Many-body calculation of the core hole spectra of PdCO

Chermcal Physics North-Holland

160 ( 1992) 353-36 1

Many-body

calculation

of the core hole spectra of PdCO

P. Decleva Dlpartrmento dr Sclenze Chlmlche, Umverslta dr Tneste, Via Valeno 38, 34127 Trreste. Italy

and M. Ohno Dwrtmenfof Physics. Received

Uppsala Ckrverstty. Box 530, S-751 21 Uppsala, Sweden

30 August 199 1

To mvestlgate the dependence of hgand (adsorbate) core hole spectra on the electronic structure of the metal substrate, we performed ab mltio 2hlp and 2h2p/3h2p CI calculations of the core hole spectra of the linear PdCO molecule using an extended basis set. The main hne 1s the one-hole state and takes a much larger intensity than for NICO and NiN2 but stdl smaller than for free CO. As in the case of NiCO and NiN2, also for PdCO the K charge transfer (CT) shakeup satellite of a small mtensity is obtained near the main line peak. The most striking spectral feature of PdCO which differs from NiCO and NiN2 is the absence of the 5 eV giant o shakeup satellite m the carbon spectrum. In the oxygen spectrum the corresponding satellite of a small satellite intensity. is shifted toward the higher energy (around 8 eV). However, with an increase of the bond length this satellite also disappears. As in the case of NZO, the x to K* shakeup satellites are obtained around 9 eV for both carbon and oxygen spectra. These dramatic spectral feature changes are explained in terms of the different degree of the do-s hybridization and s-do promotion in the local metal configuration in the ground and Ionized states. We point out the possibility that the DES spectrum becomes sumlar to the AES spectrum in the spectator Auger decay energy region even when initial core hole state differs.

1. Introduction The appearance of giant satellites in the core hole spectra of molecules adsorbed on metal surfaces has attracted great interest and has been satisfactorily explained by a model Hamiltonian approach [ 1 ] which has become the standard description of these phenomena. Essentially the same interpretation, based on a charge transfer (CT) from the metal sp and d band (orbitals) to the empty A* orbital of the ligand, pulled below the Fermi level by the attraction of the core hole, was further obtained by quantum chemical molecular cluster calculations [ 2 ] based on the Hartree-Fock ASCF approach. Recent high-resolution XPS core hole spectra of the CO/Ni( 100) and NJ Ni ( 100 ) systems show, however, a number of newly resolved satellite lines as well as significantly large shakeup/off satellite intensity which are not satisfactorily described by previous theoretical approaches [ 3,4]. Moreover, the simple model proposed ap0301-0104/92/$05.00

0 1992 Elsevier Science Publishers

pears largely inadequate to give detailed account of the variations of the satellite structures associated with changes in the ligand species or the metal surface, understanding of which is essential if specific chemical information, first of all the local electronic structure of the adsorption site and the nature of the bonding, is to be extracted from the spectral data. Recently, a good reproduction of the new experimental data has been achieved by ab initio many-body calculations based on the minimal linear clusters NiCO and NiN2 [ 5,6]. It is pointed out that inadequate reproduction of the experimental features or even different interpretations afforded by older cluster approaches may well be attributed to limitations of the scheme employed, notably insufficient treatment of many-body effects of both ground and ionized states, rather than to an intrinsic fault of the finite cluster model [ 5,6]. Moreover, the present approach shows the potential for a systematic understanding of the chemically significant changes in-

B.V. All rights reserved.

354

P. Decleva, M Ohno / Core hole spectra of PdCO

duced in the spectra by variation of the substrate/ ligand system. Of course, size effects may be important. and may be systematically examined by enlarging the cluster employed, although this is computationally expensive. The present approach has shown that for the CO/Ni system the lowest energy state (of the largest intensity) is clearly the one-hole main line state (Koopmans state) and not the rr charge transfer ( CT) 2h lp (two hole one particle) shakedown state. On the contrary, for the NJNi system the lowest energy state, which is still described by the relaxed 1h state, looses a large part of the spectral intensity to the 5.5 eV “satellite” which becomes in the case of the N,/Ni system the main line state of the largest intensity. The 5.5 eV giant satellite of these systems is interpreted as due to the o to o* shakeup excitation associated with the local metal configurational change

