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Negative ion resonances and surface vibrations
Journal ofElectron Spectrompy and Related Phenomena, 64/M (1993) 3949 036%2043/93/$06.00 0 1993- Elsevier Science Publkhem B.V. All rights reserved
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Negative ion resonances and surface vibrations R.E. Palmer Department of Physics, University of Cambridge, Cavendish Laboratory, Madingley Road, Cambridge C33 OH&UK This paper reviews some recent developments in high resolution &&on energy loss spectroscopy @IREELS) studies of adsorbed molecules. The principal theme is the negative ion resonance scattering mechanism and its usefulness in the investigation of surface vibrational states. Several examples are considered: first, the excitation of low frequency molecule-surface modes, including overtones, as illustrated by the case of physisorbed 02/pt(lll); secondly, the relationship between the CJresonauc6 energy and the bond length in HRKELS studies of chemisorbed molecules, and illustrated by the examples of chemisorb8d Ofl/pt(lll) and chemisorbed CO; finally, the utility of the resonauce scattering mechanism in unravelling the behaviour of a complex adsorption system, coadsorbed K and 02 on graphite.
1. INTRODUCTION The negative ion r8sonauce scattering mechanism is now well established in high resolution ekctron energy loss spectroscopy (HREELS) studies of adsorbed molecules Ill. Resonance scattering is a special and distinct example of short-range impact scattering, in which the scattered electron is temporarily trapped in a specific antibonding orbital of the target molecule, leading to large enhancements of the vibrational excitation cross-sections. Such resonances are well known in gas phase electron-molecule scattering experiments, and the manner in which the basic characteristics of the resonance state are modified on the surface has been the subject of considerable research. A relatively rec8nt review of the progress made in this area is given in ref. 1. The principal theme which we shall address in this paper is the usefulness of %esonance EELS” in the investigation of surface vibrations, and we shall illustrate
this theme by reviewing some recent experimental studies conducted in Cambridge. For the most part, experimental studies of resonance electron scattering by adsorbed molecules have been concerned with the excitation of relatively high frequency intramolecular vibrational Imd88. However, eSpeCidy in the ud8xt Of surface dynamics, the low frequency frustrated modes which occur on the surface are at least of 8qual interest. In section 2 we explore the excitation, via the formation of a negative ion resonance, of low frequency molecule-surface modes in the physisorbed 0 q/Pt(lll) system. Of particular importance here is the excitation of vibrational overtones in addition to the fundamental mode. 02 can also be chemisorbed on the Pt(ll1) surface, enabling us to explore, in section 3, the fundamental characteri&ics of the resonance state in (strongly) chemisorbed molecules. While it is true that negative ion resonances have now been identified in HREELS studies of a wide variety of both
physisorbed and chemisorbed molecules, our understanding of the nature of the resonance state in the former case is more advanced than in the latter (11, where the influence of the molecule-substrate interaction is expected to be more pronounced. In the case of chemisorbed 02A?t(lll) we find experimental evidence for a qualitative correlation between the bond length of the chemisorbed molecular species and the energy of the *a resonance”. In section 4 we shall illustrate the usefulness of this correlation, and, in more general terms, of an understanding of the resonance scattering mechanism, via a discussion of a rather complex spectroscopic system, co-adsorbed alkali metal (K> and 02 on graphite. In all these diBcusBionB,we shall be concerned with a specific negative ion resonance state, the “0 resonance* state of the 02 molecule. The Q resonance is a
Beam Energy = 11 eV
180
190
200
210
220
Energy Loss (meV)
Fig. 1. Detailed lineshape of the v = 0 - 1 excitation in the HREELS spectrum of a physisorbed monolayer of @/Pt(lll) at 30 K, A fitted curve using five Gaussians is also shown.
single particle shape resonance, created by captureof an electroninto the 2pau* (i.e. the 30~) orbital of the 02 molecule, This resonance has also been identified in HREELS studies of a variety of other adsorbed molecule, e.g. CO [21, NO 131 and N2 [41, in addition to 02 CM.The Q state appears at higher incident electron energies (between =7 and 20 ev) than the low lying n resonances in these molecules, which makes the cr resonance easily accessible in a standard HREELS experiment. The resonance is related to the u resonance commonly observed in X-ray absorption studies of small adsorbed molecules [61, where a multiple scattering picture of the resonance has been developed [71, a picture we shall find helpful later. 2. RESONANT EXCITATION OF LOW FREQUENCYMODESINHREELS Fig. 1 shows the detailed lineshapeof the v = 0 - 1 stretch mode in the HREELS spectrum of physisorbed 02 on Will) at a temperatureof ~30 K WI. The line is clearly asymmetric in shape, and shows fine structure which can be described by a deconvolution of the experimental curve into a series of five Gauesians. The main line is marked with an arrow in the figure and has a frequency of 196 f 1 meV, corresponding to the pure v = 0 - 1 stretch mode of the physisorbed 02 molecule. The three loss peaks appearing at highor energies (200 2 1, 206 f 1 and 210 f 1 meV) are assigned to the simultaneous excitation of the pure v = 0 - 1 mode and a Berieaof low frequency modes (t = 0 - 1, 2...), corresponding to the molecule-surface vibration and its overtones. The firat observation of such a series of coupled low frequency modea in HREELS was reported by Jacobi and Bertolo in 1990 [91.Here we explore the mechanism of excitation of these modes.
41
0.12 P 3
I
I
I
I
I
1
1
1
10 6 8 Beam energy (ev)
12
I
0.1 -
+ s 0.08 u P I 0.06 g $0.04 5 _Eo.O2 0
I 2
4
0.1
I
g
? 0.08
0
I
I
I
I
I
I
14
I
W -
1 2
I
I
4
10 6 8 Beam energy (eV)
12
14
4
6 8 10 Beam energy (ev)
12
14
I
P g 0.05
a =0.04 P $ 0.03 I e! po.02
.
g 0.01
01 2.
