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Identification of localised and delocalised adsorbate orbitals on W(100) by photoelectron spectroscopy
Volume 36, number
3
IDENTEFICATION
CHEMICAL PHYSICS LETTERS
OF LOCALISED
BY PHOTOELECTRON
AND DELOCALISED
1.5 November
ADSORBATE
1975
ORBITAL!4 ON W(roo)
SPECTROSCOPY
J.W. LINNETT, D.L. PERRY and W.F. EGELHOFF Jr.* Department
of Physical Chemistry,
Received 9 June 1975 Revised manuscript received
University of Cambridge, Cambridge Cl32
IEP, UK
10 July 1975
The comparison of the photoelectron spccka cf chemisorbed layers taken with glancing photon incidence and those with normal photon incidence permits a qualitative dcterminaticn of the degree of delocalisation of the various orbitals at the surface. For glancing photon incidence, both localised and delocalised orbit& can bc photoionised. However, for atomic sites have a much higher photoionisaLion cross normally incident photons, those orbit& localised around particular section than those orbit& extensively delocalised in the plane of the surface. Examples of this behaviour in the spectra of hydrogen and carbor .lonoxidc on W(lOO) arc presented.
In attempting to understand the electronic structure of simple chemisorbed layers, it would be very helpful to know whether a particular electronic state was loczlised, in the sense of a band structure, over a large number of adatom sites. Recent theoretical predictions and experimental results have made it apparent that electronic states which are thought to be deIocalised in the two-dimensional plane of a metal surface have a very small photoionisation cross section for photons which are polarised with the electric vector parallel to the metal surface [ 1,2]. Using 10.2 eV photons, it was shown that, as the polarisation of the electric vector changed from perpendicular to the surface to parallel to the surface, the photoemission from the eIectronic surface state on the W( 100) surface fell off much more rapidly than the photoemission from the other bands [1] . This was attributed to the lack of a potential gradient in the plane of the surface, which surfack state electrons rnus~ experience in order to be photoionised when the electric vector is parallel to the surface. In theoretical studies, it has long been noted that * ?res-ent address: Department of Chemicd Engineering, California Institute of Technology, Pasadena, CalXomia 91109, USA.
photoionisation cannot occur when the electron is completely delocalised as a free electron in the direction of the photon electric vector [3]. This is simply a consequence of the impossibility of conserving both momentum and energy. In early treatments of the photoelectric effect it was assumed that ionisation of the “free-electron” metal took place by allowing the work function potential barrier at the surface to absorb momentum sufficient to satisfy the selection rules [3]. Although this model for photoemission trast
with
is a rather
extreme
simplification,
its con-
gas phase photoionisation
extreme cases, between faces will lie.
provides two w_hich all real metallic sur-
At one extreme, in gas phase photoionisabion. each molecular orbital is local&d around the molecule and will experience a potential barrier 10 escape, analogous to the work function barrier, which ex.Ws only perpen&cular to the metal surface. Localisation of an orbital is simply a consequence of the electron in that orbital experiencing a potential barrier preventing delocalisation. Thus, an orbital at a metal surface which is somewhat Iocalised around a particnlar site will always have a potential gradient for the electron to interact with under the influence of a photon. This will even be true when tha photons are 331
Volume 36, number 3
15 November
CHEMICAL PHYSICS LETTERS
1975
uolarised with the electric [‘e&or parallel to the surface, because the orbital is not deiocalised in the
tern had established the conditions necessary to ensure the absence of unwanted contamination of the