I&61. This different spectral behavior is related to the different ground state electronic structures of these two systems. As it is shown by the authors of refs. [ 791, for the states where the do shell is completely filled as in CuCO. the metal polarization to reduce the repulsion between the metal valence s and CO 50 orbitals arises from 4s-4po hybridization. The hybridized 4s-4po orbital polarizes away from CO to reduce the overlap and repulsion with the CO 50 orbital. For NiCO and NiN, (the ground state is the ‘X+ state which consists of 3drr43da44s’ ( ‘S), 3do’3dx43dZ44s’ (ID) and 3d023drt43d644s0 (IS)) when the do and 4s are singlet coupled, they can hybridize. The s + do hybrid (o* metal orbital) increases the charge density along the metal-ligand axis, while the s-do (o metal orbital) reduces the charge density along the metal-ligand axis. Thus by increasing the occupation of the s-do hybrid, the o repulsion with the ligand can be reduced, while retaining a 3d94s1 occupation on Ni (do-s hybrid). In addition, the 3d orbital is much more compact than the 4s and the metal occupation d”+* can mix in with d”+‘s; this mixing in of the spatially compact d”+’ occupation greatly reduces the repulsion (4s-3do promotion). The 3do4s hybridization is much stronger in NiCO than in NiN2 because of the stronger bond. In NiCO there is large 4s to 3da donation, which allows the Ni to donate half an electron to the CO 2rr* orbital. If the do-s hybridization and s-3do promotion is favored also upon the ligand core ionization, it is ex-

petted that the o relaxation associated with this hybridization and promotion mechanism plays a much more significant role for NiN2 than for NiCO because of less d and more s occupation in the ground state of NiNZ in comparison to NiCO. In our recent work [ 5,6] on the core hole spectra of NiN2, it is shown that indeed the depletion of 4s and increase in metal d occupation is favored also upon the core ionization in the ligand, leading to a very large change in the metal o orbital upon ionization, giving almost equal overlaps with o and o* relaxed orbitals. However, the relaxation of the other orbitals (e.g. K orbitals) is found modest and comparable to NiCO. This indicates that the strong orbital relaxation which appears as such in the “giant 5 eV” metal o to o* satellite line, acquiring a large fraction of the spectral intensity (the largest intensity in the case of NiN2), seems to be associated with a change in the local metal configuration rather than with an effective CT, which although significant, appears to be rather similar in the both main line and satellite states. For NiCO and NiN? a definite CT is indeed associated with the x to K* shakeup excitation. However, it is not responsible for the lowest energy main line (which is not the 2hlp x CT shakedown state but the one-hole state) as has been widely assumed, but only for the first low lying newly resolved 2 eV satellite of a small intensity. If this metal configuration change is the main mechanism active for the appearance of the “5 eV giant” o to o* shakeup satellite line, one can anticipate a significant change in the core spectra of CO on a Pd metal surface because of a much less degree of the do-s hybrid and s-do promotion due to a higher d and smaller s occupation in the ground state in comparison to NiCO and NiNz (in the case of bulk metal, d band population of Pd and Ni metal is 9.6 and 9.4, respectively [ lo] ). A similar population is indeed obtained also for the linear clusters M-CO (M denotes a metal atom), showing that interaction with the ligand leads to an electronic situation close to that expected for the adsorbates, and far away from the isolated atomic ground state, d8s2 in Ni and d” in Pd. So in the present work we calculated the core hole spectra of PdCO by the ab initio 2hlp and 2h2p/3h2p CI method (with the 2h2p ground state correlation ), using an extended basis set to see whether such subtle differences in the electronic structure of the adsorption site give any significant spectral feature changes

355

P. Decleva, M. Ohno / Core hole spectra ofPdC0

compared to the core hole spectra of NiCO and NiN2, and to stimulate experimental verification, as to the best of our knowledge there are no published core hole spectra of CO adsorbed on a Pd metal surface, despite the large number of studies devoted to the ground state properties. The present calculation predicts the absence of the 5 eV giant o shakeup satellite in the core hole spectra of PdCO. In the rest of part of the article we point out the possibility that for the CO/Ni system, the DES spectrum and the AES spectrum may become very similar in the spectator Auger decay region but in the participant Auger decay energy region the DES spectrum may differ from the AES spectrum.

