Conference Summary

Journal of Electron Spectroscopy and Related Phenomena, 39 (1986) 343-352 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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CONFERENCE SUMMARY A.C. Luntz IBM Research Laboratory, San Jose, CA 95193, U.S.A.

ABSTRACT This paper attempts to summarise some of the key developments in vibrational spectroscopy that were present at this conference. This is accomplished principally by comparing to the corresponding status at the last conference in the series. It is by nature a subjective appraisal. I NTRODUCTI ON It is a frightening responsibility and virtually impossible task to try to summarize all of the new results, techniques, issues and trends presented at this conference. Since my own attention to the field of vibrations at surfaces dates only from the last conference in this series (VAS 3)1, the best I can hope to do in this summary is to compare impressions of this conference with the one held nearly- three years ago, and to emphasize those areas which are either totally new or w~ere substantial progress has occurred. Because of limitations in time, background and personal interest, even this objective represents only a subjective and probably opinionated appraisal. One overriding impression of the previous VAS 3 conference was that of the two major techniques for vibrational spectroscopy, EELS and IRRARS, because of its extended spectral range, EELS was clearly the method of choice for structural studies of adsorbates, e.g. chemical identity of surface species, binding site, orientation, etc. It was thus the favourite tool for those trying to draw analogies between UHV single crystal studies and catalytic chemistry. On the other hand, because of the higher spectral resolution, IRASS was the favourite tool for studying lateral interactions, frequency shifts, mode dispersions, steps, lineshapes, etc. While this trend has largely continued in VAS 4, there were indications that this clean separation in application is beginning to blur. Principally due to the extensions of the

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spectral range in IRRAS using FTIR spectrometers (Chabal, Chesters), IRRAS is likely to playa much more substantial role in the future in structural studies. In addition, the new dimensions added to impact scattering in EELS (Lehwald) suggest that this technique is likely to encroach on the former preserve of IRRAS, e.g. lateral interactions, dispersions, dynamics, etc. There has clearly been steady evolution and progress in applied vibrational spectroscopy, i.e. using vibrational spectroscopy as just one of the bag of tricks to solve interesting problems in surface chemistry and physics. Although such studies were largely presented in the poster sessions at this conference, it is also evident that this is the main draw of vibrational spectroscopy in surface science and the chief reason that most of the participants are here. Since it is impossible to chronicle all of the contributions of vibrational spectroscopy to all areas of surface science, I will restrict comments to developments in the experimental techniques and to new areas that have opened up or developed rapidly since the last conference. TECHNICAL DEVELOPMENTS Table I presents an updated version of the comparative techniques table originally presented by Ibach in his summary to VAS 3. 2 Those areas where substantial new capability exists since that time or techniques which are entirely new are in bold print. One of the most significant technical developments, as exemplified by the talks of Chesters and Chabal, is that the spectral range, resolution and sensitivity of IRRAS have been greatly extended using FTIR spectrometers. As of the conference date, the spectral range now extends down to 700 cm- I at a resolution of 1-2 cm- l and with a sensitivity comparable to or better than EELS. This feat has been accomplished with standard commercial FTIR spectrometers, but with careful attention and understanding of the tradeoffs necessary to optimize SiN for the surface IRRAS experiment. In the future, inclusion of longer wavelength detectors in these spectrometers should extend the useful range even further. The results presented here demonstrate their advantages over specular EELS, and in large part foreshadow the demise of specular EELS for many chemical applications. EELS instrumentation has proven largely sufficient for most studies and has been relatively static, except for the development

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of the dispersion compensated spectrometer (Dubois). The nearly 100X greater sensitivity of this instrument allows modest time resolved spectra to be obtained, but this has not yet achieved resolution comparable to conventional designs. Tunneling spectroscopy has virtually disappeared as a major focus, but mention should be made of several current attempts to observe vibrationally inelastic vacuum tunneling in the scanning tunneling microscope. 3 If this is ultimately successful, vibrational spectroscopy of a single atom on the surface could one day be possible, and this would radically change the future of the field. Unenhanced Raman spectroscopy was first reported at VAS 3 almost as a curiosity.4 The results presented by Campion at this conference indicate that it has developed into a full fledged complement to IRRAS with a variety of applications. At present, sensitivity of the technique is 10-1000 times less than the best IRRAS, but obvious instrumental developments in the next few years should overcome this limitation. Although not presented at this conference, mention should be made of the recent IR emission results using a cold spectrometer. 5 Although not as general as FTIR, this technique is capable of measuring linewidths for the low frequency metal-adsorbate modes, and this is an issue of considerable importance for understanding the dynamics of molecules on surfaces. Unfortunately, because the technique depends on IR emission at thermodynamic equilibrium, the range of useful surface temperatures is somewhat limited. One possible solution is to use a similar cooled spectrometer for conventional IRRAS. Calculations indicate that sufficient sensitivity down to 200 cm- I should be achievable. The use of lasers as sources of IRRAS has been reported for the first time at this conference, both using cw diode lasers (Bermudez) and continuously tunable pulsed IR lasers (Hoffmann). Of the two approaches, the continuously tunable laser is likely to prove much more useful. The advantages of using a tunable pulsed laser source are high spectral resolution, an extended spectral range, and most importantly the possibility of doing time resolved spectroscopy with nanosecond time resolution. However, since continuously tunable IR lasers are not commercially available, a large investment in laser engineering is unavoidable. In addition such lasers are inherently noisy making it difficulty to achieve

