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Chemical bonding effects in the inverse photoemission spectra of chemisorbed CO
Volume
118, number3
CHEMICAL
4ZHEMIcAL BONDING EFFECl-S IN THE INVEBSE PHOTOEMISSION P.S. BAGUS,
K_ HERMANN
SPECl-RA
OF CHEMISOBBED
26 July
1985
CO
’
San Jose, Cl4 95193. USA
IBM Research bbcramy,
Ph. AVOURIS,
PHYSICS LETTERS
A R. ROSS1 2
IBM Warson Raearch
Cester,
Yorktown
He&s.
NY 10598. USA
and
K_C. PRINCE Fritz -H&r
Im~rlut der Max - Planck
- GeselLrchaf$ Faradayweg
4 - 6, D - I OGil Berlin 33, Wear Gemmy
Received 5 February 1985
The mverse photoemission spectra of CO chemisorbcd on several transition mcral surfaces shows a broad, = 3 eV full-width at half-maximum, resonance aEsociated with Lhe co 2 T* level. For CO/Cu(l00), there is a doublet rather than a single broad peak We interpret Ihe cheousorbed CO structure as being due to chemical bonding errecu in rhe anionic system An electron may be added into one of LWOdistinct kinds of orbit&. In Lhe first kind, associated with lower energy ioruc stales, Lhe orbital is bonding between CO(2T*)
and Lhesurface conduction band; in the second kind, the orbital is anti-bonding
is based on ab initio Hat-tree-Fock
wavefunctions for Cu,CO
The negative ion states of adsorbates have been receiving considerable attention largely because of developments in inverse photoemission spectroscopy or BIS (Bremsstrahlung isochromat spectroscopy) which aUow the direct expenmental study of these anion states [l] , Such states are believed to play very important roles in a variety of surface dynamical processes. Chemisorbed CO is being studed as a prototype system in order to develop an understanding of the mechanisms responsible for the spectra. For example, there have been observations of well-defined 2a-derived features in the BIS spectra of CO/Nl(lll) [2], CO/Cu(llO) 131, and CO/l?d(lll) [4] _In general, a broad feature centered at =3 eV above the Fermi level,EF , with a full-width at half-maximum, fwhm, also of a3 eV has been found. 1 Permanent address- Institut fclr Theorebsche Physik, TU Claus&al, D3392 ClausthaI-ZelIerfeld, West Germany.’ ’ Pcrmanenr address: Chemistry DepmenL University of fiMnecticUL, Storrs, CT 96268, USA
Tbis interpretation
clusters.
In this paper, we report theoretical molecular orbital studies for negative ion states of CuCO, CusCO, and CU,~CO clusters; the clusters are all chosen to model on-top site, normal-mcidence, C-end down chernisorption of CO on Cu(100). We consider negative ion states for which the orbital of the added electron has substantial amounts of CO 2n* character. This C to 0 anti-bonding orbital is the lowest-lying unoccupled orbital of free CO. The cluster wavefunctions give strong evidence that there are two classes of negative ion states which involve significmt mixtures of CO 2s* and metal (conduction band) 4plr character. These classes of states differ in the nature of their interaction between CO and the Cu surface; the lower-iying class of states is bonding between CO and Cu wbiIe the higher lying class is anti-bonding. The calculated separation of the bonding and anti-bonding states in CuCO and Cu,,CO is ==2 eV. Thus, this different bonding behavior may account, in large measure, for the broad fkhm observed for the Zn-derived features in the BIS 311
Vohme
118, number 3
CHEhfICAL
PHYSICS
