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Photoemission of backbonding electrons from CO chemisorbed on Cu(III)
Volume 84. number 3
CHEMICAL.
PHOTOEMISSION
Sk
LINDGREN,
OF BACKBONDBUG
J. PAULand
PHYSICS
ELECTRONS
15 December
LETTERS
FROM CO CHEMISORBED
1981
ON Cu(l11)
L. WALLDeN
Received 12 August 1981
Angle-resolved photoelectron energy spectra recorded III the normal due&on for a CO-exposed Cu( 111) sample show a peak at the Femu cdgc for &h monolayer coverages We assomte tti peak v&h a filled part of a resonance-broadened CO 2n IeveL
Wlthm
the commonly
chemlsorptlon, al mvolves
mamIy
and the lowest
accepted
the bmdmg
picture
of CO
of the molecule
the highest occupied
to a met-
level, 50,
empty level, 2~. The mteraction
with
the metal modifies
the orbltals and there is a charge transfer which IS described as a donation of 50 electrons to the substrate and, posnbly, a back donation of metal valence electrons resulting in some occupancy of 2n derived states [l-4] _ The highest ftied levels of chemlsorbed CO have been studled extenswely by photoelectron spectroscopy. A good quahtatwe understandmg has emerged of the excitation spectra associated with these levels when CO IS adsorbed on different metals [5]. For CO strongly cherrusorbed on a transitIon metal the photoemisslon spectra generally show two peaks. One 1s due to the eJection of 4u electrons wMe both 5a and In electrons contribute to a peak at somewhat lower bmding energy. For CO on Cu, a case of weaker chemlsorption, a thud peak has been observed and as-
cnbed
to a shake-up
Regardmg
process
[6,7].
the 21r level, not much has been l-t
from the photoemisslon measurements. The lack of adsorbate-mduced structure which can be unamblguously interpreted in terms of a 2a enussion either means that the level is empty, the emission mtenslty very weak or that the interaction with the substrate has broadened the level to an extent that the 2s derived states are spread out over most of the valence d-band range of energes for a transition-metal substrate. In the latter cases it will obviously be difficult 0 009-2614/81/0000_0000/Zi
02.75
0 1981
North-Holland
to identify adsorbate-induced changes of the electronic structure smce this must be baxd on adsorbate-mduced changes of photoemission spectra in a range of energes where there is strong emission from the substrate d bands. The difficulty arises from the fact that this background emission may be strongly modified for reasons not directly related to electromc structure changes. The spectra may for lose some of their angle-dependent characteristics as a result of scattenng by the adsorbate of
example
electrons
photoexcited
in the bulk of the substrate
PI Here we report on the observation of an emission peak at the Fenm edge m angle-resolved photoelectron energy spectra from CO-covered Cu( 111). We associate tlus peak with the ermssion of electrons out of a fiUed part of the CO 27r resonance. The peak is observed only at Hugh CO coverages. We suggest that it IS associdted with molecules having different adsorption sites than the on-top sites which are the likely ones at low coverage. The interpretation in the present case Is more straightforwar
d than for a transi-
tlon metal smce the substrate background emission is weak in the sp baud initial state energy range between the Fermi level and the upper 3d band edge 2 eV below it. We record angle-resolved photoelectron energy spectra from liquid-nitrogen-cooled Cu(l1 I) exposed
to CO using LEED and work-function change measurements to characterize the sample. Unpolarized light from an inert-gas discharge strikes the sampIe at
42” incidence. Our cylindrical mirror electron
eneq?~ analyzer accepts an emission cone having 3 2* opcning angle. The LEED pattern obtains addition31 spots after el.2 L CO exposure (1 L = IO-6 Tort s) due to the formation of 3n ordered 3y2 X 31j2, R 30” uvcrlnyer. For this overlayer the coverage 6) is one third if 0 is defined as the fraction of adsorbed CO molecules per surf&e Cu atom. At around this coverage the work function has a minimum value which is 0.45 cV b&w that of Cu(t 11). At saturation coverage which rcquircs 4--S L gas exposure the work function is nmnnd O-3 eV above the minimum. A comparison with previous work on Cu(l I 1) [9f and Cu( 100) [ I fill sltgpsts
that our saturation
coverage
is in the
0.4--0.5. Considcr3blc clianges of UV photoemission spcctra XC cvirlent 3~ srnaIl CO cavernges (fig. 1). The spcctrn were recorded in the direction normal to the s:ttnplc at Jw = i&8 cV, A CO exposure of *OS t is range
Volume 84, number 3
CtlEMICAL
[ 111 P-0. Gartland and B. Slagsvold, Phys. Rev. 812 (1975) 4047. [ 121 J.F. Janak, A.R. Wilbams and V.L. Moruzz~, Phys Rev. Bll(1975) 1522. (131 S.k Ltidgren and L. Wtidin, Mater. Sm. Eng. 42 (1980) 127. [ 141 K. Horn and J. Pntchard, Surface SCL 55 (1976) 701. [ 151 J. Pritchard, T. Cattenck and R.K. Gupta, Surface Sn 53 (1975) 1.
490
PHYSICS
LETTERS
15 December
[ 161 [ 171 [ 181 [ 191
R Ryberg, pnvate commumcatlon. S Andersson, Surface Sn 89 (1979) S. Andersson and J. Pendry, J. Phys. EN.J. Persson and R. Ryberg, Sohd (1980) 613. [201 S. Andersson, Sohd State Commun.
[Zl]
H. Froilzhem,
AppL Phys
H. Hopster,
13 (1977)
147.
1981
477. Cl3 (1980) 3547. State Commur~ 36 21 (1977)
75.