plane of +e surface. However, near the other extreme, as in the case of the surface state on W(kOO), the orbital is very extensively delocalised in the plane of the stirface, and only a relatively small potential gradient parallel to the surface will be experienced. If the electric vector is parallel to the surface, the photoionisation cross section will be relatively small. However, if the electric vector is perpendicular to the surface, the delocalised electron can interact with the work function potential gradient and the photoionisation cross section will be relatively large. In the present work, a comharison between photoelectron spectra with near-normally incident unpolarised photons (in which c:ase the electric vector of the photons is nearly parall: to the surface) and glancing incidence photons (in which case the electric vector ofsome photons will be nearly perpendicular to the surface) has been applied to determine the
crystal surface [4,5]. The analyser accepts electrons emitted in a cone with apex angle 4O, permitting
degree of deloczlisation
of :)rbitals in two chemisorb-
ed systems on W(I 00). The experimental geometry is illustrated in fig. 1. The angle between the photon beam (11~ = 21.2 eV) and the axis of the 127” parallel plate electrostatic analyser is fixed at 90’ at all times. The crystal is the only movable part of the system. The axis about which it can be rotated is indicated by the dct on the side of the crystal in fig. 1. This axis is perpendicular to bot!l the photon beam and the axis of the analyser. Thus the crystal can be rotated for either glancing photon incidence (denoted here 8 = 0”) or for norma! photon incidence (0 = 90”). The W(100) crystal was prepared and cleaned by well-established methods. The backgcund pressure of reactive gases was 2 X 10-l’ torr during these experiments, and previous work on the present sys-
hu
\
11001
Fig. 1. The geometry of the photoemission expximent showin,o the photon beam, the .Ixis of acceptance of the analyser, and the rotation oxk of the crystal, which are mutua!ly pxpendic+r. .33i
good resoluticn of the angular dependence of the spectra. The photoeiectron spectrum of the clean W(100) surface displays a marked variation with change of angle. The analysis of this is a complex theoretical problem which would require the calculation of the band structure along a !arge number of lines in the Brillouin zone. Feuerbacher and Christensen have been able to interpret the energy distribution of photoelectrons emitted normally to the (loo), (110) and (11 I) faces of single crystal tungsten, in terms of the one-dimensional electronic properties along the syminetry line corresponding to each crystal face [6]. In this study we are concerned only with the angular variation of the adsorbate induced energy levels. In a previous paper the authors examined the angular dependence of the photoelectron spectrum of hydrogen adsorbed on W( 100) [G]. A method of analysis
of the data
led to the proposal
different
from
the present
of a model for the electronic
one
structure of the surface complex. The peak due to hydrogen adsorption which had the largest binding enera (% 7 eV below E,) was concluded to be formed from the W-6s and I-I-Is orbitals. It was thought to be extensively delocalised into a two-dimensianal band structure in the plane of the surface. The next hydrogen-induced peak (z 5 eV below Ef) was attributed to an orbital which was localised around each hydrogen atom. It seemed that the absence of a two-dimensionai band structure was a result of the orbital being composed of W-5d hybrids, each of which pointed a lobe at one hydrogen atom and a node at the adjacent hydrogen atom. In this sense each orbita! consisted of two hybrids and a H-1s orbital would not interact with adjacent orbitals of the same type. Such an orbital is illustrated in fig. 2. The hybrids are composed of the Sdrz and 5d,z _Yz orbitals, and the hydrogen is in the two-coordinate bridge site which is thought to 5e the binding site at saturation coverage. In our previous paper, the other hydrogen-induced peaks in the spectrum (between -3 eV and Ef) were interpieted as being due !argely to orbitals having rather small e!ectron density on the hydrogen atom, but having large
Vo!ume 36, number
3
CHEMICAL
PHYSICS
15 November
LETTERS
1975
Fig. 2. An illustration
of the proposed formation of a localised orbital around each hydrogen atom from dxz_y?-rdyz hybrids and the H-1s orbita:.