spectra of chemisorbed CO 1241. The broad peak observed may contain unresolved contributions from these two classes of states_ Experimental BIS spectra [5] for CO/&(100 show a clear doublet due to CO; at normal eletion incidence to the surface, the components are separated by 1.3 eV. This doublet occurs because the contributions of the bonding and antibonding states cm be resolved_ The fwhm of each mmponent of the CO/Cu(lOO) doublet [5] is ~15 eV or about one-half of the fwhm observed when there is no resolved doublet. The case of CuCO provides a very simple model for the negative ion states formed during the inverse photoemission process for CO chemisorbed on a Cu surfacz. In particular, this system has only two low-lying orbit& of ITsymmetry which can accept an electron_ These two n orbitals arise from CO- __.ln427r1 and Cu__.4s14p1 _ The nature of the changes which these orbitals undergo through their interaction with the other unit are clearly shown by the character of the negative ion states of CuCO-. We consider the propeties of self-consistent field, SCF, wavefunctions for this cluster. The initi, neutral state of CuCO is 2X+ wth the configuration . ..3da23dsr43d644s~1...5~21n4. where the perturbed orbit& of Cu and CO are represented by their separated unit notation_ The nature of the interaction of Cu with CO has been discussed at length elsewhere [6] ; two features of the interaction were stressed_ (1) The overlap and consequent repulsion between the Cu 4s and the CO 50 orbitals is large; the polarization and hybridization of the Cu 4s orbital away from CO reduces this repulsion [6,7]. And (2), the Cu ds donation to CO 2~7~ is the main contriiution to the dative bonding. The negative ion states of CuCO- are formed by adding a T electron to CuCO into an orbital denoted wr. The configuration of cuco1.5
. ..3d023d~43d644spu1wr1...5c?ln4. This configuration has llI and 311 spin couplings. The average of configuration, fq(311) + $*(lll), is used for the SCF calculation; the spin coupling of the Cu open 4spu shell with the negative ion, yr~, orbitd is a property of CuCO which has no connection with CO/CU. Wavefunctions for the two lowest states of this configuration have been obtained. The Cu-C and C-O dis312
LEITERS
26 July 1985
tances are R(Cu-C) = 3.70 bohr = 196 A andR(C-C ~2.15 bohr. These are the dismces used in earlier work on neutral CuCO 163 and are close to the bond distances for on-top site adsorption of CO/Cu(lOO) [g] . The SCF wavefunctions are based on & expansic of the orbit& with con&acted Gaussian-type basis set and symmetry and equivalence restrictions are used; details are given elsewheie [6] _ The two states of CuCO- dissociate at large R&u-C) to Cu- and CO and to Cu and CO-. However at R(Cu-C) = 3.70 bolu, there is a considerable mixing of Cu- and CO- character in both states. The general nature of the wr negative ion orbital will be considered first and then the energetics. A Mulliken population analysis of the YITorbital is used to describe its general character. AlthougJ~ a pops lation analysis may have serious artifacts [7], it is appropriate for qualitative features. The gross populations for Cu and CO and the overlap populations between Cu and CO and between C and 0 are given for the VITorbitals of both CuCO- states in table 1. For Cu, the gross population is divided into pn and drr character_ Fo; state 1, the lower of the CuCO- states, the vn orbital is 75% CO and 25% Cu. The C-O overla population, -0.5, clearly shows the C-O antibonding 27r*, character of this orbital. The orbital is, however, bonding between Cu and CO as shown by the CuXO overlap population. The VT orbital of the higher, state 2, CuCO- state is also strongly hybndized, 67% Cu ant 33% CO 2rr*_ However, it is anti-bonding between Cu and CO; the Cu-CO overlap population is -0.3 _In both cases. the Cu character of the orbital is almost en tirely of valence pn character; the mixture of dn is ver small. These populations give clear evidence that there is significant chemistry occurring for the negative Ion states of CuCO