Fig. 2. Intensities of the coupled low modes in the v = 0 - 1 line profile of physisorbed O@t(lll) as a function of electron impact energy. fbquency
Fig. 2 shows the intensities cd the individual loss peaks extracted from the deconvolution as a function of electron impact 8nergy, while fig. 3 shows the mode intensities as a function of detection angle. The most importantpoint t0 not43i8 that tb8 energyand angular dependence of the coupled modes (e.g. v = 0 - 1, t = 0 - 1) track that of the pure intramolecularstretch mode (v = 0 - 1). The energy and angular dependence of the pure v = 0 - 1 mode is consistent with negative ion resonance scatteringvia the *&- eingle particle shape resonance of molecular 02, also known a8 the “a ehape resonance”, observed in the gas phase at 19.6 eV, and shifted down in energy on the surface to =7 eV by the image potential of the metallic substrate. The angular dependence of the mode intensity arises from the ps/f8 symmetryof this resonancestate 1101,which lead8 t0 a minimum in the i&x&y normal to the surface, i.e. perpendicular to the molecular axis of the physisorbed molecule which is approximately parallel to the surface 1111.The results of figs. 2 and 3 demonstratethat the coupled low frequency mode8 are excited by the same negative ion resonance mechanism. Such low frequency mode8 play an important role in surface dynamical processes, such as surface diffusion and desorption. They also represent a window onto the potential energy ,surface (PES) describing the molecule-surface interaction.Since the calculationof the full, multi-dimensional molecule-surface PES is a very demanding task, experimental insight8 into the nature of this eurface are particularly useful 1121.With the kind of resolution now available in HREELS, -1 meV, it is interesting to speculate whether one could use resonance scattering to determine the anharmonicityof the PES from the frequenciesof the overtones of the molecule-surface modes. Even with a resolution of 1 meV, this ta8k appears
42
.
: a)
I
I
Beam energy-7eV
,l,,,I,.,I,
0
2
.I,,,
,111,
4
6
8
10
12
Beamenergy (eV) Fig. 4. Intensity of the v = 0 - 1 vibrational excitation (frequency 106 meV) of chemisorbed Os/Pt(lll) at 90 K as a function of incident electron energy. insurmountable at present in the case of physisorbed molecules, but in the case of chemisorbed molecules, where the frequency of the molecrde-surface vibration for example, can be much higher (40 mev), the prospect is appealing.
50
3. BOND LENGTHS WITEARULEB EIRJZKIS?
0 -5
5
15
25 35 45 55 Detection Angle (“)
65
75
Fig. 3. Intensities of the coupled low frequency in the v = 0 - 1 line profile of physisorbed 02/Pt(lll) as a function of detection angle. The electron beam energy is 7 eV.
IN
Fig. 4 shows the intensity of the intramolecular stretch mode (frequency 106 meV) of chemisorbed molecular 02/pt(lll), produced by adsorption at -90 K, as a function of electron impact energy C131. The origin of the sharp peak in the crosssection at =l eV is uncertain, but it may arise from a threshold resonance 1141. The broad resonance feature centred at 3.75 f 0.25 eV is assigned to the Q shape resonance of the chemisorbed molecule, an assignment which is supported by the close correlatibn’between the angular distribution of fig. 5, for the chemisorbed molecule, and
43
the angular distributions of fig. 3, for the physisorbed molecule (the chemisorbed molecule also lies approximatelyparallel to the surface[ill). The assignment of the resonance observedat 3.75 eV to the u shape resonance of the chemisorbed 02 molecule implies a shift in resonance energy of -3.25 eV between the physisorbed and chemisorbed state, or, in other words, a shift in energy of 5.76 eV between the gas phase molecule and the chemisorbed state. There are several factors which we need to considerin order to understand the origin of this shift in energy. (i) The resonance energy shift in the physisorbedmolecule(with respect to the gas phase) can be assigned to the (attractive) image potential (more properly, the exchange-correlationpotential); in the case of the chemisorbsd molecule, a similar shift is expected. However, the chemisorbed molecule is so close to the surface (and hence the image plane) that the familiar, asymptotic image potential form (-l/42) of
2ot
-
-20
E= 4.4 eV
0
20 40 Angle(W)
60
80
Fig. 5. Intensity of the v = 0 - 1 vibrational excitation of chemisorbed 02/Pt(lll) as a function of detection angle (labelled with respect to the surface normal). The electron beam energy is 4.4 eV.
the exchange-correlation potential is not valid. Instead, the value of the exchangecorrelationpotential is probably close to the *saturated” value, i.e. the “inner potential’ of the Pt bulk, -6.6 eV M. (ii) The chemisorbed02 molecule on pt(lll) is thought to bear a net negative charge as a result of charge “back donation” from the substrate till. This will give rise to a repulsive interaction between the adsorbed molecule and the incoming electron - this interaction is described by the exchangecorrelation potential of the adsorbed molecule, which incorporates the uon-site Coulomb potential”. This repulsive interaction will increase the resonance energy,thus opposing the effect of the image potential. (iii) Studies of the G shape resonance in X-ray absorption spectroscopy have suggested an empirical correlation between the shape resonance energy and the bond length of the (free or adsorbed) molecule /6,161.Indeed, it has been proposed that this correlation allows the determination of “bond lengths with a ruler”, i.e. the extraction of the bond length from the resonance energy, to which it is related in an approximately linear fashion. The question of whether a similar correlation might be established in HREELS is intriguing. To explore further the last effect, i.e. the relation between Q resonance energy and bond length in HREELS,we have calculated the v E 0 - 1 vibrational excitation crosssection for the free 02 molecule as a functionof the bond length. The calculation employs the Xa treatment of the exchangecorrelation potential and gives good quantitative agreement with the experimental gas phase vibrational excitationcross-sectionsDOI.Fig. 6 is a plot of the energy of the calculated peak in the v = 0 - 1 cross-section as a function of the bond length of the free molecule. It can be
seen that the resonance energy decreases, in almost linear fashion, as the bond length is increased. Using the bond length of chemisorbed 02/Pt(lll) obtained from NEXAFS WI, 1.37 f 0.5 A,the calculation predicts a a resonance energy of 3.5 f 0.5 eV in a free 02 molecule with this bond length. This value is close to the experimental resonance energy of the chemisorbed 02 molecule on the Pt(ll1) surface, 3.76 It 0.5 eV. The question then arises “is correspondence merely a this coincidence?“. Fig. 7 shows a compilation D71 of the experimental 0 resonance energies of a sequence of molecules containing the C-O functional group as a function of the C-O bond length, determined independently. The resonance energies are obtained from HREELS studies of gas phase CO 1181 and (CH3)2O [191, and chemisorbed CO/Ni(llO) [ZOI and HCOOMi(110) 1213.The C-O bond lengths in the free molecules are taken from tables [223. The value for CO/Ni(llO) comes from LEED E231.No bond length is available for HCOO/Ni(llO), so we use the value for HCOO/Cu(llO) 1241, which should be
similar. This value is actually, obtained from the cr resonance energy - bond length correlation proposed in NEXAFS, but it is also consistent with photoelectron diffraction results [251. The solid line in fig. 7 is the result of an Xu calculation for the free CO molecule, similar to that described above for 02. What we see in the figure is a striking level of agreement between the calculated curve and the experimental points - which, even disregarding the calculated theory curve, appear to lie approximately on a straight line. Taken together with the results for 02, this correlation seems to indicate that the change of the bond length is the principal cause of the shift in the Q resonance energy of the chemisorbed molecule. One further topic needs discussion in connection with the results presented in fig. 7, and concerns those molecules in which there are not one, but two, C-O bonds, i.e. the
30 L
1
I
I
I
1.1
1.4
C-O dd2,d tt&h
1.2
1.25 1.3 1.35 O-O bond length (A)
1.4
1.45
Fig. 6. Q resonance energy versus bond length from Xa calculations for the free 02 molecule.