W-W overlap populations [4]. This would indicate that they are bulk tungsten d-bands at the surface
which are somewhat modified by the adsorption of hydrogen. Such bands would be extensively delocalised in the two-dimensional p!ane of the surface as well as extending into the bulk. Fig. 3 illustrates the photoelectron spectra of the clean and hydrogen-saturated surfaces for near g!ancing (a and b) photon incidence, and near-normal photon incidence (c and d). The shaded areas indicate the peaks induced by the adsorption of hydrogen. The peaks of the three types described above are clearly observed in (b). For glancing incidence photons, both localised (labelled (2) in fig. 3) and delocalised (labelled (3) in fig. 3) orbitals are observed in the photoelectron spectrum. However, approaching normally incident photons, the intensity of the peaks due to delocalised bands should fall off much more rapidly than that of localised orbitals. Consistent with this principle, only the peak assigned to localised orbitals is seen in fig. 3 d, in which near-normal incidence of the photons was used. It should be noted that our method of varying the angle of photon incidence is not ideal. By tilting the crystal, the angle at which electrons are collected varies with the angle of photon incidence (see fig. 1). Ideally, the crystal and detector should be held in fixed positions and the helium resonance lamp moved to vary the angle of incidence. This is not possible in the present apparatus. Nevertheless, the basic principles of the experiment remain valid; the experimental results indicating exactly the behaviour expected from the previously sugested model of the electronic structure of this system. One further example of this type of behaviour hx been identified in the CO on W( 100) system. Expoiurc of the cIean surface at room temperature to = IOtorr of CO produces a tightly bound state of CO, de-
I
I
I
-15
1
I
I
,
,
1
,
,
’
1
I
,
-10 -5 EF Electron Binding Energy /eV
Fig. 3. The photoclcctron
spectra
(/IV= 21.2
cV) of (a) the
clean npd (b) the hydrogen-saturated W(100) surfxc at glancing photon incidence (0 = 10”) and the spectra of (c) the clean and (d) the hydrogen-saturated W(100) surface zt near-normal photon incidence (0 = 74’). Pcnks indicated 1 and 3 arc due to dclocalised orbit&; peak 2 is due to a more localiscd
orbital.
signatcd fl-CO, and a weakly bound state, designated cr-CO [7]. It hcs been susested that in the B state the molecule is dissociated and bound as separate carbon and oxygen atoms [8]. Indeed, the UV photoelectron spectra of P-CO are essentially identical to those of the undoubtedly dissociated fl state of nitrogen on W(lO0) [5]. In such a system, in rvh&zh the carbon and oxygen atoms would be strongly bound to the tungsten atoms, it is easily imagined that the valence orbitals at the surface would be delocalised into twodimensional bands. In contrast, the a: state of CO is molecular, being similar to a transition metal carbonyl [7]. In such a state the valence molecular orbitals would certainly be localised around each CO molecule. Figs. 4s and 4b show the spectra of the clean surface, the PC0 layer, and the LY-COpeak (shaded) at glanc333
.Voiume 36. number 3
CHEMICAL
PHYSICS LETTERS
15 November
1975
Iocahsed around each CO molecule than the vdence of P-CO which is quite likely to have a rather deloczlised band structure. It should be nozed that tile o-CO, P-CO comparison involves distinctly different surface species, and this necessarily complicates the interpretation. Et could be that the valence orbit& of both states are highly localised but that the mean free path of the electrons from @CO is much greater than those from /MX near the crystal surface for normalty incident photons. The design of the present apparatus prevents the elimination of this pcssibility. However, this shortcoming is likely to be much less irnportxrt in the case of hydra. gen. In conclusion, it is felt that the above method of anahysis, applied in this case to two quite different examples, has great potential for facilitating the orbitals
I
I
I -15
! i I I I I , , I -10 -5 Etectrrxr Binding Energy /eV I
I
I
I
,
I EF
Fig. 4. The photoelectron spectra (IIV = 21.2 eV) of (a) the clean and (b) the CC?-wturnt?d W(tO0) surface at glancing photon incidence (6 = 10’) and the spectra of(c) the clean and (d) CO-saturated surface at near-normal photon incidence (ff = 74”). The shaded areas i?dicatc emission from c&O, obtained with the crystal at 300 K in 1 X 10m6 torr of CO.
