- Email: [email protected]
Electron-molecule scattering and photodynamics at surfaces
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Surface
ELSEVIER
Science 307-309
(1994) 335-343
Invited paper
Electron-molecule
scattering and photodynamics
at surfaces
R.E. Palmer Cacendish Laboratory, UniL:ersiryof Cambridge, Madingley Road, Cambridge CB3 OHE, UK (Received
20 August
1993)
Abstract This article discusses the role of electron-molecule scattering in the photodynamics of adsorbed molecules. In comparison with gas phase photochemistry, adsorption on the surface opens up new channels for light absorption. Specifically, the interaction of electrons photoexcited in the substrate with the adsorbed molecule may lead to molecular dissociation and desorption as well as to chemical reactions between co-adsorbed molecules. As a consequence, the physics of the electron-molecule scattering event is a key element in the interpretation of the photodynamics. These issues are illustrated by a comparison of recent electron stimulated and photon stimulated desorption studies of a model system, physisorbed O/graphite.
1. Introduction
The primary purpose of this article is to demonstrate the connection between the two topics mentioned in the title - “electron-molecule scattering” and “photodynamics” - which at first sight may seem rather unconnected. Surface photodynamics encompasses a variety of molecular processes occurring on the surface as a result of the absorption of light, including molecular desorption and dissociation as well as photochemical reactions [l]. In comparison with gas phase photochemistry, the role of the surface is primarily twofold. First, the surface may organise the adsorbed molecules such that the orientation of the molecular axes is well defined, creating the possibility of photochemical reactions between aligned photooriented molecules - “surface chemistry”. Secondly, the surface may open up new channels for light absorption compared with the case of the isolated molecule. This can hap0039.6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(93)E0803-3
Science
pen in two ways: (i> the modification of the valence electronic structure of the adsorbate may alter the photoabsorption spectrum of the molecule, especially in the case of chemisorption; (ii) light may be absorbed in the substrate itself, leading to the creation of hot electrons (or holes) which may then interact with the adsorbed molecule. This latter possibility is the basis of the connection between surface photodynamics and electron-molecule scattering, and is illustrated schematically in Fig. 1. One important type of electron-molecule interaction is electron attachment. Here an electron, generated, e.g., by photoabsorption in the substrate, attaches to an adsorbed molecule leading to the formation of a molecular negative ion. Such negative ion resonances are now known to play a significant role in a variety of molecular processes at the gas-surface interface [2]. These include vibrational excitation of both intramolecular modes and low frequency molecule-surface
B.V. All rights reserved
336
R. E. Puimrr
/ Surface
Science 307-309
modes [3], as manifest, for example, in high resolution electron energy loss spectroscopy (HREELS) and state-selective molecular beam scattering experiments, together with molecular desorption and dissociation, evident, e.g., in the electron stimuIated desorption (ESDI and photon stimulated desorption (PSD) experiments which form the subject of this review. The intramolecular dynamics driven by electron attachment depend on the propagation of the nuclear wavepacket over the new potential energy surface (PIES) corresponding to the negative ion state which is formed. In this context, vibrational excitation and molecuIar dissociation can be seen as competing decay channels for the negative ion state, as illustrated schematically in Fig. 2. In the case of an adsorbed molecule, the PES is a multi-dimensional surface spanning the molecule-surface co-ordinate system as well as the intramolecular co-ordinates. As a result, desorp-
Fig 1. Schematic illustration of direct and substrate-mediated photochemistry. Light absorption by the molecule leads to an intramolecular electronic transition (a), whereas light absorption in the substrate generates a distribution of electrons with maximum energy E, + hv, which can tunnel, (b), into unoccupied orbitals of the adsorbed molecule.
(19941 335-343
&A-A) Fig. 2. Schematic diagram showing alternative intramolecular decay channels for a molecular negative ion resonance state. (1) Electron capture projects the nuclear wavepacket onto the new PES; (2) the wavepacket propagates over the negative ion PES, followed either (3) by electron emission, which leads to vibrational excitation of the neutral molecule, or (4) by molecular dissociation.
tion of the (intact) molecule from the surface and excitation of the low-frequency molecule-surface modes can also be viewed as competing channels for resonance decay following the propagation of the nuclear wavepacket over the excited state (negative ion) PES. The existence of a new channel for light absorption (i.e. in the substrate) is arguably the most general result of the many recent experiments in the flourishing field of surface photodynamics [I]. To date, these experiments have mainly used lasers or discharge lamps as the light source, restricting studies to the visible and near UV, i.e. hv < 6 eV. This energy happens to be approximately equal to the work function of most surfaces, and therefore has some important consequences. (i) The maximum energy of the photoexcited eIectrons generated in the substrate is given by E, + hv, so that light absorption creates a distribution of hot electrons reaching up in energy to the vacuum level or thereabouts. As a result, the elucidation of the electron-molecule interaction required by the photoexcited electron mechanism is difficult, because the vacuum level is also the lower bound in electron impact experiments, which might otherwise shed useful light on
337
R.E. Palmer /Surface Science 307-309 (19941335-343
this interaction. (ii) If the photoexcited electrons generated in the substrate have energies E < E,,, then their interaction with molecules on the surface, which arises through electron tunnelling into unoccupied levels of the adsorbate, is limited to the first monolayer. On the other hand, if E > E,,, then the photoexcited electrons can propagate for a larger distance through, say, a condensed film of adsorbed molecules, determined by the mean free path of the electrons in the film. These ~ansiderations motivate a study of surface photodynamics in the VUV region of the spectrum, i.e. with hv > 6 eV. In this article we shall review some recent photodissociation experiments [4,5] using the Daresbury synchrotron radiation source, which provides access to easily tunable light in the VUV. For simplicity, a single model system, physisorbed O,/graphite, will be considered, and the photodissociation results will be interpreted with the help of electron impact studies of this same system conducted in the laboratory [6-81. These ESD studies provide new insights into the role of electron-molecule interactions in surface dynamics. A particular feature of the experiments we shall consider is the generation of atomic negative ions, O-, from the condensed 0, molecule. In the gas phase, there are two principal routes to the creation of the O- ion: dissociative attachment (DA), which may be represented by the following scheme, e-+0,+0;
-+o+o-
(1)
ing to the specific optical transitions of the molecule which appear as the thresholds in electron driven DD. The adsorption system on which we shall focus is O,/graphite. This physisorption system has a rich structural phase diagram, allowing the preparation of oriented 0, molecules in various lattice structures as a function of temperature and coverage. At temperatures of around 25 IS there are two distinct monolayer phases - the S phase, in which the molecular axes are (approximately) parallel to the surface, and the 6 phase, the more dense, saturated monolayer phase in which the molecules stand up (approximately) normal to the surface. Resonance EELS [ 111 and NEXAFS [ 121 studies indicate a significant degree of librational motion in both these phases, with mean tilt angles of order 20”. In the multilayer regime, the molecular axes are also oriented approximately normal to the surface [12].