and by extension for CO/Cu. Because of their signifimnt CO 2~’ character, both states woul lead to CO related BIS resonances. There are two major reservations for the calculated energies of the CuCO- states. The first is that SCF electron affinities, EAs, are, in general, too small compared to experiment [9]. Our calculated SCF-EEA for Cu is -13 eV (average of Cu 3P and lP states) and for CO it is -2.6 eV; both EAs are too negative_ However, the experimental EA(C0) is -15 eV [lo] ; thus, our SCF EA(C0) is not too badly in error. The error for EA(Cu) is compounded by the fact that our Cu basis set has not beem optirmzd to represent the dif-
Volume 11% number 3
(xu?xIcAL
PHYSICS LlzTTERs
26 July 1985
Table 1 Gross end 0yerLappopulations for the orbital of the added elrxfron in the lowest SCF negative ion states of the C!u&O clustas, n = 1.5, atid 13. The electron afliuity. EA. of these states is also given Gross
cu CllCO‘state 1
Cucostate2
CUg0-
P
total P d total
0.67 0.00 0.67
s
0.02 0.13 0.02 0.16
P d to&l cll,~C6 state1
S P d total
m3co
S
state2
P d total
CU,$W state 3
s P d total
co
0.85 0.15 -0.01 1.00 0.13 0.23 0.02 0.38 0.27 064 0.00 0.92
0.75
c-o
0.18
-050
-0.20
-
-
-
0.33
-0 30
-0-33
-
-
-2 20
0.84
0.13
-0.56
-0.61
0.00
0.62
0.00
0.17
0.00
-0.40
-
-
0.08
-0.01
-0.06
-
-
P d total
0.68
0.32
fuse character of this Cu anion. The second reservatiou is that the valence 4s and 4p orbitals of the Cu atom can ghe only a very hmited representation of the empty part of the Cu conduction band. For these reasons,
we will only be concerned with the qualitative character orour cluster energetics. Both of the states of CuCO- are ~bo~d with respect to the energy of
-0.09
0.22
-
-
-
S
state4
EA (ev)
-
0.21 0.44 0.03
w3m-
CU-CO
-
0.22 0.02 0.25
d
OWliIP
-0.28
0.21
-0.96
-250
CUCO; state 1 has an EA of -020 eV end state 2 has an EA of -220 eV. The separation of the two states is 2.00 eV and arises from the change from bonding to anti-bonding character of the wr orbital. We def?.neinteraction energies,Em, by wmparhg the energy of CuCO with the sum of the energies of appro~ately charged Cu and CO units. The specific 313
CHEMICAL PHYSICS LETIERS
Volume 118. number 3 choices arc: cuco,
EINT = E(Cu) + E(C0)
CUCO-,
state 1:
E*
= E(Cu)
+E(CO-)
- E(CuCO),
- E(CuCO-),
state 2: EINr = E(Cu-)
+ E(C0)
- Jz(CuCO-).
> 0, the system is bound end for EINT < 0, For E,, repulsive. The SCF Ena for CuCO is -0.55 ; this repulsive interaction has been explained [6] as being due to the strong Cu 4s to CO 5u repulsion;limitations of the SCF model also contribute [1 1,121. Even though the CuCO cluster does not ~W-IZan attractive interactron between Cu and CO, the differenfor the neutral and anionic states will ces of the E,, indicate the nature of the anion bonding. The Em for CuCO- state 1 is +1.83 eV; it is strongly bound with respect to Cu and CO-. This has three origins. (1) The VITorbital for this state is Cu to CO bonding. (2) Negatively charged CO unit polarizes the Cu charge and interacts positively with the induced Cu dipole [ 131. This is the single atom analogue of a metal image charge induced by CO-. (3) There is some Penetration of the Cu charge by CO- [6] ; this penetration reduces the screening of the Cu nucleus and leads to an attractive Coulomb interaction which reduces the frozen orbital for CuCO- state 2 is -1.49 eV; repulsion. The Em it is 092 e-V more repulsive than CuCO neutral. This is largely because the additional electron is placed in an anti-bonding orbital. The principal features of these low-lying CuCOstates should be reIevant for chemisorbed CO. The low-lying state is bondmg between CO(27r) end Cu(4p~); it is further stabilized by charge penetration and by polarization