ing photon incidence: This assignment of the peaks to dissociated and undisscciated states of adsorbed CO resembles that for CO adsorbed on a number of different metal surfaces [9--l I]. The a-CO peak was distinguished by pumping away the background CO to = lW1’ torr and warming the crystal to 400 K This removes the (Ystate but not the 0 state [7j. Clearly, at glancing photon incidence the a-CO peak height is considerabIy smaller than the @-COpeak heighrx. However, in figs. -kc and 4d, at nearnorma photon incidence, the a-CO peak height is considerably lsrger than the &CO peak heights. Again, the *s‘iii$e model discussed above would imply that this indicates that the valence orbit& of a-CO are more
334
interpretation species.
of photoelectron
spectra
of adsorbed
References [I] B. Feucrbacher and B. Fitton, Solid State Commun. 15 (1974) 295. 121 J.G. Endriz, Phys. Rev. B7 (1973) 3444. K. Mitchell, Proc. Roy. Sot. Al46 (1934) 442. W.F. Egeihoff and D.L. Perry, Phys. Rev. Letters 34
(1975) 93. W.F. Egelhoff, J.W. Linnett and D.L. Perry, Faraday Discussions Chen. Sot. No. 58 (1974). B. Fcuerbacher and N.E. Christensen, Phys. Rev. BlO (1974) 2373. I.T. Yates, R.G. Grcenlcr, 1 Ratajczykowa and D.A. King, Surface Sci. 36 (1973) 739. C.G. Goymour and D.A. King, J. &em. Sot. Faraday 169 (1973) 736,749. D. Eastman and J.K. Cashion, Phys. Rev. Letters 27 (1971) 1520. S.J. Atkinson, CR. Brundlc and R1.W. Roberts, Faraday Discussicns Chcm. Sot. No. 58 (1974). T-A. Clarke, J.D. Gay. B. Law and R. Mason, Chern. Phys. Letters 31 (1975) 29.
3
IDENTEFICATION
CHEMICAL PHYSICS LETTERS
OF LOCALISED
BY PHOTOELECTRON
AND DELOCALISED
1.5 November
ADSORBATE
1975
ORBITAL!4 ON W(roo)
SPECTROSCOPY
J.W. LINNETT, D.L. PERRY and W.F. EGELHOFF Jr.* Department
of Physical Chemistry,
Received 9 June 1975 Revised manuscript received
University of Cambridge, Cambridge Cl32
IEP, UK
10 July 1975
The comparison of the photoelectron spccka cf chemisorbed layers taken with glancing photon incidence and those with normal photon incidence permits a qualitative dcterminaticn of the degree of delocalisation of the various orbitals at the surface. For glancing photon incidence, both localised and delocalised orbit& can bc photoionised. However, for atomic sites have a much higher photoionisaLion cross normally incident photons, those orbit& localised around particular section than those orbit& extensively delocalised in the plane of the surface. Examples of this behaviour in the spectra of hydrogen and carbor .lonoxidc on W(lOO) arc presented.
In attempting to understand the electronic structure of simple chemisorbed layers, it would be very helpful to know whether a particular electronic state was loczlised, in the sense of a band structure, over a large number of adatom sites. Recent theoretical predictions and experimental results have made it apparent that electronic states which are thought to be deIocalised in the two-dimensional plane of a metal surface have a very small photoionisation cross section for photons which are polarised with the electric vector parallel to the metal surface [ 1,2]. Using 10.2 eV photons, it was shown that, as the polarisation of the electric vector changed from perpendicular to the surface to parallel to the surface, the photoemission from the eIectronic surface state on the W( 100) surface fell off much more rapidly than the photoemission from the other bands [1] . This was attributed to the lack of a potential gradient in the plane of the surface, which surfack state electrons rnus~ experience in order to be photoionised when the electric vector is parallel to the surface. In theoretical studies, it has long been noted that * ?res-ent address: Department of Chemicd Engineering, California Institute of Technology, Pasadena, CalXomia 91109, USA.