2. Electron stimulated
desorption
Fig. 3 shows the yield of desorbed 0’ ions from a 3 monolayer (ML) film of physisorbed O,/graphite as a function of electron impact energy. The signal shows a threshold at about 20 eV, above which the O+ yield rises monotonically. This behaviour is attributed to the dipolar dissociation mechanism. Fig. 4 shows the yield of O- ions, and in this case much more complex behaviour is observed. In the monolayer phases,
and dipolar dissociation CDT)), e-t- 0, j e-+ 0: + e-+ 0++ O-.
(2)
The former process, DA, is a resonant process, and is an example of electron attachment and subsequent decay of the negative ion state - in the free 0, molecule a well-defined resonance is observed at _ 7 eV 191. The latter, DD, which also generates positive ions, O’, occurs above a minimum threshold energy ( _ 17 eV1, and corresponds to an inelastic electron scattering process resulting in molecular electronic excitation [lo]. Note that while the DA process has no parallel in photoabsorption, photon driven DD can occur in this case resonances are observed, correspond-
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30 .F cn 20 b
.i
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-
Ot: 0
‘lJlr’ 10 15 Electron
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20 25 Energy
30
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(eV)
Fig. 3. Yield of Of ions desorbed from 3 ML O2 /graphite a function of electron impact energy.
as
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I
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.
-
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.
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:
; 400
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(c) 3ML l
~
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400000 200000 0
Electron Energy (eV) Fig. 3. Yield of O- ions desorbed from (a) the 8 phase. (b) the < phase and (c) 3 ML of O?/graphite as a function of electron impact energy. Also shown in (cl are the results obtained when a retarding potential of 2 V is applied to the desorbed ions prior to detection (open circlesf.
Figs. 4a and 4b, the signal is dominated by DD (though in the 6 phase weak resonances are aIso observed at around 7.5 and 13 eV). However, in the case of the 3 ML film, Fig. 4c, the yield of Oions (filled circles) is dominated by a very strong resonance feature at u 7 eV, which is a factor IO”-lo4 more intense than the resonance at 7.5 eV in the i phase. In fact, the signal at 20 eV (where DD dominates) from the 3 ML film is similar to that from the monolayer [ phase, but is completely overshadowed by the 7 eV peak and also by a second, much weaker resonance at 13 cV. A primary question raised by the results of Fig. 4 is why the 7 eV resonance is so strongly sup-
pressed in the monolayer phases. A clue to the answer comes from the signal obtained from the 3 ML film when a retarding potential is applied to the desorbed ions (open circles in Fig. 4c) before they are detected by the mass spectrometer. In this experiment, only ions with more than a certain kinetic energy (here 2 eV) are detected. Fig. 4c shows that the application of this retarding potential extinguishes the 7 eV resonance. whereas the DD signal at 20 eV, though diminished, is preserved. The 13 eV resonance also survives, and a new peak appears at 10 eV. These results must reflect the kinetic energy distributions of the ions created: specifically, the KE distribution from the 7 eV resonance is more weighted to lower kinetic energies than that from DD at 20 eV. This conclusion suggests that the suppression of the 7 eV resonance in the monolayer phases may also arise from the KE distribution of the ions created, since, in order to be detected, these ions have to escape from the image/polarisation potential in the monolayer which has a value of 1.5 eV [13]. In the case of a thicker film, where the outermost layer of molecules is further away from the surface. the value of the pofarisation potential is reduced (to about 0.6 eV), allowing ions of lower kinetic energy to escape. This hypothesis is consistent with results obtained from a monolayer of 0, displaced from the graphite surface by a spacer layer of Ar, Fig. 5. As the thickness of the Ar film is increased, the resonance peak in the OP yield grows strongly, while the DD signal at 20 eV remains roughly constant. Another possible explanation of the suppression of the resonance in the monolayer phases is that the negative ion resonance state is quenched (i.e. its lifetime is reduced) by interaction with the surface. Various studies indicate that, in the case of a physisorbed molecule on a metal surface, the lifetime of the resonance state is reduced by a factor of about 2-3 (or possibIy more [14]), e.g. as a result of the lowering of the centrifugal barrier which traps the incident electron [3]. Since the probability of DA depends exponentially on the lifetime, such a factor might lead to a significant reduction in the probability of ion production. It is difficult to distinguish between these two possi-
R.E. Palmer /Surface
Science 307-309
ble mechanisms of resonance suppression, i.e. ion trapping by the image potential and resonance quenching, since both predict a similar coverage dependence - indeed, both may be contributing at the same time. One interesting possibility would be to search for an alternative signature of DA which did not require ion desorption, e.g. detection of molecular fragments (or the products of subsequent reactions) on the surface with vibrational spectroscopy, as a test of the survival of the DA process in the monolayer regime. The angular distributions of O.- ions created in ESD from O,/graphite are shown in Fig. 6. The most striking feature of these results is that the distributions show a peak normal to the surface irrespective of the initial orientation of the molecule on the surface, i.e. from both the [ phase and the 6 phase (where the molecules lie parallel to the surface!). These results suggest that the inital orientational order of the molecules is lost in the process of dissociation, in contrast with ESD studies of chemisorbed molecules where the angular distribution of ions does reflect the adsorption geometry [15]. In order to explain this behaviour, consider first the influence of the image potential on the angular distributions. As we have already discussed, the image potential presents a barrier to ion desorption, and it favours
(b) 1ML
02/5L
ArlGraphite
Fig. 5. Yield of O- ions desorbed from 1 ML of 0, adsorbed on (a) 1 L and (b) 5 L spacer layers of Ar on graphite.
(1996) 335-343
339
64 Cphase 8eV electrons
D
/
-t
.