I
(A)
1.5
Fig. 7. Compilation of measured 0 resonance energies and bond lengths for gas and chemisorbed molecules phase containing the C-O functional group. For sources see text. The solid line is the Q resonance energy for free CO as a function of the bond length calculated using the Xa scheme.
free (CH3)2O molecule and the chemisorbed HCOO species on Ni(ll0). In principle, one would expect a splitting of the Q resonance into two components as a consequence of the interaction between the two C-O bonds 1261, which would certainly complicate the (I resonance energy - bond length correlation. Since no such splitting has yet been observed, we have plotted the resonance energies which have been measured, but note this point as a topic for further investigation. If the apparent correlation between Q resonance energy and bond length in chemisorbed molecules is to hold (at an approximate level), then the other two factors which might be expected to influence the resonance energy, i.e. the image potential and the on-site Coulomb repulsion, must, approximately, cancel each other out. Evidently, a detailed theoretical investigation of these effects would be most desirable, but in the meantime we can consider a (qualitative) plausibility argument which suggests that the net effect of these two opposing terms may indeed be small - leaving the effect of the bond length to dominate. First, we note that the separation between the exchangecorrelation potential of the substrate and the exchange-correlation potential of the adsorbate is rather artificial. One should really consider the overall electron density of the system, since in the chemisorption regime (i.e. close to the surface) the local density approximation (LDA) to the exchange-correlation potential seems to work rather well 1271(this is not true of the phyeisorption regime - the image potential is a non-local effect). The plausibility argument is then based on the ideas embodied in effective medium theory, developed by Norskov and others in the early 1980’s and applied to chemisorbed atoms 1281.A central pillar of this scheme is the notion of a Yavoured” electron density -
independent of the substrate - in which the adsorbed atom likes to sit, leading to a minimum in the adsorption energy. This determines the position of the chemisorption minimum outside the surface, and is basically a treatment of the sp electron density of the substrate. The substrate d band is then treated by a hybridisation term. It is the former term which is of interest here. Suppose that the 0 atom favours an electron density which is close to the density provided by the other 0 atom in the 02 molecule. As the molecule approaches the substrate, each 0 atom feels the electron density of the substrate and compensates for this increased density by moving further away from the other 0 atom in the molecule. In this scenario, it is plausible that the charge density in the vicinity of the chemisorbed molecule is not going to be so different from that of the free molecule. On the basis of the LDA, the exchangecorrelation potential experienced by the probe electron will then be similar to that of the free molecule, so that the resonance energy difference between the gas phase molecule and the chemisorbed molecule may be dominated by the bond length change (an effect which arises from the multiple scattering, Oparticle in a box” picture of the 0 resonance [71). Of course, as already stated, these are speculations of a qualitative nature, but they are motivated by the need to explain empirical results. These considerations also suggest that the approximate correlation between resonance energy and bond length in HREELS may be quite a general feature. This is clearly a notion that can only be established or disproved by further experimental studies of the cr resonances of chemisorbed molecules. Is the apparent correlation between resonance energy and bond length in HREELS of any use? People certainly use the resonance energies measured in NEXAFS to derive bond lengths in
46
chemisorbed species, though to the best of our knowledge these results have yet to be crosschecked by other, and more established, structural techniques, e.g. LEED. We would certainly be most reluctant to treat bond lengths derived from HREELS resonance energies as quantitative structural parameters, since the cancellation of the other factors influencing the bond length can surely only be approximate - though the trends obtained may be useful qualitative indicators. However, there is another use for the correlation, which seems to us important and which is nicely illustrated by another specific example, the coadsorption of K and 02 on graphite, as discussed in the following section.
. . . . l
. .
@I **
K/gr+lLCJ
1
K/gr+lLq
1
4. SEARCHING FOR CHEMICAL SPECIES lT+JFigs. 8 and 9 show a selection of HREELS spectra obtained when the “7x7” phase of K&a hite (where the K-K separation is -16 1 ) is exposed to 02 at 25 K [291. Fig. 8(a), obtained with an electron beam energy of 9 eV and in the specular scattering direction, shows a single discrete loss feature at -350 meV - corresponding to the low frequency surface plasmon of the graphite substrate to which the adsorbed alkali atoms donate electronic charge [303 superimposed on an intense background of electron-hole pair excitations arising from the detailed Fermi surface characteristics of semimetallic graphite [311. When the surface is exposed to 1 L 02 the surface plasma frequency is reduced to ~150 meV, fig. 8(b). This suggests that some of the charge donated by the adsorbed alkali atoms to the substrate is withdrawn from the substrate when 02 is co-adsorbed on the surface. The question which then arises is “what is the nature of the co-adsorbed 02 species causing this behaviour”?
1(c)
l
**
I
-0.2
:
‘:
d<
‘. 8
. ‘-/ .. ;* . . . . .. . : . t. f.
0
I
I
I
0.4 0.6 0.2 Energy Loss (ev)
1
0.8
Fig. 8. Electron energy loss spectra from coadsorbed K and 02 on graphite at 30 K, obtained with an incident electron energy of 9 eV. (a) spectrum from K/graphite with specular scattering geometry (inset), showing the K/graphite surface plasmon. (b) as (a) following exposure to 1 L 02. (cl spectrum obtained from K+O2/graphite with off specular scattering geometry (inset), showing vibrational energy loss peaks characteristic of physisorbed 02.
47
.
.