ration of the bridge bonding also show the absence of the giant o to o* shakeup satellite line. There is not any significant difference in the lower shakeup energy region between the calculated core hole spectra of PdCO and Pd?CO. Recent accurate ab initio calculations of the binding energy of the CO/Pd system [ 161 show that good agreement with experiment is achieved by calculating PdCO rather than PdzCO because of a smaller bonding strength of the Pd-Pd in the Pd dimer than in the metal surface. This implies that PdCO may be more suitable than Pd&O for the description of the essential part of the bonding of CO on a Pd metal surface, and that the quantitative behavior is not strongly influenced by the adsorption site.

2. Theory 3. Results and discussion We refer to ref. [ 111 for the details of the present scheme which has been proved to give a reliable semiquantitative description for the core hole spectra of small molecules. It is therefore expected that the picture obtained is at least qualitatively accurate for the isolated molecule, and trends are well reproduced. The Gaussian type [ 17s 11 p,8d] basis set of Huzinaga [ 121 for Pd is enlarged by an additional d function and two p-type functions to take into account the 5p orbital of Pd [ 13 1. Then it is contracted to (8s 6p, 5d). For CO the [9s,5p] basis set of Huzinaga for C and 0 [ 141 is contracted to (5s 3p) and s, p Rydberg plus a d polarization component (a,(C)=0.75andcru,(0)=0.85) isadded.Twosets of the calculations were performed for PdCO, keeping the CO distance at 2.2 au and the Pd-C distance at 3.65 au and 4.0 au. The former distance is the experimental distance of CO adsorbed on a Pd( 100) surface (bridge bonding) [ 151 and is also close to the results of accurate ab initio studies on linear PdCO [ 161, whereas the latter is close to the nonrelativistic SCF value [ 17 1, and provides a measure of the effect of a reduced Pd-CO interaction, which might occur in CO adsorbed on top of a Pd surface, which is forced at high coverage [ 18 1. So the present results are obtained, assuming the top site chemisorption, although the bridge bonding is the most dominant for the CO adsorption on a Pd ( 100) surface [ 19 1. The preliminary calculations of the core hole spectra of Pd,CO with the experimental geometrical configu-

For the sake of simplicity, we discuss mainly the results of core hole spectra calculated at the Pd-C distance of 3.65 au. We refer to ref. [ 5 ] for the results of NiCO at an Ni-C distance of 3.5 au. In tables 1 and 2 we list the present 2hlp and 2h2p/3h2p CI calculations of the core hole spectra of PdCO (at R= 3.65 au). For the dominant configurations we denote 1s- ’ i- ‘j by i-j. The comparison between the 2hlp and 3h2p CI results allows an estimate of the importance of the double excitations included in the Table 1 Theoretical method Level

lo 01,

20 C,S

core hole spectra of PdCO calculated

Theory

(2hlpCl)

by the 2hlp CI

at 3.65 au

E(eV)

I

I TCI

configuration

0.0 2.91 6.58 8.67 10.73 11.63

0.649 1 0.0161 0.0486

100.0 0.2 2.5 1.5 1.0 0.4

Is-’ K-i? o-o* n-n* o-o* (x-(n)x*) same

0.0 1.96 6.52 7.20 8.66 10.16 15.90

0.6845

100.0

Is-’ n-x* o-o* 1x--K* x--K* rr-(n)x* z-Jr*

0.05 14

0.8 3.7 1.5 1.5 1.1 3.1

P. Decleva, M. Ohno / Core hole spectra ofPdC0

356 Table 2 TheoretIcal core hole spectra 3h2p CI method Level

lo 01,

20 C,*

Theory

of PdCO calculated

by the 2h2p/

(3h2p CI) at 3.65 au

E(eV)