of the Cu charge. The second state is 2 .O eV higher in energy because, m large part, the additional electron is in an anti-bonding orbital. Part of the difference between these two states is an initial state effect arising from bonding present in the virtual orbit& of neutral CuCO. The low-lying rr virtuals have sigmfic=ant Cu 41x7 and CO ~YT* mixing. The lowest is 65% Cu and Cu-CO bonding while the second virtual is 48% Cu and anti-bonding. The orbital energies, inverse Koopmans’ theorem, are se-ted by 1.8 eV. Thus, a large part of the features of the CuCO- states are due to initial state effects. 314
26 July 1985
We have also examin ed low-lying negative ion state of clusters containing more Cu atoms, Cu,CO and CuI,CO. The Cu13 cluster contains 9 atoms of the first layer and 4 of the second layer of Cu(100); rt 1s denoted Cu13(9,4). The CO molecule is added above the central frost layer atom to form CuI,(9,4)CO. The CUCJ1,4)CO cluster, described elsewhere C6.141, contains only the Cu adsorption site atom m the fnst laye and 4 Cu atoms of the second layer. A pseudopotentia is used to describe the environmental Cu atoms not directly involved in the bonding with CO [ 141. The anion configumtions are formed by adding en electron in a R orbital which is unoccupied in the ground&ate neutral configuration. population analyses for the add tional anion orbital are given in table 1; the Cu contrib tion is s un-uued over all Cu atoms of the cluster_ For CusCO-, the lowest state which has sigruficant 277’ ch acter is shown; it is clearly bonding between Cu end Cc as seen from the positive Cu-CO overlap populations For Cu13CO-, we report results for the four lowest states with au added n electron. The anion orbital of the lowest state, state 1, has no CO character: it corresponds to an electron added to the bottom of the unoccupied STpart of the Cu(lO0) conduction band. State 2 has significant 27r* character and is Cu-CO bonding. Its EA is +021 eV; the CuI, representation of the image charge of CO- is sufficient to make this state stable. States 3 and 4 have some 2~’ character; they are dominantly metal and are anti-bonding between Cu and CO. Their EAs are -096 and -2.50 eV respectively. The Cu-CO bonding and anti-bonding features of the anion states of Cu5C0 and Cu,aCO are consistent with those found with the simpler CuCO. In conclusion, our cluster results show that there is signifkant chemical interaction and mixing between Cu and CO for the anion states of the cluster. The lowest state W&L s@ii%ant CO 27r* character is bonding between Cu and CO and contains a reasonable amount of Cu character. Higher-lying states which have CO 2n* character are anti-bonding between Cu and CO. AU the clusters studied support a quahtative prediction that there will be two classes of anion states the lower-lying bonding between Cu end CO and the higher-lying anti-bonding. Our calculations are consis-
tent with the observation of a doublet in the CO induced BIS spectra for CO/Cu(loO) [S] and explain thF origin
of this doublet.
Volume 118, number 3
c!HF.MICAL PHYSICS LEl-rERS
We wish to acknowledge cussions
with Professor
AM.
useful and stimulating disBradshaw.
Referenoep [l]
V. Dose, W. Albnann, k Goldmann, U. Kolac and J. Rogozik, Fhys. Rev. Letters 52 (1984) 1919, D. Straub end FJ. Himpsel, Phys Rev. Letters 52 (1984)
1922. [2] Th Fauster and FJ_ Hunpsel.Phys Rev. B27 (1983) 1390. [3] J. Roguzitr, H. Scheidt, V. Dose, K.C. prince and A.M. Bradshaw, Surface Sci 145 (1984) L481. [4] P.D. Johnson, DA. Wesner. J.W. Davenport and N.V.
Smith, Phys. Rev. B30 (1984) 4860. [S] J. Rogozik, V. Dose, K.C.Prince, AM. Bradsbaw. P.S. Bagus, K. Hermann and Ph. Avouris, submitted for publication.