photoionisation cannot occur when the electron is completely delocalised as a free electron in the direction of the photon electric vector [3]. This is simply a consequence of the impossibility of conserving both momentum and energy. In early treatments of the photoelectric effect it was assumed that ionisation of the “free-electron” metal took place by allowing the work function potential barrier at the surface to absorb momentum sufficient to satisfy the selection rules [3]. Although this model for photoemission trast
with
is a rather
extreme
simplification,
its con-
gas phase photoionisation
extreme cases, between faces will lie.
provides two w_hich all real metallic sur-
At one extreme, in gas phase photoionisabion. each molecular orbital is local&d around the molecule and will experience a potential barrier 10 escape, analogous to the work function barrier, which ex.Ws only perpen&cular to the metal surface. Localisation of an orbital is simply a consequence of the electron in that orbital experiencing a potential barrier preventing delocalisation. Thus, an orbital at a metal surface which is somewhat Iocalised around a particnlar site will always have a potential gradient for the electron to interact with under the influence of a photon. This will even be true when tha photons are 331
Volume 36, number 3
15 November
CHEMICAL PHYSICS LETTERS
1975
uolarised with the electric [‘e&or parallel to the surface, because the orbital is not deiocalised in the
tern had established the conditions necessary to ensure the absence of unwanted contamination of the
plane of +e surface. However, near the other extreme, as in the case of the surface state on W(kOO), the orbital is very extensively delocalised in the plane of the stirface, and only a relatively small potential gradient parallel to the surface will be experienced. If the electric vector is parallel to the surface, the photoionisation cross section will be relatively small. However, if the electric vector is perpendicular to the surface, the delocalised electron can interact with the work function potential gradient and the photoionisation cross section will be relatively large. In the present work, a comharison between photoelectron spectra with near-normally incident unpolarised photons (in which c:ase the electric vector of the photons is nearly parall: to the surface) and glancing incidence photons (in which case the electric vector ofsome photons will be nearly perpendicular to the surface) has been applied to determine the
crystal surface [4,5]. The analyser accepts electrons emitted in a cone with apex angle 4O, permitting
degree of deloczlisation
of :)rbitals in two chemisorb-
ed systems on W(I 00). The experimental geometry is illustrated in fig. 1. The angle between the photon beam (11~ = 21.2 eV) and the axis of the 127” parallel plate electrostatic analyser is fixed at 90’ at all times. The crystal is the only movable part of the system. The axis about which it can be rotated is indicated by the dct on the side of the crystal in fig. 1. This axis is perpendicular to bot!l the photon beam and the axis of the analyser. Thus the crystal can be rotated for either glancing photon incidence (denoted here 8 = 0”) or for norma! photon incidence (0 = 90”). The W(100) crystal was prepared and cleaned by well-established methods. The backgcund pressure of reactive gases was 2 X 10-l’ torr during these experiments, and previous work on the present sys-
hu
\
11001
Fig. 1. The geometry of the photoemission expximent showin,o the photon beam, the .Ixis of acceptance of the analyser, and the rotation oxk of the crystal, which are mutua!ly pxpendic+r. .33i
good resoluticn of the angular dependence of the spectra. The photoeiectron spectrum of the clean W(100) surface displays a marked variation with change of angle. The analysis of this is a complex theoretical problem which would require the calculation of the band structure along a !arge number of lines in the Brillouin zone. Feuerbacher and Christensen have been able to interpret the energy distribution of photoelectrons emitted normally to the (loo), (110) and (11 I) faces of single crystal tungsten, in terms of the one-dimensional electronic properties along the syminetry line corresponding to each crystal face [6]. In this study we are concerned only with the angular variation of the adsorbate induced energy levels. In a previous paper the authors examined the angular dependence of the photoelectron spectrum of hydrogen adsorbed on W( 100) [G]. A method of analysis