I

I re,

configuration

0.0 4.31 8.03 8.63 11.30 12.68 12 78 13.48

0.5023

100.0 0.2 10.6 13.6 0.5 0.2 0.5 1.0

Is-’ x--x* o-o* a-n* (o-o*) a-(n)o* Same Same =1-j+

0.0 4.81 9.08 II.54 12.32 14.63

0.5469

100.0 1.1 23.3 1.3 0.2 0.2

Is-1 n-r* x--x* In-n* j+n**

0.0533 0.0684

0 1274

+

1nz_$*

o-o*

latter and of the convergence of the results reported. The carbon and oxygen core hole energy difference calculated by the 2hlp and 3h2p CI methods is 243.6 and 245.7 eV, respectively. The energy difference is fairly independent of the change of bond length. The experimental energy difference for the CO/Ni system is 246.2 eV [ 3,4]. Unfortunately, the experimental value is not available. It is most likely that the core hole energy splitting is fairly independent of the substrate. Then, as in the case of NiCO. the agreement with experiment is most likely improved by the 3h2p CI method. The carbon and oxygen main line intensity obtained by the 2hlp (and 3h2p) CI methods is 0.69 (0.55) and 0.65 (0.50), respectively. The main line intensity is also fairly independent of the change of the bond length. The C and 0 main line intensity for PdCO is much larger than the corresponding experimental one for the CO/Ni ( 100 ) system (0.29 and 0.36, respectively [ 3,4] ) but still smaller than the one for free CO (0.67 and 0.60, respectively [ 3,201). As in the case of NiCO, the main line intensity obtained by the 3h2p CI method is much reduced from the one by the 2hlp CI method. This also shows the importance of the higher order valence excitations neglected by the 2hlp CI method, namely the 2h2p excitations in the presence of a core hole, the relaxation of the hole and particle in the 2h 1p configuration and the screening of the hole-hole and

hole-particle interactions. For both C 1s and 0 1s levels, the overlap between the ground state and ionic ASCF orbitals is larger than 0.955 and 0.92 1, respectively. The ionic state orbitals are still very similar to the ground state ones. The main line peak definitely corresponds to the lh configuration. The squared overlap between Koopmans and relaxed 1h conliguration for C 1s and 0 1s levels is 0.7066 and 0.6578, respectively. The relaxation takes away 30 to 35% of the total intensity. This also indicates that the conliguration mixing such as double excitations is important. For PdCO as well as NiCO, it turns out that the total spectral intensity obtained within a certain scheme (2hlp or 3h2p CI method) for the energy region where the spectral feature is mainly determined by the metal-ligand CT shakeup excitations and local metal excitations, is fairly independent of the changes of the bond length. As for NiCO, the C and 0 satellite of a small intensity by the rr( metal) to 7c*ligand CT shakeup excitation (with respect to the lowest energy main line state) is obtained by th 2hlp CI method at 1.96 and 2.91 eV, respectively, and by the 3h2p CI method at 4.8 1 and 4.3 1 eV. respectively. The satellite energies obtained by the 2hlp CI method are comparable to the corresponding ones for NiCO (2.0 and 1.6 eV, respectively). but those by the 3h2p CI method are much larger. The satellite intensity obtained by the 3h2p CI method is as small as 0.006 and 0.00 1, respectively, whereas for NiCO it is 0.02 and 0.004, respectively. For the CO/N1 system only the C satellite is observed on the shoulder of the main line peak. As in the case of NiCO, the satellite intensity for the carbon is much larger than the one for the oxygen because of a larger localization of the 2rc* orbital on the carbon site than the oxygen site. With an increase of the bond length, the satellite energy position is shifted toward the lowest main line peak and at the same time the intensity is increased. This is also the case with NiCO. This spectral behavior can be understood in terms of the changes of the lhlh and lhlp interactions in the 2h lp (3h2p) configurations as discussed in detail in ref. [ 51 for NiCO. As in NiCO with an introduction of the higher order valence excitations, the satellite energy is shifted toward the larger binding energy. For the CO/Ni system the 1s to 2x* resonant excitation energy is very close to this satellite energy because the 1s to 27r* resonantly excited 1h 1p