.
i
280 380 f #* IIIIIIIIII!tIl,,,,l,,I,I,,,,
-0.1
0.0
0.1 0.2 0.3 0.4 Energy Loss (eV)
0.5
Fig. 9. Electron energy loss spectrum from co-adsorbed K and 02 (1.5 L) on graphite at 30 K, obtained with an incident electron energy of 4.0 eV and off specular scattering geometry (inset). The loss peak at 190 meV corresponds to physisorbed 02, while the peak appearing at ==140meV is indicative of a more strongly bound 02 species. Overtones of both these vibrational excitations are also seen at 280 meV and 380 meV. No loss peaks which could be associated with an 02 species are observable in the specular HREELS spectrum of fig. 8(b). To “find” the coadsorbed species we utilise our understanding of the resonance scattering cross-sections, as described previously. First, fig. 8(c), we look well away from the specular direction, to “sieve out” the loss features excited by dipole scattering (including both the surface plasmon and the electron-hole pair continuum), but at the same beam energy, 9 eV, since the cr shape resonance of the physisorbed 02 molecule
should be accessible at this energy (resonance peak energy 9.5 eV in the gas phase, 8 eV on clean graphite [al). What we are now able to see is the v = 0 - 1 vibration of physisorbed 02 at =196 meV, together with thev=O-Zovertone(andaweakv=O-3 overtone), from which we are able to conclude that even at this low 02 exposure physisorbed 02 is already present on the surface. However, the existence of physisorbed 02 cannot account for the apparent withdrawal from the substrate of the electronic charge donated by the alkali. If the alkali charge is donated to a coadsorbed 02 species instead of the substrate, we might expect to find some negatively charged peroxo or superoxo type species in the HREELS spectrum. Now the ground state of the gas phase 02- species has a bond length of 1.34 A (and a vibrational frequency in the range of -132 - 140 meV) [323. Using this bond length as a guide, the resonance energy - bond length calculation of fig. 6 predicts a o resonance energy of x4.4 eV. We therefore take an HREELS spectrum at this beam energy, again going well away from specular to remove the dipole excitations. The spectrum so obtained, fig. 9, clearly shows a new loss peak, at =14ClmeV, in addition to the peak at 190 meV observed with an impact energy of 9 eV and attributed to physisorbed 02. The first overtone of the new loss feature is also visible at ~280 meV. The 140 meV peak is assigned to a molecular 02 species akin to 02-, in harmony with cluster calculations for this co-adsorption system, which predict a molecular 02 species lying parallel to the surface on top of an adsorbed alkali atom. We believe that the physisorbed 02 molecules are adsorbed on the bare surface between these stable “KO2”moieties.
48 The analysis of this complex adsorption system illustrates in a convincing fashion the usefulness of an understanding of the resonance scattering mechanism - and, specifically, of the approximate resonance energy - bond length correlation - in the investigation of chemical species on surfaces. It also illustrates the usefulness of resonance scattering in maximising the sensitivity of the HREELS technique. This latter point is further confirmed by a detailed study of the resonance scattering cross-section for the chemisorbed 02 species in the K+Oggraphite system, shown in fig. 10. This compares the energy dependence of the v = 0 - 1 intensities for the chemisorbed 02 species (frequency 140 meV) and the physisorbed molecule (frequency 190 meV). The resonance observed in the chemisorbed molecule, centred at -4 eV with a full-width half-maximum of only about 3 eV, leads, 6
, ,
I, I I I,
AE = 140meV 0
r”““l”““““““‘I’
2
4
6 8 10 12 Electron Energy(eV)
like the resonance in the physisorbed molecule, to a considerable enhancement of the vibrational mode intensity.
6. CONCLUSIONS Negative ion resonances play an important role in a variety of dynamical processes occurring at the gas-surface interface 1121.However, the issues discussed in this paper - excitation of low frequency modes and their overtones, the relationship between resonance energy and bond length in chemisorbed molecules, and the application of resonance scattering to the analysis of complex systems - have been chosen to illustrate the utility of the negative ion resonance scattering mechanism in practical vibrational spectroscopy at surfaces; specifically, in maximising the vibrational excitation cross-sections for both intramolecular and molecule-surface modes in both physisorbed and chemisorbed molecules. This capability complements the use of the angular distributions in resonance scattering as a probe of molecular orientation 111, and equips the vibrational spectroscopist with a powerful tool for surface analysis.
R.E. Palmer and P.J. Reus, Rev. Mod. Phys. 64,383 (1992). 2. E.T. Jensen and R.E. Palmer, Surf. Sci. 233,269 (1990). 3. 1. Nandhakumar, Z.Y. Li and R.E. Palmer, to ba published. 4. J.L. Wilkes, PhD Thesis, University of Cambridge, England, 1990. 5. E.T. Jensen, R.E. Palmer and P.J. Rous, Phys. Rev. Lett. 64, 1301(1990); E.T. Jensen, R.E. Palmer and P-J. Rous, Surf. Sci. 237,153 (1990). 1.
14
Fig. 10. Resonance energy profiles for 02 (1.5 L) co-adsorbed with potassium (x0.1 ML) on graphite at 30 K. The intensities’ of the 140 and 190 meV loss features (normalised to the diffuse elastic intensity) are plotted as a function of incident electron energy.
49
6. 7.
8. 9. 10. 11.
12. 13.
14. 15. 16.
17.
J. Stshr, NEXAFS Spectroscopy, (Springer-Verlag, Berlin, 1992). J. Stehr and K.R. Bauchspiess, Phys. Rev. Lett. 67, 3376 (1991); J.A Sheehy, T.J. Gil, C.L. Winstead, BE. Farren and P.W. Langhoff, J. Chem. Phys. 91, 1796 (1989). L. Siller, J.F. Wendelken and R.E. Palmer, J. Chem. Phys. in press. K. Jacobi and M. Bertolo, Phys. Rev. B 42,3733 (1990). KM. Hock and R.E. Palmer, J. Chem. Phys. 97,8736 (1992). W. Wurth, J. Stshr, P. Feulner, X. Pan, K.R. Bauchspiess, Y. Baba, E. Hundel, G. Rocker and D. Menzel, Phys. Rev. L&t. 65,2426 (1990). R.E. Palmer, Prog. Surf. Sci. 41, 51 (1992). L. bller, J.F. Wendelken, K.M. Hock and R.E. Palmer, Chem. Phys. Lett. in press. L.A. Morgan, P.G. Burke and C.J. Gillan, J. Phys. B 23,99 (1990). D.M. Ceperley and B.I. Alder, Phys. Rev. Lett. 45,566 (1980). J. Stohr, F. Sette and A.L. Johnson, Phys. Rev. Lett. 53, 1684 (1984); D.A. Outka, J. Stiihr, W. Jark, P. Stevens, J. Solomon and R.J. Madix, Phys. Rev. B. 35,4119 (1987). L. Siller, J.F. Wendelken, K.M. Hock and R.E. Palmer, Surf. Sci. 287/288, 165(1993).
