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Photoemission study of no chemisorption on Pd (111) using synchrotron radiation with energy of 30–130 eV
Surface Science 176 (1986) L841-L846 North-Holland, Amsterdam
L841
S U R F A C E S C I E N C E LETTERS P H O T O E M I S S I O N S T U D Y OF N O C H E M I S O R P T I O N O N Pd (111) U S I N G S Y N C H R O T R O N RADIATION WITH ENERGY OF 3 0 - 1 3 0 eV Eizo M I Y A Z A K ! , Isao K O J I M A * , Masahiro ORITA, Kiyotaka SAWA, Noriaki S A N A D A Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan
Tsuneaki M I Y A H A R A and Hiroo K A T O Photon Factory, National Laboratoryfor High Energy Physics, Oho-machi, Tsukuba-gun, Ibaraki 305, Japan Received 18 March 1986; accepted for publication 8 July 1986
A synchrotron radiation study of NO chemisorption on P d ( l l l ) was carried out at room temperature in the hv = 30-130 eV range which covers the "Cooper minimum" of Pd4d electrons. The relative enhancement in photoemission sensitivity of the NO-derived 40, 50 + Dr and 2~r molecular orbitals was dramatic at h v = 1 2 0 - 1 3 0 eV, and NO was confirmed to chemisorb in molecular form on the surface at room temperature. The bonding shifts of the 50 and lrr orbitals were found to be smaller than 0.2 eV. The 2~ level showed a si'~nificant bonding shift of 0.7-1.9 eV, suggesting that the unpaired electron in the 2~- orbital is mainly responsible for the chemisorption bonding through a charge transfer mechanism; i.e., electron donation from the 2rr up-spin level into the metal and simultaneous back-donation into the down-spin 2~r state from the metal.
Chemisorption of diatomic molecules such as NO, CO and N 2 on a solid surface has been studied extensively because of interest both in the similarity of valence levels of these molecules and in the difference in electron occupation of the 27r antibonding orbitals, i.e. the 2~r molecular orbital is singly occupied for NO, whereas it is vacant for CO and N 2. Further, the N O molecule is known to be adsorbed more stably on Pd metal as compared with other transition d-metals, i.e. on Pd, N O exists in molecular form at room temperature and the dissociation of the adsorbed N O occurs at a higher temperature. Many techniques for surface analysis such as UPS [1-6], XPS [4-8], AES [6-9], L E E D [1,5-11] and EELS [12] have been applied to the investigation of the interaction of N O with metal surfaces, but few investiga* Present address: National Chemical Laboratory for Industry, Yatabe-machi, Tsukuba-gun, lbaraki 305, Japan.
0039-6028/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L842
E. Miyazaki et al. / Photoemission study of NO on P d ( l l l )
tions using synchrotron radiation as an exciting probe have been reported [3,131. The presence of the unpaired electron in N O enhances its chemical reactivity with solid surfaces, leading to a decrease in the dissociation energy of the molecule as compared with those of CO and N 2. The N O / P d chemisorption system has been investigated by Conrad et al. [1] using the conventional UPS method, however, some ambiguities in photoemission data from the adsorbed N O seem to remain in the energy positions of the 2~ level which is superimposed with a broad and intense Pd4d valence band and in the position of 1~" and 5o levels which mostly appear in the same energy range [14,15]. These ambiguities, in particular the former, can be removed by using synchrotron radiation as the excitation source and by choosing excitation energies where the emission from the substrate valence band is suppressed relative to that from the adsorbate-derived levels. In such metals as Pd and Ag, a strong decrease in the emission occurs at a photon energy of - 80 eV, leading to a minimum at - 1 3 0 eV (Cooper minimum [16-18]). Shirley et al. clearly demonstrated this advantage in studying