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sufficient sensitivity for surface lRRAS. At present, sensitivity is adequate for CO or other strong absorbers, but an order of magnitude more sensitivity is required for general use. Because such sources may ultimately provide high temporal resolution in vibrational spectroscopy, serious efforts continue to try to enhance sensitivity using these laser based sources. 6 TABLE 1 Summary of techniques in surface vibration spectroscopy Sensitivity in Monolayers

Surface Area

Spectral Regime

Resolution

>1000 cm- l

5cm- l

<1000 cm- I

variable finely dispersed material

cm 2

>700 cm- I (today)

10-1_10-4

10-l cm2

>100 em-I

enhanced Raman

variable

10-2cm-2 >100 cm- I

tunnel spectroscopy

10-2

10-2cm2

>200 cm- 1

He-scatteri ng

10-17

10-lcm2

500 cm- I (today)

1-2 em-I ambient pressure I >30 cm- vacuum impact scattering 5 cm- 1 special systems & surface conditions 5-20 em-I specially prepared samples 2 cm- I current ly used for Rayleigh-phonons

1O-4em2

>400 cm- I

10 em-I

spatially resolved vacuum, limited Ts range

lR-transmission

many

neutron scattering

many

IR-reflection absorption

1-10-4

electron energy loss

unenhanced Raman 1_10-2

m2

IR emission

1_10-1

1 em 2

>300 em-I

2 cm- l

laser IR

1-10-2

IQ-Ici

>450 em-I


finely dispersed material

nanosecond

time

resolved

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NEW AREAS IN VIBRATIONAL SPECTROSCOPY I.

EELS impact scattering Certainly one of the most impressive developments since VAS 3 is in the application of impact scattering. At VAS 3, only impact scattering involVing hydrogen atom motions had been observed. Now, by increasing the electron energy, impact scattering cross sections have been increased to allow measurements of dispersion curves for surface phonons, surface resonances, and parallel modes of heavy adsorbates. With the advances in IRRAS presented at this conference, impact scattering may become the main emphasis of EELS studies in the future. A.

Phonon spectra At VAS 3, surface phonon studies were restricted to molecular beam experiments on Rayleigh mode3 of nonreactive crystals such as LiF or Au.? As discussed by Lehwald, electron impact scattering is a much more general technique for phonon studies than molecular beam scattering, albeit at much lower energy resolution. This does not mean that inelastic He scattering is finished, as the experiments of Sibener illustrate. Emphasis on low frequency phonon modes (rare gas crystals, soft modes in phase transitions) or complicated surface Brillouin zones (where many phonon modes exist) should guarantee the importance of the molecular beam studies. Particularly impressive were the phonon dispersion curves for adsorbates on Ni(IOO) observed by electron impact scattering (Lehwald), especially the observation of Rayleigh mode softening for c(2x2) 0 on Ni(IOO), implying the buildup of stresses prior to adsorbate induced reconstructions. In fact, the whole theoretical emphasis on surface reconstructions and relaxations and its connection to phonon spectra was a major new theme for this conference. However, a much voiced concern was that lattice dynamics interpretations of the phonon spectra based on central force, and especially nearest neighbour central force models, may be too simplistic, especially for covalent adsorbates. (This concern is amplified since surface relaxations or linear derivative terms of the pair potentials are not usually included in the calculations and the dynamical matrix lacks rotational invariance). Thus, the connection between the observed phonon spectra and a unique surface force field still appears weak.

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B.