UN< phase 20eV electrons
:
20eV electrons
0 10 20 30 40 50 60 70 60 90 Emission Angle 8 (“) Fig. 6. Angular distributions of O- ions desorbed from (a) the [ phase of 0, /graphite, with electron impact energy 8 eV (DA), (b) the [ phase with electron impact energy 20 eV (DD) and (c) the 6 phase with electron impact energy again 20 eV (DD).
desorption by those ions which have a higher component of kinetic energy perpendicular to the surface. Thus, the image potential tends to create an angular distribution with a peak normal to the surface from an angular distribution which otherwise would be isotropic. Therefore, if there is a mechanism for randomising the inital trajectories of the ions, then the image potential can account for the “universal” normal peak observed in Fig. 6. One can imagine at least two mechanisms which might generate the requisite, approximately isotropic, angular distribution: (i> collisions of the ions generated in the adsorbed layer with neighbouring molecules, and (ii) the excitation of high lying rotational states during the
molecular dissociation process. The latter possibility has been explored by Rous and co-workers [6,7], as illustrated by Fig. 7, which shows the angular distributions obtained from a classical molecular dynamics simulation of the O,/graphite system. The calculated distributions show a peak normal to the surface from both the 6 and < phases, and, in fact, the widths of the distributions are in good quantitative agreement with the experimental results (including the reproducibly larger width in the 6 phase distribution compared with the 5 phase). What is happening in the simulations is as follows: unless the molecule is exactly parallel to the surface, dissociation of the molecule tends to drive the lower atom into the repulsive wall of the surface potential, off which it bounces, setting the molecule spinning. Quantum mechanically, this would correspond to the population of high lying rotational states of the molecule on the time scale for molecular dissociation, leading to a randomisation of the trajectories. As the ions move away from the surface, the image potential selects for desorption those ions with sufficient KE normal to the surface to overcome the image barrier, leading to angular distributions of the type which are observed, Fig. 6. In addition to desorption of the fragments generated by molecular dissociation, the ESD experiments also reveal chemical reactions between these dissociation fragments and neighbouring molecules in the physisorbed film [Xl. This feature is illustrated by Fig. 8, which shows the yield of 0, ions desorbed from a 4 ML film of O,/graphite as a function of electron impact energy. The 0, ions are produced by the following ion-molecule reaction: 0
+02+o,.
(e)
I
(3)
The yield of this reaction product shows a welldefined resonance at 13 eV, which can be identified with the 13 eV resonance which produces OP ions from the multilayer, Fig. 4(c). What is striking is that the dominant 7 eV resonance in the O- yield is only present as a comparitively weak shoulder in the 0, signal. It seems that this effect can also be traced to the image/ polarisation potential in the condensed phase.
l
I>,
102030405060706090
Emission Fig. 7. Calculated
angular
Pi-1
Angle to Normal distributions
from (a) the 6 phase of O2 /graphite on the lower
atom
mixture
(b) the i phase
In (c) the electron
in the molecule.
upper atom and in (e) a random
(“1
of Ok- ions desorhrd
via DD.
via DD and (c) - (e) the 6 phase via DA. localised
h
i\
in Cd) on the
of the two
Adopting a “binary collision model”. i.e. assuming that momentum is conserved in reaction (3) above, the kinetic energy of the 0.; ion produced
R. E. Palmer /Surface
Science 307-309
341
(1994) 335-343 r
z.Fn * ’
0
.
.
..
. 5
(1
.
-WI&.-
.= .
-f
“““l”“““““II” 5
10 Electra:
30 E”&
Photon Energy I eV
(Z, from a 4 ML film of impact energy.
Fig. 9. Yield of O- ions photodesorbed from 4 L ( - 2 ML) of 0, /graphite as a function of photon energy in the VUV.
is one third of the KE of the O- ion, allowing estimates to be made of the maximum KE of the 0; ions created via the 7 and 13 eV resonances, respectively [S]. The values so obtained are N 1.25 eV for the 13 eV resonance, which is enough to escape from the polarisation potential of the multilayer, and N 0.75 eV in the case of the 7 eV resonance, which is comparable with the value of the polarisation potential, leading to the suppression of this feature in the yield of desorbed 0; ions. Of course, reaction products which remain on the surface do not manifest themselves in the ESD experiment, and their detection would require analysis of the adsorbed film (e.g. using HREELS) following electron beam irradiation.
resonance feature at 17 eV in the gas phase. Since the molecule is physisorbed, and the molecular electronic structure is therefore only weakly perturbed by adsorption on the surface, such an effect immediately suggests a substrate driven process. This is confirmed by the coverage dependence of the ion yield (hv = 22 eV), shown in Fig. lOa, which shows a sharp rise at low 0, exposures followed by a steady fall - consistent with
Fig. 8. Yield of 0; ions desorbed 0, /graphite as a function of electron
3. Photon stimulated
hu = 22 CV
desorption
The ESD studies discussed in the previous section represent an invaluable resource in interpreting the results of the corresponding PSD experiments. Here we will consider the results of recent investigations [4,5] using the Daresbury synchrotron radiation source to generate tunable UV light in the photon energy range 13-35 eV, where photoelectron driven processes may be anticipated. Fig. 9 shows the yield of O- ions desorbed 4 L O,/graphite (N 2 ML) as a function of photon energy. The results are quite different from the corresponding gas phase photodissociation yield [16], resulting from direct molecular photoabsorption - for example, there is a sharp
b g
. (b) 0’
5
hu = 22 eV
< 5
8 . 09
0
20 Exposure
40
60
IL
Fig. 10. Yield of (a) O- ions and (b) O+ ions photodesorbed from 0, /graphite as a function of 0, coverage with a photon energy of 22 eV.
the attenuation of electrons, photoexcited in the substrate. as they travel through the molecular film towards the outermost surface (from where desorption can occur). With a coverage of - 2 ML, we expect the yield of 0 ions to be dominated by the DA resonance at - 7 eV (Fig. 4~). from which we can predict a threshold for photoelectrot? drillen dissociatir’e attachment at - 11.5 eV ( = 7 eV + 4.5 eV, the work function of graphite). This predicted threshold is in reasonable agreement with the observed threshold, although the latter does seem to be shifted to higher energy, - 14 eV. The shift in the threshold for photoelectron driven DA seems to be traceable to the detailed band structure of the substrate, a subject which has so far received relatively little attention in surface photodynamics. Graphite is a semi-metal with a very low density of states at the Fermi level; in fact, the density of occupied states rises sharply with energy below E,, and shows a (first) maximum 2.5 eV below El,, associated with flat bands at the M point on the edge of the Brillouin zone [17]. The threshold for DA induced by electrons photoexcited from these states occurs at 14 eV (= 11.5 eV + 2.5 eV), in excellent accord with the experimentally observed value. The experimental yield of O- ions also shows a second threshold at - 20 eV, and this too can be associated with a specific feature of the occupied band structure of the substrate, in this case a second maximum in the density of states (the largest), located 7.5 eV below E,. The threshold for DA from these states is 19 eV ( = II .5 eV + 7.5 eV). again in good accord with the experiments. Thus, the experimental PSD results not only expose a new mechanism for surface photodissociation, i.e. photoelectron driven DA, but also highlight the critical role of the band structure of the substrate in substrate-mediated photodynamics at surfaces. The importance of the band structure of the substrate is also illustrated by the yield of 0’ ions from O,/graphite as a function of photon energy, Fig. 11. Again, the data bear little relation to the corresponding gas phase photodissociation measurements, except for the shoulder observed in Fig. 11 at - 22 eV, which may be associated with a resonance in the free molecule
15
IO
3)
Photon Fig. of 0,
Il.