26 July 1985
[6] P.S. Bagus, K. Hermauu and C.W. Bausldicher Jr.. J. Chem Phya 81(1984) 1966. [7] P.S. Begus, CJ. Nehn and C.W. Bauschkher Jr.. Phys. Rev B28 (1983) 5423. [8] S. Andersson and J. Pendry, Phys Rev. Letters 43 (1979) 360. [9] HF. Schaefer IU, The electronic ticture of atoms and
molecules (Addison-Wesley,
Reading, 1972).
[ 101 M. Eberhsrdt, L. Langhans, F. Linda and HS. Taylor, Phys. Rev. 173 (1968) 222. [ 111 P.S. Bagus, CJ. Nelin and C.W. Bauschiicher Jr.. J. Vat.
Sci TschnoL A2 (1984) 905. ilZ] W. MiiIltx and P.S. Bagus, to be pubbxhed. 1131 J. Broughton and PS. Bagus, J. Chem. Phys. 77 (1982)
3627. [ 141 PS. Bagus, C.W. Bauschlicher Jr., CJ. Nelin, B.C. Laskowskf and M. Seel, J. Chem. Phys. 81(1984) 3594.
Phys. Rev. Letters,
315
118, number3
CHEMICAL
4ZHEMIcAL BONDING EFFECl-S IN THE INVEBSE PHOTOEMISSION P.S. BAGUS,
K_ HERMANN
SPECl-RA
OF CHEMISOBBED
26 July
1985
CO
’
San Jose, Cl4 95193. USA
IBM Research bbcramy,
Ph. AVOURIS,
PHYSICS LETTERS
A R. ROSS1 2
IBM Warson Raearch
Cester,
Yorktown
He&s.
NY 10598. USA
and
K_C. PRINCE Fritz -H&r
Im~rlut der Max - Planck
- GeselLrchaf$ Faradayweg
4 - 6, D - I OGil Berlin 33, Wear Gemmy
Received 5 February 1985
The mverse photoemission spectra of CO chemisorbcd on several transition mcral surfaces shows a broad, = 3 eV full-width at half-maximum, resonance aEsociated with Lhe co 2 T* level. For CO/Cu(l00), there is a doublet rather than a single broad peak We interpret Ihe cheousorbed CO structure as being due to chemical bonding errecu in rhe anionic system An electron may be added into one of LWOdistinct kinds of orbit&. In Lhe first kind, associated with lower energy ioruc stales, Lhe orbital is bonding between CO(2T*)
and Lhesurface conduction band; in the second kind, the orbital is anti-bonding
is based on ab initio Hat-tree-Fock
wavefunctions for Cu,CO
The negative ion states of adsorbates have been receiving considerable attention largely because of developments in inverse photoemission spectroscopy or BIS (Bremsstrahlung isochromat spectroscopy) which aUow the direct expenmental study of these anion states [l] , Such states are believed to play very important roles in a variety of surface dynamical processes. Chemisorbed CO is being studed as a prototype system in order to develop an understanding of the mechanisms responsible for the spectra. For example, there have been observations of well-defined 2a-derived features in the BIS spectra of CO/Nl(lll) [2], CO/Cu(llO) 131, and CO/l?d(lll) [4] _In general, a broad feature centered at =3 eV above the Fermi level,EF , with a full-width at half-maximum, fwhm, also of a3 eV has been found. 1 Permanent address- Institut fclr Theorebsche Physik, TU Claus&al, D3392 ClausthaI-ZelIerfeld, West Germany.’ ’ Pcrmanenr address: Chemistry DepmenL University of fiMnecticUL, Storrs, CT 96268, USA
Tbis interpretation
clusters.