chemisorption of CO on Pd [18,19], though the 2~r level was not observed in the case of CO. Here we report synchrotron radiation photoemission results for the N O / P d system using photon energies between 30 and 130 eV where the occupied 2~" level in the chemisorbed N O was clearly observed near the Fermi level. The angle integrated photoemission measurements were carried out at the Photon Factory of the National Laboratory for High Energy Physics, using a constant deviation monochromator [20] and a cylindrical mirror analyzer (CMA). Throughout the whole experiment, p-polarization light was used and the sample surface was fixed in an orientation of 45 ° off normal to both the incident light and the CMA center axis. The clean surface of Pd(111) was prepared by cycles of Ar ion bombardment and annealing at 350°C. An AES measurement of the surface showed no trace of contaminants, and, the LEED analysis exhibited a sharp (1 × 1) pattern. Fig. 1 illustrates the variation of the photoemission spectra with photon energy for a clean Pd(111) surface and for the surface after exposure to 7 L of N O at room temperature; the relative enhancement in adsorbate sensitivity is dramatic for photon energies of 120-130 eV, where the NO-derived molecular orbitals clearly dominate the spectra. Thus, the relative depression of intensity in the valence region of the substrate for the Pd 4d electrons due to the Cooper minimum is clearly seen for 120-130 eV photons. Fig. 2a shows the difference spectrum for the NO-covered P d ( l l l ) surface where the curve for the clean surface (dotted line in fig. 1 at h~, = 120 eV) has been subtracted from that with N O adsorption (solid line in fig. 1 at h~, = 120 eV). The spectrum consists of three adsorbate-derived peaks, i.e. two large peaks centred at - 9 . 2 and - 1 4 . 6 eV and a broader peak having a maximum at - 2 . 7 eV below the Fermi level (EF). Fig. 2b shows the energy levels of
E. M i y a z a k i et al. / Photoemission study o f N O on P d ( l l l )
L843
5 cr+l~r 4~
2~
t
.................
............
". . . . . . . . . . . . . . . . . .
"""
,\ !2oev
J °ev
....
30eV -15.0
-i0,0
-5.0
0.0 (El)
BINDING ENERGY / eV
Fig. 1. Angle integrated photoemission spectra of a clean (dotted curves) and a NO-covered Pd(111) (solid curves) surface at various photon energies. Enhancement of the NO-derived feature is clearly seen at -2.7, -9.2, and - 14.6 eV for h 1,= 120-130 eV.
gaseous N O o b t a i n e d b y Edqvist et al. using high-resolution H e p h o t o e m i s s i o n s p e c t r o s c o p y [21], where the fine structure seen a r o u n d - 1 7 eV arises from the m u l t i - e l e c t r o n processes associated with the u n p a i r e d electron in the 27r orbital. A c o m p a r i s o n of the m e a s u r e d spectra in c h e m i s o r b e d p h a s e with that in gas phase leads to the a s s i g n m e n t of the p e a k s at - 2.7, - 9.2 a n d - 14.6 eV to the 2~r, 50 + 1~ a n d 4 a m o l e c u l a r orbitals respectively, of the c h e m i s o r b e d N O . T h e p r e s e n t results on PD(111) are consistent with the previous U P S d a t a o b t a i n e d on a Pd surface b y C o n r a d et al. using H e II r a d i a t i o n [1]. T h e y f o u n d the p e a k s at - 2 . 6 , - 9 . 2 a n d - 1 4 . 6 eV for the 2~r, 50 + l~r a n d 40 orbitals, respectively. The present e x p e r i m e n t clearly c o n f i r m s that, as is p r e d i c t e d theoretically in a previous p a p e r [22], N O is n o t dissociated on the Pd surface at r o o m t e m p e r a t u r e , since no p e a k or s h o u l d e r was o b s e r v e d n e a r - 5.5 eV which is the characteristic energy of a d i s s o c i a t e d a t o m i c nitrogen or oxygen derived 2p orbital. This c o n t r a s t s with the N O / N i system where there
L844
E. Miyazaki et al. / Photoemission study of NO on P d ( l l l ) a)
." .,'_.
ej
.'"