Dispersion of adsorbate modes With impact scattering, the dispersion of both perpendicular and parallel polarized modes is now measurable. Observation of both modes prOVides considerably more insight into both lateral interactions and the structure of the adsorbate. The hopefully final resolution of the bond length controversy in c(2x2) 0 on Ni(IOO) has provided this author with a valuable lesson. 8 At least half of the frequency shift in the speCUlar EELS spectra for the Ni-O mode relative to that in the p(2x2) overlayer has now been ascribed to a symmetry allowed mixing of the p(2x2) mode with substrate Phonons 9, while the rest is ascribed to complicated changes in the force constants in the c(2x2) case due to incipient adsorbate induced reconstruction. I O It thus seems prudent when trying to assign or interpret low frequency modes, to refrain until lattice dynamics calculations are included in the analysis, and even then to be extremely cautious given the present state of knowledge of surface force fields and their mode of incorporation in these calculations. C.

Hindered librational and parallel translational modes for molecular adsorbates These have not yet been observed, but this author enters a special plea to attempt such measurements. Knowledge of these modes and their dispersion is critical in understanding the dynamic processes at surfaces. As Landman pointed out, in thermally activated surface processes (diffusion, desorption and dissociation), it is just these modes which provide a 'doorway' to couple energy from the substrate into the molecule. Further, it is Just these same modes which have been implicated in dephasing processes for line broadening of molecular adsorbates. l l 2.

Time resolved spectroscopy The beginnings of time resolved vibrational spectroscopy appeared at this conference with both dispersion compensated EELS (Dubois, Ellis) and fast FTIR (Chesters). At the present, time resolution of apprOXimately 10 msec is achieved, and it is not yet clear whether faster time scales will be possible with these techniques in the future. One unsettled issue, however, is what time resolution is reqUired for totally new kinds of experiments. In the results presented here, adsorption-desorption kinetics can equally as well be studied by molecular beam techniques, and the studies of

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desorption limited species can also be studied with conventional EELS at lower surface temperatures. It is anticipated that to observe true chemical transients, species that are unstable at any surface temperature, will need less than microsecond time resolution and will probably require the laser based techniques. It is probable, however, that the limited time resolution available today will prove fruitful for measuring kinetic parameters of stable adsorbed species or in kinetic studies of phas2 transitions. 3.

Electrochemical IRRAS and SERS Although only represented by the talk of Bewick, it should be emphasised that electrochemical IRRAS has grown enormously in the past three years and is beginning to have a major impact in electrochemistry. The principal reason for this is that it is the only spectroscopic technique which can measure the concentration of the majority species on the electrode and this allows correlations with electrochemical behaviour. SERS, since it monitors minority species on the electrode, has not been particularly successful in correlating with the electrochemistry. In a large part, this accounts for the diminished interest in SERS. In addition, although some controversy still remains on the mechanisms of enhancement, the shear weight of evidence now favours some participation of the charge transfer mechanism (Otto). 4.

Vibrational lineshapes At VAS 3, the prevailing sentiment seemed to be that because vibrational bands were broad (5-10 cm~l) on metal surfaces, this implied a short vibrational lifetime. Because damping of high frequency vibrations into surface phonons is unlikely to account for this lifetime, this implied damping into electronhole pairs. Since that time, it has been realized that this is a naive view of lineshape theory as practiced in other fields, and other line broadening mechanisms have been suggested. 11 At this conference, a whole variety of lineshape mechanisms have been demonstrated. Dephasing by both phonons (Gadzuk, Persson) and electron-hole pairs (Morawitz) has been discussed theoretically, and demonstrated experimentally for bridge bonded CO on Ni(III) (Persson) and H(2xl) on Si(IOO) (Chabal). Inhomogeneous contributions arising from order-disorder phenomena were observed

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for CO on Ru(IOO) (Hoffmann). Electron-hole pair damping was unequivocally demonstrated by Chabal for the wag overtone of H on W(IOO) by the observation of a Fano resonance lineshape originally predicted by Langreth. 12 Electron-hole pair damping has also been invoked, in a less convincing manner, to explain the linewidths observed for CO on Cu{IOO) (Ryberg). Ultimately, picosecond time domain experiments like those presented by Cavanagh will be necessary to sort out the various dissipative and dephasing contributions in the general case, and in second generation experiments to probe specifics of energy transfer pathways. 5.

