
29
order nonlinear susceptibility of a solid (vijk(2x)). Here we extend the model in Ref. [12] to include the adsorbate coverage dependence. It has been shown throughout literature that the weak binding of adsorbates to a metal surface leads to a shift in SS energies that may or may not be linear with coverage [2–5]. This delocalized energy shift is analogous to the temperature-induced shift reported in Refs. [12,13]. We make the assumption that the effective masses of the surface states do not change significantly upon adsorption. It is expected that in the case of chemisorption, the dispersion of the states can be grossly affected by adsorption. Thus the model presented here is applicable only for the case of weak adsorption (physisorption) in which the nature of the states (i.e. dispersion, symmetry etc.) does not change significantly. From Ref. [12] the surface state transition enhanced SH intensity can be expressed as 32 2 Z F ðk ; T Þ a k 7 6 I 2x ðx; T Þ ¼ I 0 ðx; T Þ4 dk k qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi5 2 2 f hx Eba ðk k ; T Þg þ fCðT Þg
ð1Þ
where I0(x,T) is a parameter that accounts for the dispersion effects of the dielectric constant and transition matrices, ki is the wave vector parallel to the surface, Fa(ki,T) is the value of the Fermi distribution function at state jaæ, hx is the incident photon energy, Eba(ki,T) is the energy separation between occupied state jaæ and unoccupied state jbæ, and C is the linewidth of the transition. The adsorbates are assumed to be in a pattern of adsorption which is either random or repulsive without resulting in large island formation. Thus, adsorption is assumed to cause a perturbation to the surface state wave functions in a delocalized way over the entire surface. This leads to a linear shift with coverage in the surface state energies for both states jaæ and jbæ. This behavior has been observed previously by PES studies of CO adsorption on Cu for example [5]. The dispersion of the occupied (jaæ) and unoccupied (jbæ) surface states on the clean surface can be expressed as Ea ¼ Ea1 ðT ; hÞ þ
h2 k 2jj 2m a
ð2aÞ
30
Eb ¼ Eb1 ðhÞ þ
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
2 k 2jj h
ð2bÞ
2m b
where, Ea1(T,h), Eb1(h) are the minimum energies for states jaæ, jbæ, and m a , m b are their effective masses [13]. Ea1 is known to depend linearly on temperature and is given by Eq. (3) [13] Ea1 ðT Þ ¼ Ea1 jT ¼0 þ
dEa T: dT
ð3Þ dEa dT
have been The parameters Ea1jT = 0,h = 0 and determined previously to be 106 meV and 0.17 meV/K respectively [13]. In addition, Eb1 was found to be 1.65 eV above the Fermi energy and largely independent of temperature [13]. The adsorbate-induced delocalized shift in surface state energies can be expressed by an additional term to Eq. (3). Thus we have Ea1 ðT ; hÞ ¼ Ea1 jT ¼0 þ
dEa dEa Tþ h; dT dh
ð4aÞ
Eb1 ðT ; hÞ ¼ Eb1 jT ¼0 þ
dEb h: dh
ð4bÞ
From Eq. (1) we see two important factors that affect the SH intensity––the population in the occupied state as described by the Fermi distribution and the resonance condition determined by Eba. If Ea moves above EF and out of the width of the Fermi distribution, the state becomes completely unoccupied and Eq. (1) approaches zero. For the SHG study, the fundamental light was set at 712 nm where hx ¼ Eba ¼ 1:74 eV was the center of the surface state transition at 100 K for Ag(1 1 0) determined in a previous SHG study [11]. A more precise measurement of the center of the transition band by photoemission was reported in Ref. [13], which gave a slightly different value of 1.76 eV. This more precise value is used for the model description of the results here. It is assumed that the non-resonant contribution to I2x(x,T,h) is negligible. We must also consider the effect of the change of the transition linewidth on I2x(x,T,h). The linewidth of the surface state transition represents a convolution of the linewidths of states jaæ and jbæ. The linewidths of the states increase linearly with temperature via electron–phonon coupling [12,14,15]. In addition, it is reasonable to assume
that the linewidth broadens with increasing adsorbate coverage as a result of impurity scattering, and accordingly, the increase is in proportionality to the coverage [16,17]. Previously a small linewidth increase upon adsorption of one monolayer of Xe on Ag(1 1 1) has been observed [2]. Thus, we represent the temperature and coverage dependent linewidth as CðT ; hÞ ¼ C0 þ
dC dC Tþ h: dT dh
ð5Þ
In Ref. [12] the parameters C0 and dC were deterdT mined to be 31 meV and 0.072 meV/K respectively for this system. In this model several parameters remain undedE dE termined: dha1 , dhb1 , dC , and I0. They can be obtained dh from nonlinear least squares fits of the experimentally measured SH intensity at different fundamental wavelengths as a function of adsorbate coverage. The values of these parameters reveal how the adsorbates affect the energies and lifetimes of the surface state electrons and also provide a quantitative understanding of surface state induced SHG.