18. M. Tronc, R. Azria and Y. LeCoat, J. Phys. B 13,2327 (1980). 19. J.C. Giordan, J.H. Moore, J.A Tossell and W. Kaim, J. Am. Chem. Sot. 107, 5600 (1985). 20. T.S. Jones, M.R. Ashton, M.Q. Ding and N.V. Richardson, Chem. Phys. L&t. 161,467 (1989).
21. T.S. Jones and N.V. Richardson, Phys. Rev. Lett .61,1752(1988). 22. Tables of Interatomic Distances and Configurations in Molecules and Ions, edited by L.E. Sutton, (Special Publ. No. 11 and 18; The Chemical Society, London 1958 and 1965). 23. D.J. Hannaman and M.A. Passler, Surf Sci. 203,449 (1988). 24. M.D. Crapper, C.E. Riley, D.P. Woodruff, A. Puschmann and J. Haase, Surf. Sci. 171, l(l986). 25, D.P. Woodruff, C.F. McConville, A.L.D. Kilcoyne, Th. Lindner, J. Somers, M. Surman, G. Paolucci and A.M. Bradshaw, Surf. Sci. 201, 228 ( 1988). 26. T.S. Jones and N.V. Richardson, Surf. Sci. 21 l/212,377 ( 1989). 27. R.O. Jones, P.J. Jennings and 0. Jepsen, Phys. Rev. B 29, 6474 (1984); J.P.A Charlesworth, R.W. Godby and R.J. Needs, Phys. Rev. Lett. 70, 1685 (1993). 28. B. Chakraborty, S. Holloway and J.K. Nerskov, Surf. Sci. 152053 (1985). 29. K.M. Hock, J.C. Barnard, R.E. Palmer and H. Ishida, Phys. Rev. L&t. in press. 30. Z.Y. Li, KM. Hock and R.E. Palmer, Phys. Rev. L&t. 67, 1562 (1991); E.T. Jensen, R.E. Palmer, W. Allison and J.F. Annett, Phys. Rev. Lett. 66, 492 (199 1). 31. R.E. Palmer, J.F. Annett and R.F. Willis, Phys. Rev. Lett. 58,249O (1987). 32. G.J. Schulz, Rev. Mod. Phys. 45, 423 (1973). 33. C. Janiak, R. Hoffmann, P. Sjovall and B. Kasemo, submitted for publication.
39
Negative ion resonances and surface vibrations R.E. Palmer Department of Physics, University of Cambridge, Cavendish Laboratory, Madingley Road, Cambridge C33 OH&UK This paper reviews some recent developments in high resolution &&on energy loss spectroscopy @IREELS) studies of adsorbed molecules. The principal theme is the negative ion resonance scattering mechanism and its usefulness in the investigation of surface vibrational states. Several examples are considered: first, the excitation of low frequency molecule-surface modes, including overtones, as illustrated by the case of physisorbed 02/pt(lll); secondly, the relationship between the CJresonauc6 energy and the bond length in HRKELS studies of chemisorbed molecules, and illustrated by the examples of chemisorb8d Ofl/pt(lll) and chemisorbed CO; finally, the utility of the resonauce scattering mechanism in unravelling the behaviour of a complex adsorption system, coadsorbed K and 02 on graphite.
1. INTRODUCTION The negative ion r8sonauce scattering mechanism is now well established in high resolution ekctron energy loss spectroscopy (HREELS) studies of adsorbed molecules Ill. Resonance scattering is a special and distinct example of short-range impact scattering, in which the scattered electron is temporarily trapped in a specific antibonding orbital of the target molecule, leading to large enhancements of the vibrational excitation cross-sections. Such resonances are well known in gas phase electron-molecule scattering experiments, and the manner in which the basic characteristics of the resonance state are modified on the surface has been the subject of considerable research. A relatively rec8nt review of the progress made in this area is given in ref. 1. The principal theme which we shall address in this paper is the usefulness of %esonance EELS” in the investigation of surface vibrations, and we shall illustrate
this theme by reviewing some recent experimental studies conducted in Cambridge. For the most part, experimental studies of resonance electron scattering by adsorbed molecules have been concerned with the excitation of relatively high frequency intramolecular vibrational Imd88. However, eSpeCidy in the ud8xt Of surface dynamics, the low frequency frustrated modes which occur on the surface are at least of 8qual interest. In section 2 we explore the excitation, via the formation of a negative ion resonance, of low frequency molecule-surface modes in the physisorbed 0 q/Pt(lll) system. Of particular importance here is the excitation of vibrational overtones in addition to the fundamental mode. 02 can also be chemisorbed on the Pt(ll1) surface, enabling us to explore, in section 3, the fundamental characteri&ics of the resonance state in (strongly) chemisorbed molecules. While it is true that negative ion resonances have now been identified in HREELS studies of a wide variety of both
physisorbed and chemisorbed molecules, our understanding of the nature of the resonance state in the former case is more advanced than in the latter (11, where the influence of the molecule-substrate interaction is expected to be more pronounced. In the case of chemisorbed 02A?t(lll) we find experimental evidence for a qualitative correlation between the bond length of the chemisorbed molecular species and the energy of the *a resonance”. In section 4 we shall illustrate the usefulness of this correlation, and, in more general terms, of an understanding of the resonance scattering mechanism, via a discussion of a rather complex spectroscopic system, co-adsorbed alkali metal (K> and 02 on graphite. In all these diBcusBionB,we shall be concerned with a specific negative ion resonance state, the “0 resonance* state of the 02 molecule. The Q resonance is a
Beam Energy = 11 eV
180
190
200
210
220
Energy Loss (meV)
Fig. 1. Detailed lineshape of the v = 0 - 1 excitation in the HREELS spectrum of a physisorbed monolayer of @/Pt(lll) at 30 K, A fitted curve using five Gaussians is also shown.