P Declevu, M. Ohno / Core hole spectra of PdCO

state is similar to the or:CT 2h 1p shakeup satellite state except for the presence of a hole in the substrate band in the latter state. For the CO/Pd( 100) system the XAS energy is larger than the XPS (lowest energy main line state) by 2.4 eV [ 2 11, whereas for the CO/ Ni( 100) system it is by 1.5 eV. This implies that the XPS relative satellite energy position for the CO/ Pd( 100) system will be larger than that (2.1 eV) for the CO/Ni( 100) system, however, this satellite may not be observed because of a very small intensity. For the C and 0 spectrum the satellite of a small intensity which is interpreted as due to the o to o* shakeup excitation associated with the local metal configurational change is obtained by the 2hlp CI method at 6.5 and 6.6 eV, respectively. The satellite intensity is 0.03 and 0.02, respectively. When the higher order valence shakeup excitations are introduced by the 3h2p CI method, the o shakeup satellite disappears in the carbon spectrum. The oxygen o shakeup satellite is obtained at 8.0 eV and shifted toward the larger binding energy by about 1 eV. Its intensity is 0.05. With an increase of the bond length the satellite intensity becomes almost zero. For NiCO the carbon and oxygen o satellite intensity obtained by the 2hlp CI method is 0.09 and 0.08, respectively. The one by the 3h2p CI method is 0.15 and 0.11, respectively. In comparison to NiCO, for PdCO in both carbon and oxygen spectra there is a significant cr shakeup satellite intensity reduction. For the carbon and oxygen spectra, the 7[:to n* metal-ligand CT shakeup satellite of a substantial intensity is obtained by the 2hlp CI method at 8.7 and 8.7 eV, respectively. This satellite is obtained by the 3h2p CI method at 9.1 and 8.6 eV, respectively. This n shakeup satellite corresponds to the 9.5 (8.5) eV carbon (oxygen) 7~CT satellite observed for the CO/ Ni system. The carbon n shakeup satellite intensity for PdCO obtained by the 2hlp CI method is 0.05. whereas for NiCO it is 0.08. The carbon n shakeup satellite intensity for PdCO obtained by the 3h2p CI method is 0.13, whereas it is 0.11 for NiCO. The oxygen K shakeup satellite intensity for PdCO obtained by the 2hlp CI method is 0.05, whereas it is 0.08 for NiCO. The oxygen x shakeup satellite intensity for PdCO obtained by the 3h2p CI method is 0.07, whereas it is 0.13 for NiCO. As for NiCO, with an introduction of the higher order valence excitations, the satellite intensity is increased for both carbon and

357

oxygen. With an increase of the bond length, the rt satellite is shifted toward the main line and at the same time the intensity is increased. This is also the case with NiCO. The theoretical relative intensity ratio of the carbon n: shakeup satellite line to the main line for PdCO by the 3h2p CI method is 0.23, whereas for NiCO it is 0.29. For the 0 x shakeup satellite it is 0.14, whereas for NiCO it is 0.36. Concerning the n CT shakeup satellite in PdCO which corresponds to the 8.5 (9.5) eV satellite of NiCO, in both carbon and oxygen spectra of PdCO there is not a large change from NiCO, however, when it comes to the giant 5.5 eV o shakeup satellite for PdCO it is missing in the carbon spectrum. In the oxygen spectrum with an increase of the bond length the o shakeup satellite disappears. As already stated in the introduction, if the depletion of 4 ( 5 )s and the increase in the metal 3 (4)d occupation is also favored upon in the ionization, it will lead to a very large change in the metal o orbital in the ionization (appearance of the giant o shakeup satellite). However, the present results show that there is no such a giant satellite in the core hole spectra of PdCO. This is in accordance with the speculation made from the different ground state electronic structure properties for NiCO and NiNZ [ 5,6]. Even takmg into account that the Mulliken population analysis may have some ambiguous aspects in the present case, it should provide a qualitative clue to the understanding of the relaxation processes in the presence of the core hole. In tables 3 and 4 we list the Mulliken population analysis of PdCO and NiN2 in the ground state, ionized main line and satellite states (which are obtained by the 3h2p CI method). It should be noted that the d population of the bulk Pd and Ni metal is 9.6 and 9.4, respectively [ lo]. This compares well with the d population for PdCO and NiCO. In the case of NiNz the d population is much lower than NiCO [ 61. One of the striking differences between PdCO and NiNz is the difference of d and s population in the ground state. One may consider the ground state of these systems in terms of the do-s hybrid and s-do promotion. In other words the ground state of NiNz consists mainly of d8s’ ( 3dn43dS44s’) and d’s’ ( 3do’3drc43dS44s’ ) local metal conligurations and that of PdCO consists mainly of d9s’ ( 3do’3drt43dZi44s’) and d’O ( 3do’3dx43d644s0) local metal configurations. Upon the ligand core ioni-