Yield of 0
/graphite
‘?
ions photodesorbed
its a function
70
35
Energ> I eV from 4 L ( - 2 ML)
of photon energy in the VLR’.
at this energy [16]. However. even at this energy. Fig. lob, the coverage dependence of the ion yield suggests that a substrate mediated process is predominant. The onset of photuelectron drir,en dipolar dissociation occurs at a photon energy of - 21.5 eV ( = 17 eV + 4.5 eV, the graphite work function). This assumes the gas phase threshold for electron driven DD at 17 eV: however. in the case of the adsorbed film, this threshold may be lowered by the value of the image/polarisation potential. - 1 eV in the second layer of O,/graphite [ 131. A threshold at - 20.5 eV is consistent with the results of Fig. 11. A further significant feature of the O- yield. Fig. 11. is the existence of resonance features at photon energies of - 24.5 and 28.5 eV. Such features are at first rather surprising, since electron driven DD is not a resonant process. The existence of resonances in the corresponding photoelectron driven process again appears to reflect the detailed substrate band structure and, in this instance, the unoccupied states as well as the occupied states. Specifically, it seems that the resonances observed in Fig. 11 can be associated with particular interband transitions [4] at critical points in the Brillouin zone - such final .rtate effects are well known in spectroscopic studies of graphite, where the high lying, unoccupied band structure shows especially strong deviations from free electron behaviour [18]. Once more the PSD studies caution against an overly simplistic view of the substrate as an electron source for hot or photoelectron driven processes, characterised
R.E. Palmer /Surface
Science 307-309
only by the location of the Fermi level. The band structure effects observed in photoemission must also be expected in photodesorption.
(1994) 335-343
343
5. References 111X.-L. Zhou, X.-Y. Zhu and J.M. White, Surf. Sci. Rep. 13 (1991) 73.
121R.E. Palmer, Prog. Surf. Sci. 41 (19921 51. 4. Summary In this article we have considered the relationship between electron-molecule scattering and photodynamics at surfaces, as illustrated by ESD and PSD studies of a model system, O,/graphite. The connection between these two areas arises from the photoexcitation of electrons in the substrate, leading to photon driven processes which, as far as the resulting electron-molecule interaction is concerned, exactly match the corresponding electron impact processes, dissociative attachment and dipolar dissociation. In the case of PSD such processes distinguish themselves from direct, molecular photoabsorption by the wavelength dependence of the signal, which reflects the details of the photoabsorption in the substrate (band structure effects) and of the electron-molecule impact processes (both resonant and threshold processes), which are exposed by the ESD studies. These considerations illustrate the importance of PSD studies in the VUV (i.e. hv > 6 eV), which allow direct comparison with ESD investigations using electrons of the same energies as those generated by photoexcitation of the substrate. Finally, the experiments reviewed here also demonstrate the possibility of doing “tunable chemistry with low energy electrons”, a concept which might have interesting applications in the area of surface processing.
[31 R.E. Palmer and P.J. Rous, Rev. Mod. Phys. 64 (19921 383. R.G. Sharpe, R.J. Guest, J.C. Barnard, 141 R.A. Bennett, R.E. Palmer and M.A. MacDonald. Chem. Phys. Lett. 198 (1992) 241. [51 R.A. Bennett, S.L. Bennett, L. Siller. M.A. MacDonald, R.E. Palmer, H.M. Wright and J.S. Foord, submitted. [61 R.J. Guest, L.A. Silva, R.E. Palmer, D.N. Bly, D.M. Hartley and P.J. Rous, submitted. [71 R.J. Guest, I.M. Goldby, R.E. Palmer, D.N. Bly, D.M. Hartley and P.J. Rous, Faraday Discuss. No. 96, in press. [81 I.M. Goldby, R.J. Guest and R.E. Palmer. Chem. Phys. Lett. 206 (19931 181. [91 G.J. Schulz, Rev. Mod. Phys. 45 (1973) 423. Uni[lOI H.S.W. Massey, Negative Ions, 3rd ed. (Cambridge versity Press, Cambridge, 1976). [ill E.T. Jensen, R.E. Palmer and P.J. Rous, Phys. Rev. Lett. 64 (19901 1301; Chem. Phys. Lett. 169 (19901 204; Surf. Sci. 237 (1990) 153. 1121R.J. Guest, A. Nilsson, 0. Bjorneholm, B. Hernnas, A. Sandell, R.E. Palmer and N. Mirtensson, Surf. Sci. 269/270 (1992) 432. R.J. [131 A. Nilsson, R.E. Palmer, H. Tillborg, B. Hernnas, Guest and N. Mirtensson, Phys. Rev. Lett. 68 (19921982; H. Tillborg, A. Nilsson, B. Hernnas, N. M&tensson and R.E. Palmer, Surf. Sci. 295 (1993) 1. M. Bertolo and K. Jacobi, Surf. Sci. 253 [141 W. Hansen, (1991) 1. Ml R.D. Ramsier and J.T. Yates, Jr., Surf. Sci. Rep. 12 (19911 243. I161 H. Oertel, H. Schenk and H. Baumgartel, Chem. Phys. 46 (19801 251. R. Saito and H. Kamimura, J. Phys. Sot. 1171 C. Fretigny, Jpn. 58 (1989) 2098. [I81 I.R. Collins, P.T. Andrews and A.R. Law, Phys. Rev. B. 38 (1988) 13348; I. Schafer, M. Schliiter and M. Skibowski, Phys. Rev. B. 35 (19881 7663.
.:,:‘:,:; :~~$~,~.;’ ::.;:.i.i::;..;::::,;: ::
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,...::
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,.,...’ :l..‘,‘, .... ~,,,.,. ::.,. :O. .:.,, “‘l:.‘I.’....,‘.‘.“..:. .. ,..:. .:.:.: “‘:‘::::::.:,::::i::,i::i:.:ii:ii:’:iiB’:ir :.:: ~,::::::.