4. Results and analysis 4.1. Water adsorption on Ag(1 1 0) On the Ag(1 1 0) surface, water adsorbs with a relatively low binding energy of about 42 kJ/mol for all coverages [18] and also a coverage-independent, unity sticking coefficient at temperatures less than the multilayer desorption temperature (Tdes > 135 K) [19]. We have shown through nonresonant SHG that the growth of the first monolayer can be described by a Langmuir adsorption model [20] which relates monolayer coverage to exposure as: h = hs(1exp(kL/hs)) where hs is the saturation coverage, k is the sticking coefficient, and L is the exposure. The saturation coverage was taken from a fitting of the non-resonant SH signal which increases with water coverage until the first monolayer saturates at hs = 1.4 L [20]. Fig. 1 shows the effect of adsorption of water onto the Ag(1 1 0) surface on the surface state enhanced SHG signal for three different incident
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
Fig. 1. SH intensity (Isp) as a function of water exposure at 85 K for three different fundamental wavelengths (712, 722, and 736 nm) within the Ag(1 1 0) surface state transition band. The solid lines through the experimental curves are fittings to Eq. (1).
wavelengths. At all three wavelengths the SH intensity decreases monotonically with increasing water coverage. At 712 nm, the decrease appears to be more rapid than at 722 and 728 nm although all curves go to zero at approximately the same exposure. These wavelengths are chosen since at the blue side of the 712 nm surface state transition band there is substantial contribution from the bulk to SHG [11]. Also shown in Fig. 1 is the nonlinear least squares fitting of the experimental data to the model described in Section 3, assuming that only
31
the first monolayer affects the surface state resonance condition. It is a reasonable assumption in the case of physisorbed molecules, that multilayers will not perturb the metalÕs surface electronic structure. The values determined for the paramedE dE ters dha1 , dhb1 , dC , and I0 in Eqs. (1)–(5) are summadh rized in Table 1. The fittings reveal that both the occupied and unoccupied surface states shift to higher energy with coverage and the lower state eventually crosses the Fermi energy and becomes completely unoccupied. One monolayer of water increases the energies of jaæ and jbæ by +100 and +150 meV respectively. In addition, the transition linewidth broadens with increasing water coverage. This determination is consistent with the previously reported small increase of linewidth upon Xe adsorption on Ag(1 1 1) [2]. The broadening effect is most pronounced for wavelengths (+200 meV/ML for 712 nm) closer to the resonance condition. 4.2. Methanol adsorption on Ag(1 1 0) Methanol has a similar binding energy, 41 kJ/ mol [21], with the silver surface as compared to water and appears to affect the surface state resonance in a similar way. The results of the wavelength-dependent decay of the resonantly enhanced SH intensity shown in Fig. 2 for methanol adsorption are comparable to those of water––a monotonic decrease with exposure is observed,
Table 1 Fitting results using Eq. (1) for the coverage dependence of SH intensity for different adsorbates and wavelengths Adsorbate
Wavelength (nm)
dEa dh
Water Water Water
712 722 728
0.100 ± 0.0004 0.101 ± 0.0003 0.100 ± 0.0005
0.149 ± 0.011 0.136 ± 0.011 0.157 ± 0.008
0.100 ± 5 · 104
0.147 ± 0.009
0.102 ± 0.0018 0.101 ± 0.0005 0.101 ± 0.0012
0.060 ± 0.018 0.070 ± 0.018 0.072 ± 0.030
0.101 ± 5 · 104
0.067 ± 0.005
Average Methanol Methanol Methanol Average dEa dh
dEb dh
712 722 728
(eV/ML)
dEb dh
(eV/ML)
dC dh
(eV/ML)
I0(x,T)
0.197 ± 0.003 0.077 ± 0.003 0.016 ± 0.006
7.72 · 108 ± 5.0 · 106 6.84 · 108 ± 5.1 · 106 8.22 · 108 ± 1.3 · 107
0.325 ± 0.007 0.178 ± 0.005 0.065 ± 0.009
8.21 · 108 ± 6.0 · 106 6.33 · 108 ± 6.03 · 106 7.52 · 108 ± 1.1 · 107
and are the adsorbate-induced shifts of the minimum energies of states jaæ and jbæ, and dC dh gives the broadening of the transition linewidth with coverage. I0(x,T) is a parameter containing the dielectric constant and transition matrix elements.