H. lbach and S. Lcwald_
CHEMICAL.
PHOTOEMISSION
Sk
LINDGREN,
OF BACKBONDBUG
J. PAULand
PHYSICS
ELECTRONS
15 December
LETTERS
FROM CO CHEMISORBED
1981
ON Cu(l11)
L. WALLDeN
Received 12 August 1981
Angle-resolved photoelectron energy spectra recorded III the normal due&on for a CO-exposed Cu( 111) sample show a peak at the Femu cdgc for &h monolayer coverages We assomte tti peak v&h a filled part of a resonance-broadened CO 2n IeveL
Wlthm
the commonly
chemlsorptlon, al mvolves
mamIy
and the lowest
accepted
the bmdmg
picture
of CO
of the molecule
the highest occupied
to a met-
level, 50,
empty level, 2~. The mteraction
with
the metal modifies
the orbltals and there is a charge transfer which IS described as a donation of 50 electrons to the substrate and, posnbly, a back donation of metal valence electrons resulting in some occupancy of 2n derived states [l-4] _ The highest ftied levels of chemlsorbed CO have been studled extenswely by photoelectron spectroscopy. A good quahtatwe understandmg has emerged of the excitation spectra associated with these levels when CO IS adsorbed on different metals [5]. For CO strongly cherrusorbed on a transitIon metal the photoemisslon spectra generally show two peaks. One 1s due to the eJection of 4u electrons wMe both 5a and In electrons contribute to a peak at somewhat lower bmding energy. For CO on Cu, a case of weaker chemlsorption, a thud peak has been observed and as-
cnbed
to a shake-up
Regardmg
process
[6,7].
the 21r level, not much has been l-t
from the photoemisslon measurements. The lack of adsorbate-mduced structure which can be unamblguously interpreted in terms of a 2a enussion either means that the level is empty, the emission mtenslty very weak or that the interaction with the substrate has broadened the level to an extent that the 2s derived states are spread out over most of the valence d-band range of energes for a transition-metal substrate. In the latter cases it will obviously be difficult 0 009-2614/81/0000_0000/Zi
02.75
0 1981
North-Holland
to identify adsorbate-induced changes of the electronic structure smce this must be baxd on adsorbate-mduced changes of photoemission spectra in a range of energes where there is strong emission from the substrate d bands. The difficulty arises from the fact that this background emission may be strongly modified for reasons not directly related to electromc structure changes. The spectra may for lose some of their angle-dependent characteristics as a result of scattenng by the adsorbate of
example
electrons
photoexcited
in the bulk of the substrate
PI Here we report on the observation of an emission peak at the Fenm edge m angle-resolved photoelectron energy spectra from CO-covered Cu( 111). We associate tlus peak with the ermssion of electrons out of a fiUed part of the CO 27r resonance. The peak is observed only at Hugh CO coverages. We suggest that it IS associdted with molecules having different adsorption sites than the on-top sites which are the likely ones at low coverage. The interpretation in the present case Is more straightforwar
d than for a transi-
tlon metal smce the substrate background emission is weak in the sp baud initial state energy range between the Fermi level and the upper 3d band edge 2 eV below it. We record angle-resolved photoelectron energy spectra from liquid-nitrogen-cooled Cu(l1 I) exposed
to CO using LEED and work-function change measurements to characterize the sample. Unpolarized light from an inert-gas discharge strikes the sampIe at
42” incidence. Our cylindrical mirror electron
eneq?~ analyzer accepts an emission cone having 3 2* opcning angle. The LEED pattern obtains addition31 spots after el.2 L CO exposure (1 L = IO-6 Tort s) due to the formation of 3n ordered 3y2 X 31j2, R 30” uvcrlnyer. For this overlayer the coverage 6) is one third if 0 is defined as the fraction of adsorbed CO molecules per surf&e Cu atom. At around this coverage the work function has a minimum value which is 0.45 cV b&w that of Cu(t 11). At saturation coverage which rcquircs 4--S L gas exposure the work function is nmnnd O-3 eV above the minimum. A comparison with previous work on Cu(l I 1) [9f and Cu( 100) [ I fill sltgpsts
that our saturation
coverage
is in the
0.4--0.5. Considcr3blc clianges of UV photoemission spcctra XC cvirlent 3~ srnaIl CO cavernges (fig. 1). The spcctrn were recorded in the direction normal to the s:ttnplc at Jw = i&8 cV, A CO exposure of *OS t is range
Volume 84, number 3
CtlEMICAL
[ 111 P-0. Gartland and B. Slagsvold, Phys. Rev. 812 (1975) 4047. [ 121 J.F. Janak, A.R. Wilbams and V.L. Moruzz~, Phys Rev. Bll(1975) 1522. (131 S.k Ltidgren and L. Wtidin, Mater. Sm. Eng. 42 (1980) 127. [ 141 K. Horn and J. Pntchard, Surface SCL 55 (1976) 701. [ 151 J. Pritchard, T. Cattenck and R.K. Gupta, Surface Sn 53 (1975) 1.
490
PHYSICS
LETTERS
15 December
[ 161 [ 171 [ 181 [ 191
R Ryberg, pnvate commumcatlon. S Andersson, Surface Sn 89 (1979) S. Andersson and J. Pendry, J. Phys. EN.J. Persson and R. Ryberg, Sohd (1980) 613. [201 S. Andersson, Sohd State Commun.
[Zl]
H. Froilzhem,
AppL Phys
H. Hopster,
13 (1977)
147.
1981
477. Cl3 (1980) 3547. State Commur~ 36 21 (1977)
75.
H. lbach and S. Lcwald_