•".. £ ,.,...?. z"
.
/
,.. "-,.
:.-". -.
-
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-15,0
-i0.0
-5,0
0,0 (Ef)
b)
5c~
4or
l~r I
2~r I
-20
BINDING ENERGY / eV Fig. 2. (a) The difference spectrum at 120 eV. The composite peak. 5o + l~r, is deconvoluted using Gaussian distribution functions with asymmetric factors (solid lines). (b) Energy levels of the gaseous N O molecule obtained by high resolution He photoemission spectroscopy [20].
is a clear indication of a peak near - 5 . 5 eV at 300 K [23]. Other marked features in the present data are a clearly distinguishable 27r peak with large peak width and the shoulder observed on the lower binding energy side of the 5o + l~r mixed peak. Furthermore, it should be noted that the 4o peak is relatively intensified as photon energy increases. Such an enhancement of the 4o peak with increasing photon energies was also reported for chemisorbed CO [24], suggesting that the relative photoemission cross section of the o-type orbitals is increased for chemisorbed CO and NO molecules, as compared to ~r-type orbitals, at high photon energies. In order to clarify the features of the shoulder appearing on the lower energy side of the - 9 eV peak, a curve-fitting technique using Gaussian distribution functions with asymmetric factors [25] was employed. The peak width at half maximum of the 4o peak is almost identical with that of the 5o + l~r peak, suggesting that the o-type orbital seems to contribute mainly to the mixed peak. Thus, the 4o peak shape has been used as a fit function in the present analysis. The asymmetric factors were essential because the 4o and 5o peaks exhibit tails for higher binding energies. The results of our analysis is shown by the solid curves in fig. 2. The
E. Miyazaki et al. / Photoemission study of N O on Pd(l l l)
L845
peak at - 8 eV thus discriminated from the composite peak m a y be assigned to the l~r orbital of molecular NO. The characteristic feature of the o orbitals of chemisorbed N O is that the multiplet effect is less p r o n o u n c e d than in the gas phase electron emission, since (1) the outermost 2~r state is strongly delocalized due to the interaction with the metal electrons and (2) the effect of polarization for different spin states is weakened. The energy of the separated l~r peak is close to the lowest ionization energy in the let-band of the gas phase NO. This implies, introducing the work function correction of 5.2 eV, that the relaxation shift is 2.1 eV and the bonding shift is smaller than 0.2 eV for the lower lying 50 orbital, and that the l~r orbital is slighly instabilized. On the other hand, the 2Tr level of chemisorbed N O which has a b r o a d distribution at - 2 . 7 to - 3 . 9 eV shows a significant b o n d i n g shift of 0.7-1.9 eV. The n o n - b o n d i n g character of the 40, 50 and l~r orbitals is essentially consistent with the theoretical and experimental studies of the N O / N i system by Batra and Brundle [2]. In a spin-unrestricted molecular orbital description, the 2~r state of N O is given by the combination of a singly occupied up-spin level located below E F and an unoccupied down-spin level above E v. Thus the chemisorption b o n d i n g by a charge transfer mechanism can occur through electron donation from the 2~r up-spin level to the metal and a simultaneous back-donation into the down-spin 2~r state from the metal and as a result a net charge flow can occur from the metal to the N O [2]. The essential difference in chemisorption character of N O from that of C O where the 50 orbital shows a large bonding shift is due to the unpaired electron in the 2~r orbital of N O and the difference in spatial distribution of the 5o orbitals in N O and CO. Finally, the slight destabilization of the l~r level due to the N O chemisorption can be attributed to the increase in electron C o u l o m b repulsion arising from the increase occupation of the ~r-symmetry orbitals. The authors thank Professor D.E. Beck of the University of WisconsinMilwaukee for reading the manuscript and for valuable suggestions.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
H. Conrad, G. Ertl, J. Kiippers and E.E. Latta, Surface Sci. 65 (1977) 235. I.P. Batra and C.R. Brundle, Surface Sci. 57 (1976) 12. J. Kanski and T.N. Rhodin, Surface Sci. 65 (1977) 63. S. Tatarenko, M. Alnot and R. Ducros, Surface Sci. 163 (1985) 249. D.E. Ibbotson, T.S. Wittrig and W.H. Weinberg, Surface Sci. 110 (1981) 294. M.J. Breitschafter, E. Umbach and D. Menzel, Surface Sci. 109 (1981) 495. M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 136 (1984) 285. L.A. DeLouise and N. Winograd, Surface Sci. 159 (1985) 199. P.J. Goddard, J. West and R.M. Lambert, Surface Sci. 71 (1978) 447. F.P. Netzer and T.E. Madey, Surface Sci. 110 (1981) 251.