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S.M. Dounce et al. / Surface Science 565 (2004) 27–36
Fig. 2. SH intensity (Isp) as a function of methanol exposure at 85 K for three different fundamental wavelengths (712, 722, and 736 nm). The solid lines through the experimental curves are fittings to Eq. (1).
aniline on the SS resonantly enhanced SHG is quite different from the cases of water and methanol. At 712 nm, the initial adsorption causes no decrease in SH intensity until about 0.1 L at which point the signal begins to decay slowly and then more rapidly after 0.2 L. Furthermore, the wavelength dependence is dramatic. With a fundamental wavelength of 722 nm, a more rapid initial decay is observed. The model outlined above fails to predict this behavior and thus no fitting is attempted in Fig. 3. It appears that stronger adsorbates such as aniline change the nature of the surface state (for instance a change in the effective mass), which complicates any quantitative analysis.
5. Discussion with a faster decay for wavelengths near the center of the transition. The data was fit to the model and the results shown in Table 1 indicate that the energies of jaæ and jbæ shift upward by +100 and +67 meV respectively at one monolayer of methanol coverage. The transition linewidth increases with coverage with the effect most pronounced closer to resonance (+320 meV/ML at 712 nm). 4.3. Aniline adsorption on Ag(1 1 0) Aniline is known to weakly chemisorb on Ag(1 1 0) with a binding energy of 73.5 kJ/mol [22]. Fig. 3 shows that the effect of adsorption of
We can account for the decrease in SH intensity at or near the SS resonance in terms of an adsorbate-induced change in minimum energies of the surface states. From the model above it can be seen that there are two factors which determine the intensity of the SH signal––the resonance condition described by the denominator of Eq. (1), and the population of jaæ given by the Fermi distribution in the numerator. The model predicts that SH intensity will decrease if the occupied surface state moves upward toward EF or if the energy separation between jaæ and jbæ moves away from resonance with the photon energy. The exponential-like decrease in SH intensity with increasing water coverage in Fig. 1 shows the adsorbate-induced depopulation/detuning effects and eventual quenching of the surface state transition as state jaæ moves above EF. The dependence on the fundamental wavelength is clear evidence for a delocalized linear shift of the surface state energies, reminiscent of the temperature-induced linear shift deduced in Ref. [12]. 5.1. Surface states minimum energy shifts and linewidth broadening
Fig. 3. SH intensity (Isp) as a function of aniline exposure at 85 K for fundamental wavelengths of 712, and 722 nm.