of the data
led to the proposal
different
from
the present
of a model for the electronic
one
structure of the surface complex. The peak due to hydrogen adsorption which had the largest binding enera (% 7 eV below E,) was concluded to be formed from the W-6s and I-I-Is orbitals. It was thought to be extensively delocalised into a two-dimensianal band structure in the plane of the surface. The next hydrogen-induced peak (z 5 eV below Ef) was attributed to an orbital which was localised around each hydrogen atom. It seemed that the absence of a two-dimensionai band structure was a result of the orbital being composed of W-5d hybrids, each of which pointed a lobe at one hydrogen atom and a node at the adjacent hydrogen atom. In this sense each orbita! consisted of two hybrids and a H-1s orbital would not interact with adjacent orbitals of the same type. Such an orbital is illustrated in fig. 2. The hybrids are composed of the Sdrz and 5d,z _Yz orbitals, and the hydrogen is in the two-coordinate bridge site which is thought to 5e the binding site at saturation coverage. In our previous paper, the other hydrogen-induced peaks in the spectrum (between -3 eV and Ef) were interpieted as being due !argely to orbitals having rather small e!ectron density on the hydrogen atom, but having large
Vo!ume 36, number
3
CHEMICAL
PHYSICS
15 November
LETTERS
1975
Fig. 2. An illustration
of the proposed formation of a localised orbital around each hydrogen atom from dxz_y?-rdyz hybrids and the H-1s orbita:.
W-W overlap populations [4]. This would indicate that they are bulk tungsten d-bands at the surface
which are somewhat modified by the adsorption of hydrogen. Such bands would be extensively delocalised in the two-dimensional p!ane of the surface as well as extending into the bulk. Fig. 3 illustrates the photoelectron spectra of the clean and hydrogen-saturated surfaces for near g!ancing (a and b) photon incidence, and near-normal photon incidence (c and d). The shaded areas indicate the peaks induced by the adsorption of hydrogen. The peaks of the three types described above are clearly observed in (b). For glancing incidence photons, both localised (labelled (2) in fig. 3) and delocalised (labelled (3) in fig. 3) orbitals are observed in the photoelectron spectrum. However, approaching normally incident photons, the intensity of the peaks due to delocalised bands should fall off much more rapidly than that of localised orbitals. Consistent with this principle, only the peak assigned to localised orbitals is seen in fig. 3 d, in which near-normal incidence of the photons was used. It should be noted that our method of varying the angle of photon incidence is not ideal. By tilting the crystal, the angle at which electrons are collected varies with the angle of photon incidence (see fig. 1). Ideally, the crystal and detector should be held in fixed positions and the helium resonance lamp moved to vary the angle of incidence. This is not possible in the present apparatus. Nevertheless, the basic principles of the experiment remain valid; the experimental results indicating exactly the behaviour expected from the previously sugested model of the electronic structure of this system. One further example of this type of behaviour hx been identified in the CO on W( 100) system. Expoiurc of the cIean surface at room temperature to = IOtorr of CO produces a tightly bound state of CO, de-
I
I
I
-15
1
I
I
,
,
1
,
,
’
1
I
,
-10 -5 EF Electron Binding Energy /eV
Fig. 3. The photoclcctron
spectra
(/IV= 21.2
cV) of (a) the
clean npd (b) the hydrogen-saturated W(100) surfxc at glancing photon incidence (0 = 10”) and the spectra of (c) the clean and (d) the hydrogen-saturated W(100) surface zt near-normal photon incidence (0 = 74’). Pcnks indicated 1 and 3 arc due to dclocalised orbit&; peak 2 is due to a more localiscd
orbital.