L846 [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
[21] [22] [23] [24] [25]
E. Miyazaki et al. / Photoemission study of NO on P d ( l l l )
H.D. Schmick and H.W. Wassmuth, Surface Sci. 123 (1982) 471. J.L. Gland and B.A. Sexton, Surface Sci. 94 (1980) 355. E.W. Plummer, Advan. Chem. Phys. 49 (1982) 533. C. Klauber and B.G. Baker, Surface Sci. 121 (1982) L513. D.E. Ibbotson, T.S. Wittrig and W.H. Weinberg, Surface Sci. 121 (1982) L522. J.W. Cooper, Phys. Rev. 128 (1962) 681; Phys. Rev. Letters 13 (1964) 762; U. Fano and J.W. Cooper, Rev. Mod. Phys. 40 (1968) 441. I. Lindau, P. Pianetta and W.E. Spicer, Phys. Letters A57 (1976) 225. D.A. Shirley, in: Chemistry and Physics of Solid Surfaces, Vol. 3, Ed, R. Vanselow (CRC Press, Boca Raton, FL, 1982) p. 43. P.S. Wehner, S.D. Kevan, R.S. Williams, R.F. Davis and D.A. Shirley, Chem. Phys. Letters 57 (1978) 334. T. Miyahara, S. Suzuki, T. Hanyu, H. Kato, K. Naito, H. Fukutani, I. Nagakura, H. Sugawara, S. Nakai, T. lshii, H. Noda, T. Namioka and T. Sasaki, Japan. J. Appl. Phys. 24 (1985) 293. O. Edqvist, L. ~,sbrink and E. Lindholm, Z. Naturforsch 26a (1971) 1407. E. Miyazaki and I. Yasumori, Surface Sci. 57 (1976) 755. A. Schichl and N. R~Ssch, Surface Sci. 137 (1984) 261. T. Gustafsson, E.W. Plummer, D.E. Eastman and J.L. Freeoufy, Solid State Commun. 17 (1975) 391. G.K. Wertheim, J. Electron Spectrosc. Related Phenomena 6 (1975) 239.
L841
S U R F A C E S C I E N C E LETTERS P H O T O E M I S S I O N S T U D Y OF N O C H E M I S O R P T I O N O N Pd (111) U S I N G S Y N C H R O T R O N RADIATION WITH ENERGY OF 3 0 - 1 3 0 eV Eizo M I Y A Z A K ! , Isao K O J I M A * , Masahiro ORITA, Kiyotaka SAWA, Noriaki S A N A D A Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan
Tsuneaki M I Y A H A R A and Hiroo K A T O Photon Factory, National Laboratoryfor High Energy Physics, Oho-machi, Tsukuba-gun, Ibaraki 305, Japan Received 18 March 1986; accepted for publication 8 July 1986
A synchrotron radiation study of NO chemisorption on P d ( l l l ) was carried out at room temperature in the hv = 30-130 eV range which covers the "Cooper minimum" of Pd4d electrons. The relative enhancement in photoemission sensitivity of the NO-derived 40, 50 + Dr and 2~r molecular orbitals was dramatic at h v = 1 2 0 - 1 3 0 eV, and NO was confirmed to chemisorb in molecular form on the surface at room temperature. The bonding shifts of the 50 and lrr orbitals were found to be smaller than 0.2 eV. The 2~ level showed a si'~nificant bonding shift of 0.7-1.9 eV, suggesting that the unpaired electron in the 2~- orbital is mainly responsible for the chemisorption bonding through a charge transfer mechanism; i.e., electron donation from the 2rr up-spin level into the metal and simultaneous back-donation into the down-spin 2~r state from the metal.