single particle shape resonance, created by captureof an electroninto the 2pau* (i.e. the 30~) orbital of the 02 molecule, This resonance has also been identified in HREELS studies of a variety of other adsorbed molecule, e.g. CO [21, NO 131 and N2 [41, in addition to 02 CM.The Q state appears at higher incident electron energies (between =7 and 20 ev) than the low lying n resonances in these molecules, which makes the cr resonance easily accessible in a standard HREELS experiment. The resonance is related to the u resonance commonly observed in X-ray absorption studies of small adsorbed molecules [61, where a multiple scattering picture of the resonance has been developed [71, a picture we shall find helpful later. 2. RESONANT EXCITATION OF LOW FREQUENCYMODESINHREELS Fig. 1 shows the detailed lineshapeof the v = 0 - 1 stretch mode in the HREELS spectrum of physisorbed 02 on Will) at a temperatureof ~30 K WI. The line is clearly asymmetric in shape, and shows fine structure which can be described by a deconvolution of the experimental curve into a series of five Gauesians. The main line is marked with an arrow in the figure and has a frequency of 196 f 1 meV, corresponding to the pure v = 0 - 1 stretch mode of the physisorbed 02 molecule. The three loss peaks appearing at highor energies (200 2 1, 206 f 1 and 210 f 1 meV) are assigned to the simultaneous excitation of the pure v = 0 - 1 mode and a Berieaof low frequency modes (t = 0 - 1, 2...), corresponding to the molecule-surface vibration and its overtones. The firat observation of such a series of coupled low frequency modea in HREELS was reported by Jacobi and Bertolo in 1990 [91.Here we explore the mechanism of excitation of these modes.
41
0.12 P 3
I
I
I
I
I
1
1
1
10 6 8 Beam energy (ev)
12
I
0.1 -
+ s 0.08 u P I 0.06 g $0.04 5 _Eo.O2 0
I 2
4
0.1
I
g
? 0.08
0
I
I
I
I
I
I
14
I
W -
1 2
I
I
4
10 6 8 Beam energy (eV)
12
14
4
6 8 10 Beam energy (ev)
12
14
I
P g 0.05
a =0.04 P $ 0.03 I e! po.02
.
g 0.01
01 2.
Fig. 2. Intensities of the coupled low modes in the v = 0 - 1 line profile of physisorbed O@t(lll) as a function of electron impact energy. fbquency
Fig. 2 shows the intensities cd the individual loss peaks extracted from the deconvolution as a function of electron impact 8nergy, while fig. 3 shows the mode intensities as a function of detection angle. The most importantpoint t0 not43i8 that tb8 energyand angular dependence of the coupled modes (e.g. v = 0 - 1, t = 0 - 1) track that of the pure intramolecularstretch mode (v = 0 - 1). The energy and angular dependence of the pure v = 0 - 1 mode is consistent with negative ion resonance scatteringvia the *&- eingle particle shape resonance of molecular 02, also known a8 the “a ehape resonance”, observed in the gas phase at 19.6 eV, and shifted down in energy on the surface to =7 eV by the image potential of the metallic substrate. The angular dependence of the mode intensity arises from the ps/f8 symmetryof this resonancestate 1101,which lead8 t0 a minimum in the i&x&y normal to the surface, i.e. perpendicular to the molecular axis of the physisorbed molecule which is approximately parallel to the surface 1111.The results of figs. 2 and 3 demonstratethat the coupled low frequency mode8 are excited by the same negative ion resonance mechanism. Such low frequency mode8 play an important role in surface dynamical processes, such as surface diffusion and desorption. They also represent a window onto the potential energy ,surface (PES) describing the molecule-surface interaction.Since the calculationof the full, multi-dimensional molecule-surface PES is a very demanding task, experimental insight8 into the nature of this eurface are particularly useful 1121.With the kind of resolution now available in HREELS, -1 meV, it is interesting to speculate whether one could use resonance scattering to determine the anharmonicityof the PES from the frequenciesof the overtones of the molecule-surface modes. Even with a resolution of 1 meV, this ta8k appears
42
.
: a)
I
I
Beam energy-7eV
,l,,,I,.,I,
0
2
.I,,,
,111,
4
6
8
10
12
Beamenergy (eV) Fig. 4. Intensity of the v = 0 - 1 vibrational excitation (frequency 106 meV) of chemisorbed Os/Pt(lll) at 90 K as a function of incident electron energy. insurmountable at present in the case of physisorbed molecules, but in the case of chemisorbed molecules, where the frequency of the molecrde-surface vibration for example, can be much higher (40 mev), the prospect is appealing.
50
3. BOND LENGTHS WITEARULEB EIRJZKIS?
0 -5
5
15
25 35 45 55 Detection Angle (“)
65
75
Fig. 3. Intensities of the coupled low frequency in the v = 0 - 1 line profile of physisorbed 02/Pt(lll) as a function of detection angle. The electron beam energy is 7 eV.
IN
Fig. 4 shows the intensity of the intramolecular stretch mode (frequency 106 meV) of chemisorbed molecular 02/pt(lll), produced by adsorption at -90 K, as a function of electron impact energy C131. The origin of the sharp peak in the crosssection at =l eV is uncertain, but it may arise from a threshold resonance 1141. The broad resonance feature centred at 3.75 f 0.25 eV is assigned to the Q shape resonance of the chemisorbed molecule, an assignment which is supported by the close correlatibn’between the angular distribution of fig. 5, for the chemisorbed molecule, and
43
the angular distributions of fig. 3, for the physisorbed molecule (the chemisorbed molecule also lies approximatelyparallel to the surface[ill). The assignment of the resonance observedat 3.75 eV to the u shape resonance of the chemisorbed 02 molecule implies a shift in resonance energy of -3.25 eV between the physisorbed and chemisorbed state, or, in other words, a shift in energy of 5.76 eV between the gas phase molecule and the chemisorbed state. There are several factors which we need to considerin order to understand the origin of this shift in energy. (i) The resonance energy shift in the physisorbedmolecule(with respect to the gas phase) can be assigned to the (attractive) image potential (more properly, the exchange-correlationpotential); in the case of the chemisorbsd molecule, a similar shift is expected. However, the chemisorbed molecule is so close to the surface (and hence the image plane) that the familiar, asymptotic image potential form (-l/42) of
2ot
-
-20
E= 4.4 eV
0
20 40 Angle(W)
60
80
Fig. 5. Intensity of the v = 0 - 1 vibrational excitation of chemisorbed 02/Pt(lll) as a function of detection angle (labelled with respect to the surface normal). The electron beam energy is 4.4 eV.