In this paper, we report theoretical molecular orbital studies for negative ion states of CuCO, CusCO, and CU,~CO clusters; the clusters are all chosen to model on-top site, normal-mcidence, C-end down chernisorption of CO on Cu(100). We consider negative ion states for which the orbital of the added electron has substantial amounts of CO 2n* character. This C to 0 anti-bonding orbital is the lowest-lying unoccupled orbital of free CO. The cluster wavefunctions give strong evidence that there are two classes of negative ion states which involve significmt mixtures of CO 2s* and metal (conduction band) 4plr character. These classes of states differ in the nature of their interaction between CO and the Cu surface; the lower-iying class of states is bonding between CO and Cu wbiIe the higher lying class is anti-bonding. The calculated separation of the bonding and anti-bonding states in CuCO and Cu,,CO is ==2 eV. Thus, this different bonding behavior may account, in large measure, for the broad fkhm observed for the Zn-derived features in the BIS 311
Vohme
118, number 3
CHEhfICAL
PHYSICS
spectra of chemisorbed CO 1241. The broad peak observed may contain unresolved contributions from these two classes of states_ Experimental BIS spectra [5] for CO/&(100 show a clear doublet due to CO; at normal eletion incidence to the surface, the components are separated by 1.3 eV. This doublet occurs because the contributions of the bonding and antibonding states cm be resolved_ The fwhm of each mmponent of the CO/Cu(lOO) doublet [5] is ~15 eV or about one-half of the fwhm observed when there is no resolved doublet. The case of CuCO provides a very simple model for the negative ion states formed during the inverse photoemission process for CO chemisorbed on a Cu surfacz. In particular, this system has only two low-lying orbit& of ITsymmetry which can accept an electron_ These two n orbitals arise from CO- __.ln427r1 and Cu__.4s14p1 _ The nature of the changes which these orbitals undergo through their interaction with the other unit are clearly shown by the character of the negative ion states of CuCO-. We consider the propeties of self-consistent field, SCF, wavefunctions for this cluster. The initi, neutral state of CuCO is 2X+ wth the configuration . ..3da23dsr43d644s~1...5~21n4. where the perturbed orbit& of Cu and CO are represented by their separated unit notation_ The nature of the interaction of Cu with CO has been discussed at length elsewhere [6] ; two features of the interaction were stressed_ (1) The overlap and consequent repulsion between the Cu 4s and the CO 50 orbitals is large; the polarization and hybridization of the Cu 4s orbital away from CO reduces this repulsion [6,7]. And (2), the Cu ds donation to CO 2~7~ is the main contriiution to the dative bonding. The negative ion states of CuCO- are formed by adding a T electron to CuCO into an orbital denoted wr. The configuration of cuco1.5
. ..3d023d~43d644spu1wr1...5c?ln4. This configuration has llI and 311 spin couplings. The average of configuration, fq(311) + $*(lll), is used for the SCF calculation; the spin coupling of the Cu open 4spu shell with the negative ion, yr~, orbitd is a property of CuCO which has no connection with CO/CU. Wavefunctions for the two lowest states of this configuration have been obtained. The Cu-C and C-O dis312
LEITERS
26 July 1985