Chemisorption of diatomic molecules such as NO, CO and N 2 on a solid surface has been studied extensively because of interest both in the similarity of valence levels of these molecules and in the difference in electron occupation of the 27r antibonding orbitals, i.e. the 2~r molecular orbital is singly occupied for NO, whereas it is vacant for CO and N 2. Further, the N O molecule is known to be adsorbed more stably on Pd metal as compared with other transition d-metals, i.e. on Pd, N O exists in molecular form at room temperature and the dissociation of the adsorbed N O occurs at a higher temperature. Many techniques for surface analysis such as UPS [1-6], XPS [4-8], AES [6-9], L E E D [1,5-11] and EELS [12] have been applied to the investigation of the interaction of N O with metal surfaces, but few investiga* Present address: National Chemical Laboratory for Industry, Yatabe-machi, Tsukuba-gun, lbaraki 305, Japan.
0039-6028/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L842
E. Miyazaki et al. / Photoemission study of NO on P d ( l l l )
tions using synchrotron radiation as an exciting probe have been reported [3,131. The presence of the unpaired electron in N O enhances its chemical reactivity with solid surfaces, leading to a decrease in the dissociation energy of the molecule as compared with those of CO and N 2. The N O / P d chemisorption system has been investigated by Conrad et al. [1] using the conventional UPS method, however, some ambiguities in photoemission data from the adsorbed N O seem to remain in the energy positions of the 2~ level which is superimposed with a broad and intense Pd4d valence band and in the position of 1~" and 5o levels which mostly appear in the same energy range [14,15]. These ambiguities, in particular the former, can be removed by using synchrotron radiation as the excitation source and by choosing excitation energies where the emission from the substrate valence band is suppressed relative to that from the adsorbate-derived levels. In such metals as Pd and Ag, a strong decrease in the emission occurs at a photon energy of - 80 eV, leading to a minimum at - 1 3 0 eV (Cooper minimum [16-18]). Shirley et al. clearly demonstrated this advantage in studying chemisorption of CO on Pd [18,19], though the 2~r level was not observed in the case of CO. Here we report synchrotron radiation photoemission results for the N O / P d system using photon energies between 30 and 130 eV where the occupied 2~" level in the chemisorbed N O was clearly observed near the Fermi level. The angle integrated photoemission measurements were carried out at the Photon Factory of the National Laboratory for High Energy Physics, using a constant deviation monochromator [20] and a cylindrical mirror analyzer (CMA). Throughout the whole experiment, p-polarization light was used and the sample surface was fixed in an orientation of 45 ° off normal to both the incident light and the CMA center axis. The clean surface of Pd(111) was prepared by cycles of Ar ion bombardment and annealing at 350°C. An AES measurement of the surface showed no trace of contaminants, and, the LEED analysis exhibited a sharp (1 × 1) pattern. Fig. 1 illustrates the variation of the photoemission spectra with photon energy for a clean Pd(111) surface and for the surface after exposure to 7 L of N O at room temperature; the relative enhancement in adsorbate sensitivity is dramatic for photon energies of 120-130 eV, where the NO-derived molecular orbitals clearly dominate the spectra. Thus, the relative depression of intensity in the valence region of the substrate for the Pd 4d electrons due to the Cooper minimum is clearly seen for 120-130 eV photons. Fig. 2a shows the difference spectrum for the NO-covered P d ( l l l ) surface where the curve for the clean surface (dotted line in fig. 1 at h~, = 120 eV) has been subtracted from that with N O adsorption (solid line in fig. 1 at h~, = 120 eV). The spectrum consists of three adsorbate-derived peaks, i.e. two large peaks centred at - 9 . 2 and - 1 4 . 6 eV and a broader peak having a maximum at - 2 . 7 eV below the Fermi level (EF). Fig. 2b shows the energy levels of
E. M i y a z a k i et al. / Photoemission study o f N O on P d ( l l l )
L843
5 cr+l~r 4~
2~
t
.................