the exchange-correlation potential is not valid. Instead, the value of the exchangecorrelationpotential is probably close to the *saturated” value, i.e. the “inner potential’ of the Pt bulk, -6.6 eV M. (ii) The chemisorbed02 molecule on pt(lll) is thought to bear a net negative charge as a result of charge “back donation” from the substrate till. This will give rise to a repulsive interaction between the adsorbed molecule and the incoming electron - this interaction is described by the exchangecorrelation potential of the adsorbed molecule, which incorporates the uon-site Coulomb potential”. This repulsive interaction will increase the resonance energy,thus opposing the effect of the image potential. (iii) Studies of the G shape resonance in X-ray absorption spectroscopy have suggested an empirical correlation between the shape resonance energy and the bond length of the (free or adsorbed) molecule /6,161.Indeed, it has been proposed that this correlation allows the determination of “bond lengths with a ruler”, i.e. the extraction of the bond length from the resonance energy, to which it is related in an approximately linear fashion. The question of whether a similar correlation might be established in HREELS is intriguing. To explore further the last effect, i.e. the relation between Q resonance energy and bond length in HREELS,we have calculated the v E 0 - 1 vibrational excitation crosssection for the free 02 molecule as a functionof the bond length. The calculation employs the Xa treatment of the exchangecorrelation potential and gives good quantitative agreement with the experimental gas phase vibrational excitationcross-sectionsDOI.Fig. 6 is a plot of the energy of the calculated peak in the v = 0 - 1 cross-section as a function of the bond length of the free molecule. It can be
seen that the resonance energy decreases, in almost linear fashion, as the bond length is increased. Using the bond length of chemisorbed 02/Pt(lll) obtained from NEXAFS WI, 1.37 f 0.5 A,the calculation predicts a a resonance energy of 3.5 f 0.5 eV in a free 02 molecule with this bond length. This value is close to the experimental resonance energy of the chemisorbed 02 molecule on the Pt(ll1) surface, 3.76 It 0.5 eV. The question then arises “is correspondence merely a this coincidence?“. Fig. 7 shows a compilation D71 of the experimental 0 resonance energies of a sequence of molecules containing the C-O functional group as a function of the C-O bond length, determined independently. The resonance energies are obtained from HREELS studies of gas phase CO 1181 and (CH3)2O [191, and chemisorbed CO/Ni(llO) [ZOI and HCOOMi(110) 1213.The C-O bond lengths in the free molecules are taken from tables [223. The value for CO/Ni(llO) comes from LEED E231.No bond length is available for HCOO/Ni(llO), so we use the value for HCOO/Cu(llO) 1241, which should be
similar. This value is actually, obtained from the cr resonance energy - bond length correlation proposed in NEXAFS, but it is also consistent with photoelectron diffraction results [251. The solid line in fig. 7 is the result of an Xu calculation for the free CO molecule, similar to that described above for 02. What we see in the figure is a striking level of agreement between the calculated curve and the experimental points - which, even disregarding the calculated theory curve, appear to lie approximately on a straight line. Taken together with the results for 02, this correlation seems to indicate that the change of the bond length is the principal cause of the shift in the Q resonance energy of the chemisorbed molecule. One further topic needs discussion in connection with the results presented in fig. 7, and concerns those molecules in which there are not one, but two, C-O bonds, i.e. the
30 L
1
I
I
I
1.1
1.4
C-O dd2,d tt&h
1.2
1.25 1.3 1.35 O-O bond length (A)
1.4
1.45
Fig. 6. Q resonance energy versus bond length from Xa calculations for the free 02 molecule.
I
(A)
1.5
Fig. 7. Compilation of measured 0 resonance energies and bond lengths for gas and chemisorbed molecules phase containing the C-O functional group. For sources see text. The solid line is the Q resonance energy for free CO as a function of the bond length calculated using the Xa scheme.
free (CH3)2O molecule and the chemisorbed HCOO species on Ni(ll0). In principle, one would expect a splitting of the Q resonance into two components as a consequence of the interaction between the two C-O bonds 1261, which would certainly complicate the (I resonance energy - bond length correlation. Since no such splitting has yet been observed, we have plotted the resonance energies which have been measured, but note this point as a topic for further investigation. If the apparent correlation between Q resonance energy and bond length in chemisorbed molecules is to hold (at an approximate level), then the other two factors which might be expected to influence the resonance energy, i.e. the image potential and the on-site Coulomb repulsion, must, approximately, cancel each other out. Evidently, a detailed theoretical investigation of these effects would be most desirable, but in the meantime we can consider a (qualitative) plausibility argument which suggests that the net effect of these two opposing terms may indeed be small - leaving the effect of the bond length to dominate. First, we note that the separation between the exchangecorrelation potential of the substrate and the exchange-correlation potential of the adsorbate is rather artificial. One should really consider the overall electron density of the system, since in the chemisorption regime (i.e. close to the surface) the local density approximation (LDA) to the exchange-correlation potential seems to work rather well 1271(this is not true of the phyeisorption regime - the image potential is a non-local effect). The plausibility argument is then based on the ideas embodied in effective medium theory, developed by Norskov and others in the early 1980’s and applied to chemisorbed atoms 1281.A central pillar of this scheme is the notion of a Yavoured” electron density -
independent of the substrate - in which the adsorbed atom likes to sit, leading to a minimum in the adsorption energy. This determines the position of the chemisorption minimum outside the surface, and is basically a treatment of the sp electron density of the substrate. The substrate d band is then treated by a hybridisation term. It is the former term which is of interest here. Suppose that the 0 atom favours an electron density which is close to the density provided by the other 0 atom in the 02 molecule. As the molecule approaches the substrate, each 0 atom feels the electron density of the substrate and compensates for this increased density by moving further away from the other 0 atom in the molecule. In this scenario, it is plausible that the charge density in the vicinity of the chemisorbed molecule is not going to be so different from that of the free molecule. On the basis of the LDA, the exchangecorrelation potential experienced by the probe electron will then be similar to that of the free molecule, so that the resonance energy difference between the gas phase molecule and the chemisorbed molecule may be dominated by the bond length change (an effect which arises from the multiple scattering, Oparticle in a box” picture of the 0 resonance [71). Of course, as already stated, these are speculations of a qualitative nature, but they are motivated by the need to explain empirical results. These considerations also suggest that the approximate correlation between resonance energy and bond length in HREELS may be quite a general feature. This is clearly a notion that can only be established or disproved by further experimental studies of the cr resonances of chemisorbed molecules. Is the apparent correlation between resonance energy and bond length in HREELS of any use? People certainly use the resonance energies measured in NEXAFS to derive bond lengths in
46
chemisorbed species, though to the best of our knowledge these results have yet to be crosschecked by other, and more established, structural techniques, e.g. LEED. We would certainly be most reluctant to treat bond lengths derived from HREELS resonance energies as quantitative structural parameters, since the cancellation of the other factors influencing the bond length can surely only be approximate - though the trends obtained may be useful qualitative indicators. However, there is another use for the correlation, which seems to us important and which is nicely illustrated by another specific example, the coadsorption of K and 02 on graphite, as discussed in the following section.