Surface
ELSEVIER
Science 307-309
(1994) 335-343
Invited paper
Electron-molecule
scattering and photodynamics
at surfaces
R.E. Palmer Cacendish Laboratory, UniL:ersiryof Cambridge, Madingley Road, Cambridge CB3 OHE, UK (Received
20 August
1993)
Abstract This article discusses the role of electron-molecule scattering in the photodynamics of adsorbed molecules. In comparison with gas phase photochemistry, adsorption on the surface opens up new channels for light absorption. Specifically, the interaction of electrons photoexcited in the substrate with the adsorbed molecule may lead to molecular dissociation and desorption as well as to chemical reactions between co-adsorbed molecules. As a consequence, the physics of the electron-molecule scattering event is a key element in the interpretation of the photodynamics. These issues are illustrated by a comparison of recent electron stimulated and photon stimulated desorption studies of a model system, physisorbed O/graphite.
1. Introduction
The primary purpose of this article is to demonstrate the connection between the two topics mentioned in the title - “electron-molecule scattering” and “photodynamics” - which at first sight may seem rather unconnected. Surface photodynamics encompasses a variety of molecular processes occurring on the surface as a result of the absorption of light, including molecular desorption and dissociation as well as photochemical reactions [l]. In comparison with gas phase photochemistry, the role of the surface is primarily twofold. First, the surface may organise the adsorbed molecules such that the orientation of the molecular axes is well defined, creating the possibility of photochemical reactions between aligned photooriented molecules - “surface chemistry”. Secondly, the surface may open up new channels for light absorption compared with the case of the isolated molecule. This can hap0039.6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(93)E0803-3
Science
pen in two ways: (i> the modification of the valence electronic structure of the adsorbate may alter the photoabsorption spectrum of the molecule, especially in the case of chemisorption; (ii) light may be absorbed in the substrate itself, leading to the creation of hot electrons (or holes) which may then interact with the adsorbed molecule. This latter possibility is the basis of the connection between surface photodynamics and electron-molecule scattering, and is illustrated schematically in Fig. 1. One important type of electron-molecule interaction is electron attachment. Here an electron, generated, e.g., by photoabsorption in the substrate, attaches to an adsorbed molecule leading to the formation of a molecular negative ion. Such negative ion resonances are now known to play a significant role in a variety of molecular processes at the gas-surface interface [2]. These include vibrational excitation of both intramolecular modes and low frequency molecule-surface
B.V. All rights reserved
336
R. E. Puimrr
/ Surface
Science 307-309
modes [3], as manifest, for example, in high resolution electron energy loss spectroscopy (HREELS) and state-selective molecular beam scattering experiments, together with molecular desorption and dissociation, evident, e.g., in the electron stimuIated desorption (ESDI and photon stimulated desorption (PSD) experiments which form the subject of this review. The intramolecular dynamics driven by electron attachment depend on the propagation of the nuclear wavepacket over the new potential energy surface (PIES) corresponding to the negative ion state which is formed. In this context, vibrational excitation and molecuIar dissociation can be seen as competing decay channels for the negative ion state, as illustrated schematically in Fig. 2. In the case of an adsorbed molecule, the PES is a multi-dimensional surface spanning the molecule-surface co-ordinate system as well as the intramolecular co-ordinates. As a result, desorp-
Fig 1. Schematic illustration of direct and substrate-mediated photochemistry. Light absorption by the molecule leads to an intramolecular electronic transition (a), whereas light absorption in the substrate generates a distribution of electrons with maximum energy E, + hv, which can tunnel, (b), into unoccupied orbitals of the adsorbed molecule.
(19941 335-343
&A-A) Fig. 2. Schematic diagram showing alternative intramolecular decay channels for a molecular negative ion resonance state. (1) Electron capture projects the nuclear wavepacket onto the new PES; (2) the wavepacket propagates over the negative ion PES, followed either (3) by electron emission, which leads to vibrational excitation of the neutral molecule, or (4) by molecular dissociation.
tion of the (intact) molecule from the surface and excitation of the low-frequency molecule-surface modes can also be viewed as competing channels for resonance decay following the propagation of the nuclear wavepacket over the excited state (negative ion) PES. The existence of a new channel for light absorption (i.e. in the substrate) is arguably the most general result of the many recent experiments in the flourishing field of surface photodynamics [I]. To date, these experiments have mainly used lasers or discharge lamps as the light source, restricting studies to the visible and near UV, i.e. hv < 6 eV. This energy happens to be approximately equal to the work function of most surfaces, and therefore has some important consequences. (i) The maximum energy of the photoexcited eIectrons generated in the substrate is given by E, + hv, so that light absorption creates a distribution of hot electrons reaching up in energy to the vacuum level or thereabouts. As a result, the elucidation of the electron-molecule interaction required by the photoexcited electron mechanism is difficult, because the vacuum level is also the lower bound in electron impact experiments, which might otherwise shed useful light on
337
R.E. Palmer /Surface Science 307-309 (19941335-343
this interaction. (ii) If the photoexcited electrons generated in the substrate have energies E < E,,, then their interaction with molecules on the surface, which arises through electron tunnelling into unoccupied levels of the adsorbate, is limited to the first monolayer. On the other hand, if E > E,,, then the photoexcited electrons can propagate for a larger distance through, say, a condensed film of adsorbed molecules, determined by the mean free path of the electrons in the film. These ~ansiderations motivate a study of surface photodynamics in the VUV region of the spectrum, i.e. with hv > 6 eV. In this article we shall review some recent photodissociation experiments [4,5] using the Daresbury synchrotron radiation source, which provides access to easily tunable light in the VUV. For simplicity, a single model system, physisorbed O,/graphite, will be considered, and the photodissociation results will be interpreted with the help of electron impact studies of this same system conducted in the laboratory [6-81. These ESD studies provide new insights into the role of electron-molecule interactions in surface dynamics. A particular feature of the experiments we shall consider is the generation of atomic negative ions, O-, from the condensed 0, molecule. In the gas phase, there are two principal routes to the creation of the O- ion: dissociative attachment (DA), which may be represented by the following scheme, e-+0,+0;
-+o+o-
(1)
ing to the specific optical transitions of the molecule which appear as the thresholds in electron driven DD. The adsorption system on which we shall focus is O,/graphite. This physisorption system has a rich structural phase diagram, allowing the preparation of oriented 0, molecules in various lattice structures as a function of temperature and coverage. At temperatures of around 25 IS there are two distinct monolayer phases - the S phase, in which the molecular axes are (approximately) parallel to the surface, and the 6 phase, the more dense, saturated monolayer phase in which the molecules stand up (approximately) normal to the surface. Resonance EELS [ 111 and NEXAFS [ 121 studies indicate a significant degree of librational motion in both these phases, with mean tilt angles of order 20”. In the multilayer regime, the molecular axes are also oriented approximately normal to the surface [12].