The fitting of the SH intensity coverage dependences to the model has revealed that one monolayer of water induces a +100 meV shift in
S.M. Dounce et al. / Surface Science 565 (2004) 27–36
minimum energy of state jaæ and a +150 meV shift of state jbæ. Likewise, one monolayer of methanol shifts jaæ by +100 meV and jbæ by +67 meV. The magnitudes of these energy shifts are comparable to the Xe and CO induced shifts. The adsorption of one monolayer of Xe on Ag(1 1 1) causes a +119 meV shift in the occupied surface state to above the Fermi level [2]. Similarly, one monolayer of CO adsorbed onto Cu(1 1 1) causes a +250 meV energy shift to above EF [5]. The direction of the water or methanol-induced SS energy shift from our analysis is consistent with the available data for weaker adsorbates and is also consistent with predictions from quenching experiments [10]. Stronger bound adsorbers such as Na, H, and O induce downward SS energy shifts [1,6–9]. These observations combined suggest an apparent trend between the adsorption strength and the direction/magnitude of the shift. Strong bound adsorbers such as Na, where charge transfer occurs upon adsorption, induce a SS shift in the same direction as the work function change. This behavior can be understood in terms of the phase accumulation model discussed elsewhere [1,9,23]. However, this behavior clearly does not hold for weaker bound adsorbates which induce a SS shift in the opposite direction of the work function change. While it is well known that adsorbates often lead to changes in surface state energies, we can only speculate as to the mechanism for the delocalized upward shift observed for weakly bound adsorbates in our experiments. One possible way to explain the upward shift (opposite of work function change) is by the lateral confinement of the surface state electrons––an effect which can be caused, for example, by introducing abrupt barriers in the surface potential in the form of step edges or adsorbate ‘‘corrals’’. On a perfectly flat metal surface, SS wavefunctions are delocalized over the entire surface. When a one-dimensional array of regularly spaced monatomic steps is introduced, however, the SS electrons become confined to terraces by reflection off of these step edges. This effect shifts the SS energy toward EF thus causing a depopulation of the state [24]. The magnitude of the energy shift depends on the number of steps (i.e. size of the terraces). This behavior can be understood in terms
33
of a one-dimensional Kronig–Penney model [24]. A similar effect was observed when surface state electrons were confined within 48-atom Fe ‘‘quantum corrals’’ on Cu(1 1 1) [25]. There the confinement resulted in the shift of the binding energy of the eigenstates toward EF. The weakly bound water and methanol molecules in this study do not introduce sharp or high potential barriers at the surface as compared to step edges or Fe atoms. The interaction may still cause sufficient barriers to induce confinement effects. If this were the case, as coverage increases, the SS wavefunctions would become confined to smaller and smaller areas of the bare surface, causing the SS energies to shift higher and higher. The results we obtain for the delocalized upward energy shift with water or methanol coverage are consistent with surface state electron confinement and suggest that the potential energy barriers created at the adsorption sites are sufficient to produce this effect. We also extract from our model the broadening of the transition linewidth as a function of adsorbate coverage. Adsorbates can essentially act as impurities off which surface state electrons can scatter, thus reducing their lifetimes and increasing the transition width. This results in a contribution (Cimp) to the total transition linewidth which is linearly proportional to the coverage [16,17]. Scattering off the adsorbates imparts the surface state electrons with some finite mean free path. The higher the coverage, the more frequent the scattering, and therefore the smaller the electronÕs mean free path [26]. A reduced mean free path is in accordance with the lateral confinement of the SS electrons. Confinement due to scattering off adsorbates leads to an increased uncertainty in the surface wave vector and thus linewidth broadening [26]. Furthermore, the distribution of confinement sizes may also contribute to inhomogeneous broadening. 5.2. Physisorption versus chemisorption The model outlined above is only applicable to relatively weak adsorption. An assumption made in this treatment is that the adsorbates do not change the fundamental nature of the surface states. If charge transfer between the molecule
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and surface is small (as is the case in physisorption) it is reasonable to assume that the dispersion (i.e. effective mass) of the surface states is unchanged. It was found in the case of adsorption of one monolayer of Xe on Ag(1 1 1) the effective mass was unchanged compared to that of the clean surface [2]. On the other hand, the adsorption of C6F6 on Cu(1 1 1), where charge transfer is significant in this weak chemisorption case, led to significant changes in the effective masses of unoccupied electronic states [27]. Strong adsorbate–substrate bonding may significantly change the SS properties though the effect on energy shift may appear to be predictable. For example, PES shows that adsorption of Na on Cu(1 1 0) also leads to a linear shift in surface state energies, but polarization studies suggest that the symmetry of the states is altered by the nearly ionic Na–Cu bond [6]. The case of aniline adsorption on Ag(1 1 0) shown in Fig. 3 further illustrates how a strong binding interaction can affect the nature of surface states. While the adsorption behavior of water and methanol can clearly be described by the model outlined above, anilineÕs effect on the surface state transition eludes the modelÕs predictability. The delayed