............
". . . . . . . . . . . . . . . . . .
"""
,\ !2oev
J °ev
....
30eV -15.0
-i0,0
-5.0
0.0 (El)
BINDING ENERGY / eV
Fig. 1. Angle integrated photoemission spectra of a clean (dotted curves) and a NO-covered Pd(111) (solid curves) surface at various photon energies. Enhancement of the NO-derived feature is clearly seen at -2.7, -9.2, and - 14.6 eV for h 1,= 120-130 eV.
gaseous N O o b t a i n e d b y Edqvist et al. using high-resolution H e p h o t o e m i s s i o n s p e c t r o s c o p y [21], where the fine structure seen a r o u n d - 1 7 eV arises from the m u l t i - e l e c t r o n processes associated with the u n p a i r e d electron in the 27r orbital. A c o m p a r i s o n of the m e a s u r e d spectra in c h e m i s o r b e d p h a s e with that in gas phase leads to the a s s i g n m e n t of the p e a k s at - 2.7, - 9.2 a n d - 14.6 eV to the 2~r, 50 + 1~ a n d 4 a m o l e c u l a r orbitals respectively, of the c h e m i s o r b e d N O . T h e p r e s e n t results on PD(111) are consistent with the previous U P S d a t a o b t a i n e d on a Pd surface b y C o n r a d et al. using H e II r a d i a t i o n [1]. T h e y f o u n d the p e a k s at - 2 . 6 , - 9 . 2 a n d - 1 4 . 6 eV for the 2~r, 50 + l~r a n d 40 orbitals, respectively. The present e x p e r i m e n t clearly c o n f i r m s that, as is p r e d i c t e d theoretically in a previous p a p e r [22], N O is n o t dissociated on the Pd surface at r o o m t e m p e r a t u r e , since no p e a k or s h o u l d e r was o b s e r v e d n e a r - 5.5 eV which is the characteristic energy of a d i s s o c i a t e d a t o m i c nitrogen or oxygen derived 2p orbital. This c o n t r a s t s with the N O / N i system where there
L844
E. Miyazaki et al. / Photoemission study of NO on P d ( l l l ) a)
." .,'_.
ej
.'"
•".. £ ,.,...?. z"
.
/
,.. "-,.
:.-". -.
-
Be
-15,0
-i0.0
-5,0
0,0 (Ef)
b)
5c~
4or
l~r I
2~r I
-20
BINDING ENERGY / eV Fig. 2. (a) The difference spectrum at 120 eV. The composite peak. 5o + l~r, is deconvoluted using Gaussian distribution functions with asymmetric factors (solid lines). (b) Energy levels of the gaseous N O molecule obtained by high resolution He photoemission spectroscopy [20].