2. Electron stimulated
desorption
Fig. 3 shows the yield of desorbed 0’ ions from a 3 monolayer (ML) film of physisorbed O,/graphite as a function of electron impact energy. The signal shows a threshold at about 20 eV, above which the O+ yield rises monotonically. This behaviour is attributed to the dipolar dissociation mechanism. Fig. 4 shows the yield of O- ions, and in this case much more complex behaviour is observed. In the monolayer phases,
and dipolar dissociation CDT)), e-t- 0, j e-+ 0: + e-+ 0++ O-.
(2)
The former process, DA, is a resonant process, and is an example of electron attachment and subsequent decay of the negative ion state - in the free 0, molecule a well-defined resonance is observed at _ 7 eV 191. The latter, DD, which also generates positive ions, O’, occurs above a minimum threshold energy ( _ 17 eV1, and corresponds to an inelastic electron scattering process resulting in molecular electronic excitation [lo]. Note that while the DA process has no parallel in photoabsorption, photon driven DD can occur in this case resonances are observed, correspond-
G I:
\
40
aC
30 .F cn 20 b
.i
10
-
Ot: 0
‘lJlr’ 10 15 Electron
5
20 25 Energy
30
35
40
(eV)
Fig. 3. Yield of Of ions desorbed from 3 ML O2 /graphite a function of electron impact energy.
as
33x
I
‘,
.
-
1.
‘.
.
(a) IML 6 phase
;
800
I =
600
2 .s m
(b) 1ML ;
:
; 400
a?
800000 600000
1
(c) 3ML l
~
0 lm
400000 200000 0
Electron Energy (eV) Fig. 3. Yield of O- ions desorbed from (a) the 8 phase. (b) the < phase and (c) 3 ML of O?/graphite as a function of electron impact energy. Also shown in (cl are the results obtained when a retarding potential of 2 V is applied to the desorbed ions prior to detection (open circlesf.
Figs. 4a and 4b, the signal is dominated by DD (though in the 6 phase weak resonances are aIso observed at around 7.5 and 13 eV). However, in the case of the 3 ML film, Fig. 4c, the yield of Oions (filled circles) is dominated by a very strong resonance feature at u 7 eV, which is a factor IO”-lo4 more intense than the resonance at 7.5 eV in the i phase. In fact, the signal at 20 eV (where DD dominates) from the 3 ML film is similar to that from the monolayer [ phase, but is completely overshadowed by the 7 eV peak and also by a second, much weaker resonance at 13 cV. A primary question raised by the results of Fig. 4 is why the 7 eV resonance is so strongly sup-
pressed in the monolayer phases. A clue to the answer comes from the signal obtained from the 3 ML film when a retarding potential is applied to the desorbed ions (open circles in Fig. 4c) before they are detected by the mass spectrometer. In this experiment, only ions with more than a certain kinetic energy (here 2 eV) are detected. Fig. 4c shows that the application of this retarding potential extinguishes the 7 eV resonance. whereas the DD signal at 20 eV, though diminished, is preserved. The 13 eV resonance also survives, and a new peak appears at 10 eV. These results must reflect the kinetic energy distributions of the ions created: specifically, the KE distribution from the 7 eV resonance is more weighted to lower kinetic energies than that from DD at 20 eV. This conclusion suggests that the suppression of the 7 eV resonance in the monolayer phases may also arise from the KE distribution of the ions created, since, in order to be detected, these ions have to escape from the image/polarisation potential in the monolayer which has a value of 1.5 eV [13]. In the case of a thicker film, where the outermost layer of molecules is further away from the surface. the value of the pofarisation potential is reduced (to about 0.6 eV), allowing ions of lower kinetic energy to escape. This hypothesis is consistent with results obtained from a monolayer of 0, displaced from the graphite surface by a spacer layer of Ar, Fig. 5. As the thickness of the Ar film is increased, the resonance peak in the OP yield grows strongly, while the DD signal at 20 eV remains roughly constant. Another possible explanation of the suppression of the resonance in the monolayer phases is that the negative ion resonance state is quenched (i.e. its lifetime is reduced) by interaction with the surface. Various studies indicate that, in the case of a physisorbed molecule on a metal surface, the lifetime of the resonance state is reduced by a factor of about 2-3 (or possibIy more [14]), e.g. as a result of the lowering of the centrifugal barrier which traps the incident electron [3]. Since the probability of DA depends exponentially on the lifetime, such a factor might lead to a significant reduction in the probability of ion production. It is difficult to distinguish between these two possi-
R.E. Palmer /Surface
Science 307-309
ble mechanisms of resonance suppression, i.e. ion trapping by the image potential and resonance quenching, since both predict a similar coverage dependence - indeed, both may be contributing at the same time. One interesting possibility would be to search for an alternative signature of DA which did not require ion desorption, e.g. detection of molecular fragments (or the products of subsequent reactions) on the surface with vibrational spectroscopy, as a test of the survival of the DA process in the monolayer regime. The angular distributions of O.- ions created in ESD from O,/graphite are shown in Fig. 6. The most striking feature of these results is that the distributions show a peak normal to the surface irrespective of the initial orientation of the molecule on the surface, i.e. from both the [ phase and the 6 phase (where the molecules lie parallel to the surface!). These results suggest that the inital orientational order of the molecules is lost in the process of dissociation, in contrast with ESD studies of chemisorbed molecules where the angular distribution of ions does reflect the adsorption geometry [15]. In order to explain this behaviour, consider first the influence of the image potential on the angular distributions. As we have already discussed, the image potential presents a barrier to ion desorption, and it favours
(b) 1ML
02/5L
ArlGraphite
Fig. 5. Yield of O- ions desorbed from 1 ML of 0, adsorbed on (a) 1 L and (b) 5 L spacer layers of Ar on graphite.
(1996) 335-343
339
64 Cphase 8eV electrons
D
/
-t
.