Other dynamics The connections of vibrational spectroscopy to dynamic processes of molecules at surfaces has often been noted previously. A particularly cogent account of this relationship was presented by Gadzuk in his paper relating to trajectories in dynamic processes to various lineshape determining factors. The importance of charge transfer, i.e. the formation of negative ion resonances, has been noted for some time now, and its relation to resonance EELS scattering and the formation of vibrational excitation in molecule surface scattering has been emphasized (Gadzuk, Karikorpi, Newns). The elegant molecular beam experiments of HZ on Cu(IOO} (Andersson) have demonstrated the importance of rotational resonances, zone boundary phonons and parallel momentum changes in sticking, and pose many challenging theoretical issues (Holloway). In dynamics, this interplay between molecular beam experiments and vibrational spectroscopy is likely to be a recurring theme in future. 6.

Grimley'S critical issues revisited In his summary for VAS 3, Grimley discussed several critical issues in theory as it relates to vibrational spectroscopy.13 There seems to have been little progress in addressing these critical issues in the past three years, and the most that can be done is to restate them and to reemphasize some aspects in view of the results presented at this conference. A. What is the role of chemical bond formation in energy dissipation to electrons at Surfaces? At present, most theories treat a single adsorbate level interacting with jellium. One extension, stimulated by Grimley'S

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summary at VAS 3, considers the role of symmetry in the adsorbate orbitals. 14 However, this general question is still particularly relevant in trying to understand the Fano resonance and electronhole pair coupling presented by Chabal since it was observed for H on W(IOO) and not for H on jellium. B. Are cluster models adequate for the ab-initio calculation of adsorption geometries and vibration frequencies? With present day computing power, it is still impossible to do high quality quantum chemistry calculations (all electron, extensive Cl) for large or even medium size clusters. Use of pseudopotentials, limited Cl or SCF wavefunctions and limiting cluster size do make the calculations tractable, but the effects of these individual approximations on calculated properties is not well understood. Although these limited cluster calculations certainly enhance our understanding of bonding at metal surfaces and the qualitative reasons for the frequency shifts in vibrational frequencies upon surface bonding, there are still few studies with quantitative accuracy. One attempt to address this size-quality issue is a study of CO on Cu clusters. 15 This suggests that rather small clusters, CU 5 or CU g, yield reasonable frequencies for intermolecular modes (CO stretCh), but that even with Cu 50, the binding energy of Cu-CO stretch frequency is not well represented. This may imply other approximations, e.g. pseudopotentials, used in the calculations are not adequate. This is a particularly pathological case for clusters since only weak chemisorption occurs and charge transfer involving both the localized metal d bands and the delocalized valence band occurs. C. Can we imbed clusters? The attempts to calculate vibrational frequencies in the presence of an external electric field (Muller, Hermann), an issue of some importance in the electrochemical lRRAS, point out the need for such an approach. Cluster calculations are necessary to get a proper description of chemistry, but since the clusters do not have the continuum of electron-hole pair states, screening is not properly described and thus the calculated response to an external field is suspect. It is a pleasure to end this attempted summary by acknowledging the contributions of Tom Grimley to this conference. His introductory paper on mechanical renormalization, angular distributions in desorption and electron-hole pair damping has, as

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always, clarified theoretical issues and commonly accepted approximations. REFERENCES Most references are to papers presented at this conference and are labelled by the name of the first author in the text. Since they are included in this volume, they are not specifically listed here. [I] [2] [3] [4] [5] [6] [7]

[8] [9] [10] [II] [12] [13] [14] [15]

C.R. Brundle and H. Morawitz, eds., Vibrations at Surfaces, Elsevier, Amsterdam, (1983). H. Ibach, ref.l, vo1.30. p.237. G. Binnig, private communication. A. Campion, ref.l. vo. 29, p. 397. S. Chiang, R.G. Tobin. P.L. Richards and P.A. Thiel, Phys. Rev. Lett. g 648 (1984). D. Bethune and A. Luntz, to be published. G. Benedex, ref I. vol. 30, p. 71. M. Cates and D.R. Miller, ref. I. vol. 30, p. 157. T.S. Rahman, J.E. Black and D.L. Mills, Phys. Rev. Lett. ~ 1469 (1981). S. Andersson, P.A. Karlsson and M. Persson, Phys. Rev. Lett. ~ 2378 (1983). J.M. Szeftel, S. Lehwald, H. Ibach, T.S. Rahman, J.E. Black and D.L. Mills, Phys. Rev. Lett. ~ 268 (1983). J.W. Gadzuk and A.C. Luntz, Surf. Sci. 144429 (1984). D.C. Langreth, Phys. Rev. Lett. ~ 126 (1985). T. Grimley, ref. I., vol. 30, p.229. K. Shonhammer and O. Gunnarsson, Phys. Rev. B27 5113 (1983). W. Muller and P.S. Bagus, to be published.