decrease in SH intensity at a fundamental wavelength of 712 nm during aniline deposition suggests that the adsorbates change some aspect of the surface state resonance other than the minimum energies and transition width. Within the scope of the model presented here, there should be an immediate decrease in SH intensity. A similar delayed decay of surface state resonant SH intensity has been observed for the adsorption of benzene on Cu(1 1 1) [28]. The delay was attributed to the selective adsorption of benzene at step sites since the surface states at step edges are completely unoccupied and thus no change in SH intensity would be observed. However, our results for aniline adsorption show that the initial delayed decay is wavelength-dependent, making the step-adsorption explanation less likely for this system. There are several possible explanations as to why aniline adsorption is not well described by the model set up for physisorption. First, aniline itself is known to produce a SH signal [29]. The SH intensity from aniline itself may compete with the resonant SH decrease brought about by the
quenching of the surface state transition. It is more likely that stronger adsorbate–metal charge transfer in the case of aniline–Ag(1 1 0) leads to a fundamental change in the nature of the surface states––for example in the effective mass. In principle such an adsorbate-induced modification of the surface states, though complex, could be included in the model presented above. 5.3. Surface state resonant SHG as a surface coverage probe The response of surface states to the presence of adsorbates is unique for each adsorbate. If one knows a priori how surface states respond to different atoms or molecules, the measured change can in turn provide information as to the type of adsorbates present on a surface. For example, this fact has become most useful to us in the identification of the existence of impurities in our water sample. As the adsorption of water molecules leads to quenching of the surface state transition, it is expected that the resonant SH signal should be recovered upon desorption of water from the Ag(1 1 0) surface, if the water sample contains no impurity. On the other hand, a contaminated water sample would produce anomalous behavior in the SH response as the contaminant would have a different desorption temperature and may interact with the surface state differently. Surface state resonant SHG has proven to be a highly sensitive tool for measuring submonolayer surface coverage. The resonant signal is many orders of magnitude larger than any non-resonant SH signal and does not require that the adsorbates themselves produce SHG. The quantitative relationship between the SH intensity and the coverage can be rigorously established for physisorbed molecules and serves as a basis for measurement of submonolayer coverage. Significantly, the surface state enhanced SH intensity can be modeled to relate to submonolayer coverage. The SH intensity changes as the occupied surface state energy shifts in response to the presence of adsorbates, thus offering a unique probe of the covered or uncovered surface area. This is different from most other techniques for monitoring the amount of the adsorbate on the surface. This Ônot-coveredÕ sur-
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face area is in some cases a significant, but hard to measure, quantity. Furthermore, as our experiments have shown, different adsorbates affect the surface states in unique ways that can be drastically different depending on the nature of the adsorbate–surface interaction. This observation affords many possibilities from careful studies of molecule–surface interactions to the use of surface states as chemical sensors. Thus, this highly sensitive and chemically discriminative technique could, for example, monitor a surface reaction in situ as the reactants are converted to products which themselves alter the surface states in a different way.
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highly sensitive probe of submonolayer adsorbate coverage. Acknowledgment This work is supported by a grant from the Air Force Office of Scientific Research. S.M.D. acknowledges the receipt of a Graduate Assistance In Areas Of National Need Fellowship from the US Department of Education. The equipment for this research was supported by a grant from the National Science Foundation, MRSEC program, No. DMR00-79909. References
6. Conclusion The effect of adsorption of water, methanol and aniline to second harmonic generation from a Ag(1 1 0) surface, resonantly enhanced by the surface state transition, has been examined. The physisorption of water and methanol appears to induce a straightforward, exponential-like decay of the SH intensity as a function of surface coverage while the weak chemisorption of aniline results in a complex change in the SH intensity. A model based on quantum mechanical description of the nonlinear optical susceptibility in resonance with a surface state transition and empirical treatment of the adsorption as a delocalized effect has been developed to describe the adsorbate coverage dependence in the second harmonic intensity. Fitting to the experimental data is able to extract the surface state energy shifts and transition linewidth broadening as a result of water or methanol adsorption. It has been determined that water and methanol molecules cause an upward shift in the minimum energies of the pair of surface states on Ag(1 1 0) and also cause the transition width to increase. These effects could be understood in terms of adsorbate-induced lateral confinement of the surface state electrons. The model presented here can in principle be applied to any weak adsorption on a metal with a pair of surface states. The surface statesÕ sensitive reaction to the presence of adsorbates provides a basis for SHG resonantly enhanced by surface state transition as a
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