tances are R(Cu-C) = 3.70 bohr = 196 A andR(C-C ~2.15 bohr. These are the dismces used in earlier work on neutral CuCO 163 and are close to the bond distances for on-top site adsorption of CO/Cu(lOO) [g] . The SCF wavefunctions are based on & expansic of the orbit& with con&acted Gaussian-type basis set and symmetry and equivalence restrictions are used; details are given elsewheie [6] _ The two states of CuCO- dissociate at large R&u-C) to Cu- and CO and to Cu and CO-. However at R(Cu-C) = 3.70 bolu, there is a considerable mixing of Cu- and CO- character in both states. The general nature of the wr negative ion orbital will be considered first and then the energetics. A Mulliken population analysis of the YITorbital is used to describe its general character. AlthougJ~ a pops lation analysis may have serious artifacts [7], it is appropriate for qualitative features. The gross populations for Cu and CO and the overlap populations between Cu and CO and between C and 0 are given for the VITorbitals of both CuCO- states in table 1. For Cu, the gross population is divided into pn and drr character_ Fo; state 1, the lower of the CuCO- states, the vn orbital is 75% CO and 25% Cu. The C-O overla population, -0.5, clearly shows the C-O antibonding 27r*, character of this orbital. The orbital is, however, bonding between Cu and CO as shown by the CuXO overlap population. The VT orbital of the higher, state 2, CuCO- state is also strongly hybndized, 67% Cu ant 33% CO 2rr*_ However, it is anti-bonding between Cu and CO; the Cu-CO overlap population is -0.3 _In both cases. the Cu character of the orbital is almost en tirely of valence pn character; the mixture of dn is ver small. These populations give clear evidence that there is significant chemistry occurring for the negative Ion states of CuCO and by extension for CO/Cu. Because of their signifimnt CO 2~’ character, both states woul lead to CO related BIS resonances. There are two major reservations for the calculated energies of the CuCO- states. The first is that SCF electron affinities, EAs, are, in general, too small compared to experiment [9]. Our calculated SCF-EEA for Cu is -13 eV (average of Cu 3P and lP states) and for CO it is -2.6 eV; both EAs are too negative_ However, the experimental EA(C0) is -15 eV [lo] ; thus, our SCF EA(C0) is not too badly in error. The error for EA(Cu) is compounded by the fact that our Cu basis set has not beem optirmzd to represent the dif-
Volume 11% number 3
(xu?xIcAL
PHYSICS LlzTTERs
26 July 1985
Table 1 Gross end 0yerLappopulations for the orbital of the added elrxfron in the lowest SCF negative ion states of the C!u&O clustas, n = 1.5, atid 13. The electron afliuity. EA. of these states is also given Gross
cu CllCO‘state 1
Cucostate2
CUg0-
P
total P d total
0.67 0.00 0.67
s
0.02 0.13 0.02 0.16
P d to&l cll,~C6 state1
S P d total
m3co
S
state2
P d total
CU,$W state 3
s P d total
co
0.85 0.15 -0.01 1.00 0.13 0.23 0.02 0.38 0.27 064 0.00 0.92
0.75
c-o
0.18
-050
-0.20
-
-
-
0.33
-0 30
-0-33
-
-
-2 20
0.84
0.13
-0.56
-0.61
0.00
0.62
0.00
0.17
0.00
-0.40
-
-
0.08
-0.01
-0.06
-
-
P d total
0.68
0.32
fuse character of this Cu anion. The second reservatiou is that the valence 4s and 4p orbitals of the Cu atom can ghe only a very hmited representation of the empty part of the Cu conduction band. For these reasons,
we will only be concerned with the qualitative character orour cluster energetics. Both of the states of CuCO- are ~bo~d with respect to the energy of
-0.09
0.22
-
-
-
S
state4
EA (ev)
-
0.21 0.44 0.03
w3m-
CU-CO
-
0.22 0.02 0.25
d
OWliIP
-0.28
0.21
-0.96
-250
CUCO; state 1 has an EA of -020 eV end state 2 has an EA of -220 eV. The separation of the two states is 2.00 eV and arises from the change from bonding to anti-bonding character of the wr orbital. We def?.neinteraction energies,Em, by wmparhg the energy of CuCO with the sum of the energies of appro~ately charged Cu and CO units. The specific 313
CHEMICAL PHYSICS LETIERS
Volume 118. number 3 choices arc: cuco,
EINT = E(Cu) + E(C0)
CUCO-,
state 1:
E*
= E(Cu)
+E(CO-)
- E(CuCO),
- E(CuCO-),
state 2: EINr = E(Cu-)
+ E(C0)
- Jz(CuCO-).