signatcd fl-CO, and a weakly bound state, designated cr-CO [7]. It hcs been susested that in the B state the molecule is dissociated and bound as separate carbon and oxygen atoms [8]. Indeed, the UV photoelectron spectra of P-CO are essentially identical to those of the undoubtedly dissociated fl state of nitrogen on W(lO0) [5]. In such a system, in rvh&zh the carbon and oxygen atoms would be strongly bound to the tungsten atoms, it is easily imagined that the valence orbitals at the surface would be delocalised into twodimensional bands. In contrast, the a: state of CO is molecular, being similar to a transition metal carbonyl [7]. In such a state the valence molecular orbitals would certainly be localised around each CO molecule. Figs. 4s and 4b show the spectra of the clean surface, the PC0 layer, and the LY-COpeak (shaded) at glanc333
.Voiume 36. number 3
CHEMICAL
PHYSICS LETTERS
15 November
1975
Iocahsed around each CO molecule than the vdence of P-CO which is quite likely to have a rather deloczlised band structure. It should be nozed that tile o-CO, P-CO comparison involves distinctly different surface species, and this necessarily complicates the interpretation. Et could be that the valence orbit& of both states are highly localised but that the mean free path of the electrons from @CO is much greater than those from /MX near the crystal surface for normalty incident photons. The design of the present apparatus prevents the elimination of this pcssibility. However, this shortcoming is likely to be much less irnportxrt in the case of hydra. gen. In conclusion, it is felt that the above method of anahysis, applied in this case to two quite different examples, has great potential for facilitating the orbitals
I
I
I -15
! i I I I I , , I -10 -5 Etectrrxr Binding Energy /eV I
I
I
I
,
I EF
Fig. 4. The photoelectron spectra (IIV = 21.2 eV) of (a) the clean and (b) the CC?-wturnt?d W(tO0) surface at glancing photon incidence (6 = 10’) and the spectra of(c) the clean and (d) CO-saturated surface at near-normal photon incidence (ff = 74”). The shaded areas i?dicatc emission from c&O, obtained with the crystal at 300 K in 1 X 10m6 torr of CO.
ing photon incidence: This assignment of the peaks to dissociated and undisscciated states of adsorbed CO resembles that for CO adsorbed on a number of different metal surfaces [9--l I]. The a-CO peak was distinguished by pumping away the background CO to = lW1’ torr and warming the crystal to 400 K This removes the (Ystate but not the 0 state [7j. Clearly, at glancing photon incidence the a-CO peak height is considerabIy smaller than the @-COpeak heighrx. However, in figs. -kc and 4d, at nearnorma photon incidence, the a-CO peak height is considerably lsrger than the &CO peak heights. Again, the *s‘iii$e model discussed above would imply that this indicates that the valence orbit& of a-CO are more
334
interpretation species.
of photoelectron
spectra
of adsorbed
References [I] B. Feucrbacher and B. Fitton, Solid State Commun. 15 (1974) 295. 121 J.G. Endriz, Phys. Rev. B7 (1973) 3444. K. Mitchell, Proc. Roy. Sot. Al46 (1934) 442. W.F. Egeihoff and D.L. Perry, Phys. Rev. Letters 34
(1975) 93. W.F. Egelhoff, J.W. Linnett and D.L. Perry, Faraday Discussions Chen. Sot. No. 58 (1974). B. Fcuerbacher and N.E. Christensen, Phys. Rev. BlO (1974) 2373. I.T. Yates, R.G. Grcenlcr, 1 Ratajczykowa and D.A. King, Surface Sci. 36 (1973) 739. C.G. Goymour and D.A. King, J. &em. Sot. Faraday 169 (1973) 736,749. D. Eastman and J.K. Cashion, Phys. Rev. Letters 27 (1971) 1520. S.J. Atkinson, CR. Brundlc and R1.W. Roberts, Faraday Discussicns Chcm. Sot. No. 58 (1974). T-A. Clarke, J.D. Gay. B. Law and R. Mason, Chern. Phys. Letters 31 (1975) 29.