UN< phase 20eV electrons
:
20eV electrons
0 10 20 30 40 50 60 70 60 90 Emission Angle 8 (“) Fig. 6. Angular distributions of O- ions desorbed from (a) the [ phase of 0, /graphite, with electron impact energy 8 eV (DA), (b) the [ phase with electron impact energy 20 eV (DD) and (c) the 6 phase with electron impact energy again 20 eV (DD).
desorption by those ions which have a higher component of kinetic energy perpendicular to the surface. Thus, the image potential tends to create an angular distribution with a peak normal to the surface from an angular distribution which otherwise would be isotropic. Therefore, if there is a mechanism for randomising the inital trajectories of the ions, then the image potential can account for the “universal” normal peak observed in Fig. 6. One can imagine at least two mechanisms which might generate the requisite, approximately isotropic, angular distribution: (i> collisions of the ions generated in the adsorbed layer with neighbouring molecules, and (ii) the excitation of high lying rotational states during the
molecular dissociation process. The latter possibility has been explored by Rous and co-workers [6,7], as illustrated by Fig. 7, which shows the angular distributions obtained from a classical molecular dynamics simulation of the O,/graphite system. The calculated distributions show a peak normal to the surface from both the 6 and < phases, and, in fact, the widths of the distributions are in good quantitative agreement with the experimental results (including the reproducibly larger width in the 6 phase distribution compared with the 5 phase). What is happening in the simulations is as follows: unless the molecule is exactly parallel to the surface, dissociation of the molecule tends to drive the lower atom into the repulsive wall of the surface potential, off which it bounces, setting the molecule spinning. Quantum mechanically, this would correspond to the population of high lying rotational states of the molecule on the time scale for molecular dissociation, leading to a randomisation of the trajectories. As the ions move away from the surface, the image potential selects for desorption those ions with sufficient KE normal to the surface to overcome the image barrier, leading to angular distributions of the type which are observed, Fig. 6. In addition to desorption of the fragments generated by molecular dissociation, the ESD experiments also reveal chemical reactions between these dissociation fragments and neighbouring molecules in the physisorbed film [Xl. This feature is illustrated by Fig. 8, which shows the yield of 0, ions desorbed from a 4 ML film of O,/graphite as a function of electron impact energy. The 0, ions are produced by the following ion-molecule reaction: 0
+02+o,.
(e)
I
(3)
The yield of this reaction product shows a welldefined resonance at 13 eV, which can be identified with the 13 eV resonance which produces OP ions from the multilayer, Fig. 4(c). What is striking is that the dominant 7 eV resonance in the O- yield is only present as a comparitively weak shoulder in the 0, signal. It seems that this effect can also be traced to the image/ polarisation potential in the condensed phase.
l
I>,
102030405060706090
Emission Fig. 7. Calculated
angular
Pi-1
Angle to Normal distributions
from (a) the 6 phase of O2 /graphite on the lower
atom
mixture
(b) the i phase
In (c) the electron
in the molecule.
upper atom and in (e) a random
(“1
of Ok- ions desorhrd
via DD.
via DD and (c) - (e) the 6 phase via DA. localised
h
i\
in Cd) on the
of the two
Adopting a “binary collision model”. i.e. assuming that momentum is conserved in reaction (3) above, the kinetic energy of the 0.; ion produced
R. E. Palmer /Surface
Science 307-309
341
(1994) 335-343 r
z.Fn * ’
0
.
.
..
. 5
(1
.
-WI&.-
.= .
-f
“““l”“““““II” 5
10 Electra:
30 E”&
Photon Energy I eV
(Z, from a 4 ML film of impact energy.
Fig. 9. Yield of O- ions photodesorbed from 4 L ( - 2 ML) of 0, /graphite as a function of photon energy in the VUV.
is one third of the KE of the O- ion, allowing estimates to be made of the maximum KE of the 0; ions created via the 7 and 13 eV resonances, respectively [S]. The values so obtained are N 1.25 eV for the 13 eV resonance, which is enough to escape from the polarisation potential of the multilayer, and N 0.75 eV in the case of the 7 eV resonance, which is comparable with the value of the polarisation potential, leading to the suppression of this feature in the yield of desorbed 0; ions. Of course, reaction products which remain on the surface do not manifest themselves in the ESD experiment, and their detection would require analysis of the adsorbed film (e.g. using HREELS) following electron beam irradiation.
resonance feature at 17 eV in the gas phase. Since the molecule is physisorbed, and the molecular electronic structure is therefore only weakly perturbed by adsorption on the surface, such an effect immediately suggests a substrate driven process. This is confirmed by the coverage dependence of the ion yield (hv = 22 eV), shown in Fig. lOa, which shows a sharp rise at low 0, exposures followed by a steady fall - consistent with
Fig. 8. Yield of 0; ions desorbed 0, /graphite as a function of electron
3. Photon stimulated
hu = 22 CV
desorption
The ESD studies discussed in the previous section represent an invaluable resource in interpreting the results of the corresponding PSD experiments. Here we will consider the results of recent investigations [4,5] using the Daresbury synchrotron radiation source to generate tunable UV light in the photon energy range 13-35 eV, where photoelectron driven processes may be anticipated. Fig. 9 shows the yield of O- ions desorbed 4 L O,/graphite (N 2 ML) as a function of photon energy. The results are quite different from the corresponding gas phase photodissociation yield [16], resulting from direct molecular photoabsorption - for example, there is a sharp
b g
. (b) 0’
5
hu = 22 eV
< 5
8 . 09
0
20 Exposure
40
60
IL
Fig. 10. Yield of (a) O- ions and (b) O+ ions photodesorbed from 0, /graphite as a function of 0, coverage with a photon energy of 22 eV.