> 0, the system is bound end for EINT < 0, For E,, repulsive. The SCF Ena for CuCO is -0.55 ; this repulsive interaction has been explained [6] as being due to the strong Cu 4s to CO 5u repulsion;limitations of the SCF model also contribute [1 1,121. Even though the CuCO cluster does not ~W-IZan attractive interactron between Cu and CO, the differenfor the neutral and anionic states will ces of the E,, indicate the nature of the anion bonding. The Em for CuCO- state 1 is +1.83 eV; it is strongly bound with respect to Cu and CO-. This has three origins. (1) The VITorbital for this state is Cu to CO bonding. (2) Negatively charged CO unit polarizes the Cu charge and interacts positively with the induced Cu dipole [ 131. This is the single atom analogue of a metal image charge induced by CO-. (3) There is some Penetration of the Cu charge by CO- [6] ; this penetration reduces the screening of the Cu nucleus and leads to an attractive Coulomb interaction which reduces the frozen orbital for CuCO- state 2 is -1.49 eV; repulsion. The Em it is 092 e-V more repulsive than CuCO neutral. This is largely because the additional electron is placed in an anti-bonding orbital. The principal features of these low-lying CuCOstates should be reIevant for chemisorbed CO. The low-lying state is bondmg between CO(27r) end Cu(4p~); it is further stabilized by charge penetration and by polarization of the Cu charge. The second state is 2 .O eV higher in energy because, m large part, the additional electron is in an anti-bonding orbital. Part of the difference between these two states is an initial state effect arising from bonding present in the virtual orbit& of neutral CuCO. The low-lying rr virtuals have sigmfic=ant Cu 41x7 and CO ~YT* mixing. The lowest is 65% Cu and Cu-CO bonding while the second virtual is 48% Cu and anti-bonding. The orbital energies, inverse Koopmans’ theorem, are se-ted by 1.8 eV. Thus, a large part of the features of the CuCO- states are due to initial state effects. 314
26 July 1985
We have also examin ed low-lying negative ion state of clusters containing more Cu atoms, Cu,CO and CuI,CO. The Cu13 cluster contains 9 atoms of the first layer and 4 of the second layer of Cu(100); rt 1s denoted Cu13(9,4). The CO molecule is added above the central frost layer atom to form CuI,(9,4)CO. The CUCJ1,4)CO cluster, described elsewhere C6.141, contains only the Cu adsorption site atom m the fnst laye and 4 Cu atoms of the second layer. A pseudopotentia is used to describe the environmental Cu atoms not directly involved in the bonding with CO [ 141. The anion configumtions are formed by adding en electron in a R orbital which is unoccupied in the ground&ate neutral configuration. population analyses for the add tional anion orbital are given in table 1; the Cu contrib tion is s un-uued over all Cu atoms of the cluster_ For CusCO-, the lowest state which has sigruficant 277’ ch acter is shown; it is clearly bonding between Cu end Cc as seen from the positive Cu-CO overlap populations For Cu13CO-, we report results for the four lowest states with au added n electron. The anion orbital of the lowest state, state 1, has no CO character: it corresponds to an electron added to the bottom of the unoccupied STpart of the Cu(lO0) conduction band. State 2 has significant 27r* character and is Cu-CO bonding. Its EA is +021 eV; the CuI, representation of the image charge of CO- is sufficient to make this state stable. States 3 and 4 have some 2~’ character; they are dominantly metal and are anti-bonding between Cu and CO. Their EAs are -096 and -2.50 eV respectively. The Cu-CO bonding and anti-bonding features of the anion states of Cu5C0 and Cu,aCO are consistent with those found with the simpler CuCO. In conclusion, our cluster results show that there is signifkant chemical interaction and mixing between Cu and CO for the anion states of the cluster. The lowest state W&L s@ii%ant CO 27r* character is bonding between Cu and CO and contains a reasonable amount of Cu character. Higher-lying states which have CO 2n* character are anti-bonding between Cu and CO. AU the clusters studied support a quahtative prediction that there will be two classes of anion states the lower-lying bonding between Cu end CO and the higher-lying anti-bonding. Our calculations are consis-
tent with the observation of a doublet in the CO induced BIS spectra for CO/Cu(loO) [S] and explain thF origin
of this doublet.
Volume 118, number 3
c!HF.MICAL PHYSICS LEl-rERS
We wish to acknowledge cussions
with Professor
AM.
useful and stimulating disBradshaw.
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26 July 1985
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