is a clear indication of a peak near - 5 . 5 eV at 300 K [23]. Other marked features in the present data are a clearly distinguishable 27r peak with large peak width and the shoulder observed on the lower binding energy side of the 5o + l~r mixed peak. Furthermore, it should be noted that the 4o peak is relatively intensified as photon energy increases. Such an enhancement of the 4o peak with increasing photon energies was also reported for chemisorbed CO [24], suggesting that the relative photoemission cross section of the o-type orbitals is increased for chemisorbed CO and NO molecules, as compared to ~r-type orbitals, at high photon energies. In order to clarify the features of the shoulder appearing on the lower energy side of the - 9 eV peak, a curve-fitting technique using Gaussian distribution functions with asymmetric factors [25] was employed. The peak width at half maximum of the 4o peak is almost identical with that of the 5o + l~r peak, suggesting that the o-type orbital seems to contribute mainly to the mixed peak. Thus, the 4o peak shape has been used as a fit function in the present analysis. The asymmetric factors were essential because the 4o and 5o peaks exhibit tails for higher binding energies. The results of our analysis is shown by the solid curves in fig. 2. The
E. Miyazaki et al. / Photoemission study of N O on Pd(l l l)
L845
peak at - 8 eV thus discriminated from the composite peak m a y be assigned to the l~r orbital of molecular NO. The characteristic feature of the o orbitals of chemisorbed N O is that the multiplet effect is less p r o n o u n c e d than in the gas phase electron emission, since (1) the outermost 2~r state is strongly delocalized due to the interaction with the metal electrons and (2) the effect of polarization for different spin states is weakened. The energy of the separated l~r peak is close to the lowest ionization energy in the let-band of the gas phase NO. This implies, introducing the work function correction of 5.2 eV, that the relaxation shift is 2.1 eV and the bonding shift is smaller than 0.2 eV for the lower lying 50 orbital, and that the l~r orbital is slighly instabilized. On the other hand, the 2Tr level of chemisorbed N O which has a b r o a d distribution at - 2 . 7 to - 3 . 9 eV shows a significant b o n d i n g shift of 0.7-1.9 eV. The n o n - b o n d i n g character of the 40, 50 and l~r orbitals is essentially consistent with the theoretical and experimental studies of the N O / N i system by Batra and Brundle [2]. In a spin-unrestricted molecular orbital description, the 2~r state of N O is given by the combination of a singly occupied up-spin level located below E F and an unoccupied down-spin level above E v. Thus the chemisorption b o n d i n g by a charge transfer mechanism can occur through electron donation from the 2~r up-spin level to the metal and a simultaneous back-donation into the down-spin 2~r state from the metal and as a result a net charge flow can occur from the metal to the N O [2]. The essential difference in chemisorption character of N O from that of C O where the 50 orbital shows a large bonding shift is due to the unpaired electron in the 2~r orbital of N O and the difference in spatial distribution of the 5o orbitals in N O and CO. Finally, the slight destabilization of the l~r level due to the N O chemisorption can be attributed to the increase in electron C o u l o m b repulsion arising from the increase occupation of the ~r-symmetry orbitals. The authors thank Professor D.E. Beck of the University of WisconsinMilwaukee for reading the manuscript and for valuable suggestions.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
H. Conrad, G. Ertl, J. Kiippers and E.E. Latta, Surface Sci. 65 (1977) 235. I.P. Batra and C.R. Brundle, Surface Sci. 57 (1976) 12. J. Kanski and T.N. Rhodin, Surface Sci. 65 (1977) 63. S. Tatarenko, M. Alnot and R. Ducros, Surface Sci. 163 (1985) 249. D.E. Ibbotson, T.S. Wittrig and W.H. Weinberg, Surface Sci. 110 (1981) 294. M.J. Breitschafter, E. Umbach and D. Menzel, Surface Sci. 109 (1981) 495. M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 136 (1984) 285. L.A. DeLouise and N. Winograd, Surface Sci. 159 (1985) 199. P.J. Goddard, J. West and R.M. Lambert, Surface Sci. 71 (1978) 447. F.P. Netzer and T.E. Madey, Surface Sci. 110 (1981) 251.
L846 [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
[21] [22] [23] [24] [25]
E. Miyazaki et al. / Photoemission study of NO on P d ( l l l )
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