. . . . l
. .
@I **
K/gr+lLCJ
1
K/gr+lLq
1
4. SEARCHING FOR CHEMICAL SPECIES lT+JFigs. 8 and 9 show a selection of HREELS spectra obtained when the “7x7” phase of K&a hite (where the K-K separation is -16 1 ) is exposed to 02 at 25 K [291. Fig. 8(a), obtained with an electron beam energy of 9 eV and in the specular scattering direction, shows a single discrete loss feature at -350 meV - corresponding to the low frequency surface plasmon of the graphite substrate to which the adsorbed alkali atoms donate electronic charge [303 superimposed on an intense background of electron-hole pair excitations arising from the detailed Fermi surface characteristics of semimetallic graphite [311. When the surface is exposed to 1 L 02 the surface plasma frequency is reduced to ~150 meV, fig. 8(b). This suggests that some of the charge donated by the adsorbed alkali atoms to the substrate is withdrawn from the substrate when 02 is co-adsorbed on the surface. The question which then arises is “what is the nature of the co-adsorbed 02 species causing this behaviour”?
1(c)
l
**
I
-0.2
:
‘:
d<
‘. 8
. ‘-/ .. ;* . . . . .. . : . t. f.
0
I
I
I
0.4 0.6 0.2 Energy Loss (ev)
1
0.8
Fig. 8. Electron energy loss spectra from coadsorbed K and 02 on graphite at 30 K, obtained with an incident electron energy of 9 eV. (a) spectrum from K/graphite with specular scattering geometry (inset), showing the K/graphite surface plasmon. (b) as (a) following exposure to 1 L 02. (cl spectrum obtained from K+O2/graphite with off specular scattering geometry (inset), showing vibrational energy loss peaks characteristic of physisorbed 02.
47
.
.
.
i
280 380 f #* IIIIIIIIII!tIl,,,,l,,I,I,,,,
-0.1
0.0
0.1 0.2 0.3 0.4 Energy Loss (eV)
0.5
Fig. 9. Electron energy loss spectrum from co-adsorbed K and 02 (1.5 L) on graphite at 30 K, obtained with an incident electron energy of 4.0 eV and off specular scattering geometry (inset). The loss peak at 190 meV corresponds to physisorbed 02, while the peak appearing at ==140meV is indicative of a more strongly bound 02 species. Overtones of both these vibrational excitations are also seen at 280 meV and 380 meV. No loss peaks which could be associated with an 02 species are observable in the specular HREELS spectrum of fig. 8(b). To “find” the coadsorbed species we utilise our understanding of the resonance scattering cross-sections, as described previously. First, fig. 8(c), we look well away from the specular direction, to “sieve out” the loss features excited by dipole scattering (including both the surface plasmon and the electron-hole pair continuum), but at the same beam energy, 9 eV, since the cr shape resonance of the physisorbed 02 molecule
should be accessible at this energy (resonance peak energy 9.5 eV in the gas phase, 8 eV on clean graphite [al). What we are now able to see is the v = 0 - 1 vibration of physisorbed 02 at =196 meV, together with thev=O-Zovertone(andaweakv=O-3 overtone), from which we are able to conclude that even at this low 02 exposure physisorbed 02 is already present on the surface. However, the existence of physisorbed 02 cannot account for the apparent withdrawal from the substrate of the electronic charge donated by the alkali. If the alkali charge is donated to a coadsorbed 02 species instead of the substrate, we might expect to find some negatively charged peroxo or superoxo type species in the HREELS spectrum. Now the ground state of the gas phase 02- species has a bond length of 1.34 A (and a vibrational frequency in the range of -132 - 140 meV) [323. Using this bond length as a guide, the resonance energy - bond length calculation of fig. 6 predicts a o resonance energy of x4.4 eV. We therefore take an HREELS spectrum at this beam energy, again going well away from specular to remove the dipole excitations. The spectrum so obtained, fig. 9, clearly shows a new loss peak, at =14ClmeV, in addition to the peak at 190 meV observed with an impact energy of 9 eV and attributed to physisorbed 02. The first overtone of the new loss feature is also visible at ~280 meV. The 140 meV peak is assigned to a molecular 02 species akin to 02-, in harmony with cluster calculations for this co-adsorption system, which predict a molecular 02 species lying parallel to the surface on top of an adsorbed alkali atom. We believe that the physisorbed 02 molecules are adsorbed on the bare surface between these stable “KO2”moieties.
48 The analysis of this complex adsorption system illustrates in a convincing fashion the usefulness of an understanding of the resonance scattering mechanism - and, specifically, of the approximate resonance energy - bond length correlation - in the investigation of chemical species on surfaces. It also illustrates the usefulness of resonance scattering in maximising the sensitivity of the HREELS technique. This latter point is further confirmed by a detailed study of the resonance scattering cross-section for the chemisorbed 02 species in the K+Oggraphite system, shown in fig. 10. This compares the energy dependence of the v = 0 - 1 intensities for the chemisorbed 02 species (frequency 140 meV) and the physisorbed molecule (frequency 190 meV). The resonance observed in the chemisorbed molecule, centred at -4 eV with a full-width half-maximum of only about 3 eV, leads, 6
, ,
I, I I I,
AE = 140meV 0
r”““l”““““““‘I’
2
4
6 8 10 12 Electron Energy(eV)
like the resonance in the physisorbed molecule, to a considerable enhancement of the vibrational mode intensity.
6. CONCLUSIONS Negative ion resonances play an important role in a variety of dynamical processes occurring at the gas-surface interface 1121.However, the issues discussed in this paper - excitation of low frequency modes and their overtones, the relationship between resonance energy and bond length in chemisorbed molecules, and the application of resonance scattering to the analysis of complex systems - have been chosen to illustrate the utility of the negative ion resonance scattering mechanism in practical vibrational spectroscopy at surfaces; specifically, in maximising the vibrational excitation cross-sections for both intramolecular and molecule-surface modes in both physisorbed and chemisorbed molecules. This capability complements the use of the angular distributions in resonance scattering as a probe of molecular orientation 111, and equips the vibrational spectroscopist with a powerful tool for surface analysis.
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14
Fig. 10. Resonance energy profiles for 02 (1.5 L) co-adsorbed with potassium (x0.1 ML) on graphite at 30 K. The intensities’ of the 140 and 190 meV loss features (normalised to the diffuse elastic intensity) are plotted as a function of incident electron energy.
49
6. 7.
8. 9. 10. 11.
12. 13.
14. 15. 16.
17.
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