the attenuation of electrons, photoexcited in the substrate. as they travel through the molecular film towards the outermost surface (from where desorption can occur). With a coverage of - 2 ML, we expect the yield of 0 ions to be dominated by the DA resonance at - 7 eV (Fig. 4~). from which we can predict a threshold for photoelectrot? drillen dissociatir’e attachment at - 11.5 eV ( = 7 eV + 4.5 eV, the work function of graphite). This predicted threshold is in reasonable agreement with the observed threshold, although the latter does seem to be shifted to higher energy, - 14 eV. The shift in the threshold for photoelectron driven DA seems to be traceable to the detailed band structure of the substrate, a subject which has so far received relatively little attention in surface photodynamics. Graphite is a semi-metal with a very low density of states at the Fermi level; in fact, the density of occupied states rises sharply with energy below E,, and shows a (first) maximum 2.5 eV below El,, associated with flat bands at the M point on the edge of the Brillouin zone [17]. The threshold for DA induced by electrons photoexcited from these states occurs at 14 eV (= 11.5 eV + 2.5 eV), in excellent accord with the experimentally observed value. The experimental yield of O- ions also shows a second threshold at - 20 eV, and this too can be associated with a specific feature of the occupied band structure of the substrate, in this case a second maximum in the density of states (the largest), located 7.5 eV below E,. The threshold for DA from these states is 19 eV ( = II .5 eV + 7.5 eV). again in good accord with the experiments. Thus, the experimental PSD results not only expose a new mechanism for surface photodissociation, i.e. photoelectron driven DA, but also highlight the critical role of the band structure of the substrate in substrate-mediated photodynamics at surfaces. The importance of the band structure of the substrate is also illustrated by the yield of 0’ ions from O,/graphite as a function of photon energy, Fig. 11. Again, the data bear little relation to the corresponding gas phase photodissociation measurements, except for the shoulder observed in Fig. 11 at - 22 eV, which may be associated with a resonance in the free molecule
15
IO
3)
Photon Fig. of 0,
Il.
Yield of 0
/graphite
‘?
ions photodesorbed
its a function
70
35
Energ> I eV from 4 L ( - 2 ML)
of photon energy in the VLR’.
at this energy [16]. However. even at this energy. Fig. lob, the coverage dependence of the ion yield suggests that a substrate mediated process is predominant. The onset of photuelectron drir,en dipolar dissociation occurs at a photon energy of - 21.5 eV ( = 17 eV + 4.5 eV, the graphite work function). This assumes the gas phase threshold for electron driven DD at 17 eV: however. in the case of the adsorbed film, this threshold may be lowered by the value of the image/polarisation potential. - 1 eV in the second layer of O,/graphite [ 131. A threshold at - 20.5 eV is consistent with the results of Fig. 11. A further significant feature of the O- yield. Fig. 11. is the existence of resonance features at photon energies of - 24.5 and 28.5 eV. Such features are at first rather surprising, since electron driven DD is not a resonant process. The existence of resonances in the corresponding photoelectron driven process again appears to reflect the detailed substrate band structure and, in this instance, the unoccupied states as well as the occupied states. Specifically, it seems that the resonances observed in Fig. 11 can be associated with particular interband transitions [4] at critical points in the Brillouin zone - such final .rtate effects are well known in spectroscopic studies of graphite, where the high lying, unoccupied band structure shows especially strong deviations from free electron behaviour [18]. Once more the PSD studies caution against an overly simplistic view of the substrate as an electron source for hot or photoelectron driven processes, characterised
R.E. Palmer /Surface
Science 307-309
only by the location of the Fermi level. The band structure effects observed in photoemission must also be expected in photodesorption.
(1994) 335-343
343
5. References 111X.-L. Zhou, X.-Y. Zhu and J.M. White, Surf. Sci. Rep. 13 (1991) 73.
121R.E. Palmer, Prog. Surf. Sci. 41 (19921 51. 4. Summary In this article we have considered the relationship between electron-molecule scattering and photodynamics at surfaces, as illustrated by ESD and PSD studies of a model system, O,/graphite. The connection between these two areas arises from the photoexcitation of electrons in the substrate, leading to photon driven processes which, as far as the resulting electron-molecule interaction is concerned, exactly match the corresponding electron impact processes, dissociative attachment and dipolar dissociation. In the case of PSD such processes distinguish themselves from direct, molecular photoabsorption by the wavelength dependence of the signal, which reflects the details of the photoabsorption in the substrate (band structure effects) and of the electron-molecule impact processes (both resonant and threshold processes), which are exposed by the ESD studies. These considerations illustrate the importance of PSD studies in the VUV (i.e. hv > 6 eV), which allow direct comparison with ESD investigations using electrons of the same energies as those generated by photoexcitation of the substrate. Finally, the experiments reviewed here also demonstrate the possibility of doing “tunable chemistry with low energy electrons”, a concept which might have interesting applications in the area of surface processing.
[31 R.E. Palmer and P.J. Rous, Rev. Mod. Phys. 64 (19921 383. R.G. Sharpe, R.J. Guest, J.C. Barnard, 141 R.A. Bennett, R.E. Palmer and M.A. MacDonald. Chem. Phys. Lett. 198 (1992) 241. [51 R.A. Bennett, S.L. Bennett, L. Siller. M.A. MacDonald, R.E. Palmer, H.M. Wright and J.S. Foord, submitted. [61 R.J. Guest, L.A. Silva, R.E. Palmer, D.N. Bly, D.M. Hartley and P.J. Rous, submitted. [71 R.J. Guest, I.M. Goldby, R.E. Palmer, D.N. Bly, D.M. Hartley and P.J. Rous, Faraday Discuss. No. 96, in press. [81 I.M. Goldby, R.J. Guest and R.E. Palmer. Chem. Phys. Lett. 206 (19931 181. [91 G.J. Schulz, Rev. Mod. Phys. 45 (1973) 423. Uni[lOI H.S.W. Massey, Negative Ions, 3rd ed. (Cambridge versity Press, Cambridge, 1976). [ill E.T. Jensen, R.E. Palmer and P.J. Rous, Phys. Rev. Lett. 64 (19901 1301; Chem. Phys. Lett. 169 (19901 204; Surf. Sci. 237 (1990) 153. 1121R.J. Guest, A. Nilsson, 0. Bjorneholm, B. Hernnas, A. Sandell, R.E. Palmer and N. Mirtensson, Surf. Sci. 269/270 (1992) 432. R.J. [131 A. Nilsson, R.E. Palmer, H. Tillborg, B. Hernnas, Guest and N. Mirtensson, Phys. Rev. Lett. 68 (19921982; H. Tillborg, A. Nilsson, B. Hernnas, N. M&tensson and R.E. Palmer, Surf. Sci. 295 (1993) 1. M. Bertolo and K. Jacobi, Surf. Sci. 253 [141 W. Hansen, (1991) 1. Ml R.D. Ramsier and J.T. Yates, Jr., Surf. Sci. Rep. 12 (19911 243. I161 H. Oertel, H. Schenk and H. Baumgartel, Chem. Phys. 46 (19801 251. R. Saito and H. Kamimura, J. Phys. Sot. 1171 C. Fretigny, Jpn. 58 (1989) 2098. [I81 I.R. Collins, P.T. Andrews and A.R. Law, Phys. Rev. B. 38 (1988) 13348; I. Schafer, M. Schliiter and M. Skibowski, Phys. Rev. B. 35 (19881 7663.