P. Decleva, M. Ohno / Core hole spectra of PdCO

358 Table 3 Mulliken

population

analysis of PdCO

State

Pd atom

4d

5s

ground state a) ground state b’

45.668 45.65 1

9.651 9.592

0.032 0.085

mam lure c Is Sl c) S2d’

45.216 45.025 45.423

9.147 8.860 9.099

0.059 0.084 0.265

main lure 0 lsS3” s4 r’ s5 g’

45.159 45.008 45.656 45.404

9.082 8.883 8.715 9.127

0.03 1 0.079 0.833 0.146

a’ Ground

state: wrthout the 2h2p ground

state correlation

b, Ground state*: wrth the 2h2p ground state correlation. ” d, ‘) ‘) *)

Sl: S2: S3: S4: S5.

4.8 9.1 4.3 8.0 8.6

Table 4 Mulliken

eV eV eV eV eV

x n x o n

satellite. satellite. satellite. satelhte. satellite.

population

analysis of NiN, Ni atom

3d

4s

state

28.103

8.705

1.496

mam lme N a Sl a.b’ s2 c’

27.424 26.826 27.426

9.356 8.732 8.480

0.183 0.171 0.993

main line N, Sl a.b’ s2 c’

27.218 26.786 27.293

9.172 8.692 8.332

0.155 0.149 0.955

State ground

‘I N, and Nb denote the inner respectively. b, Sl: first x satellite. ” S2: “giant” o satellite.

and the outer

mtrogen

atom,

zation there is an overall CT from the metal to the ligand. While in the case of the 7crelaxation this causes only a minor change in the occupied orbitals, the o relaxation can lead to a dramatic change in the metal do orbital, associated with a depletion of the s population due to the do-s hybridization and s-do promotion. Consequently,the o relaxation can occur mainly when the d’s’ and (or) d9s’ local metal configurations dominate the ground state. The n relaxation is then mainly associated with the d’O configuration, leading to a final d9 (3d023drr33dS4) occupation, and to the corresponding metal-ligand x to TC*satellites. On the other hand the o relaxation

involves the d%’ and d9s’ configurations in the initial ground state, giving d9s’ ( 3do’3drr43dF44s’) and d” ( 3d023dn43dS4) configurations, respectively by the s-do hybridization and s-do promotion, which turn into d’s’ and d9 configurations, respectively by loosing 1 dn electron through d-x* CT. The d’s’ configuration is forced again to the d9 (3d023drt33dS4) configuration in the lowest energy state by the cr repulsion with the ligand orbitals, but which appears as such in the metal o to o* satellites, acquiring a large fraction of the intensity associated with the initial d’s’ and d9s’ configurations. So the main line intensity dramatically decreases with the increase of d’s2 and d9s’ configurations in the ground state, and there is a corresponding increase of the o satellites, and a parallel decrease in the corresponding rt satellites. Indeed this is observed behavior along the series PdCO, NiCO and NiN2 where the weight of the d” increases and that of d8s’ decreases in the order of NiN2, NiCO and PdCO. So in NiNz the o relaxation is dominating in the lowest energy state, causing an increase in the d population despite the large CT towards the ligand, which empties the s orbital, giving a metal configuration close to d9. For the o shakeup state, the d9 configuration of the lowest energy state changes to the d’s’. Indeed the Mulliken population analysis of the o CT shakeup state of NiNz shows that the d-s population can be approximated in terms of a linear sum of d8s’ and d’; less d population and less (but a substantial) s population in comparison to the ground state [ 5,6]. On the other hand, for the n CT shakeup excitation, the excitation from the d” is favored. However, for NiN2 in the ground state the d” is much less dominating than for NiCO and PdCO. This explains particularly a much smaller intensity of the 8.5 eV n shakeup satellite for NiN2 in comparison to NiCO and PdCO. The population analysis indeed shows that d9 is dominating for the 7cCT shakeup satellite. This kind of analysis is also valid for PdCO. Indeed the population analysis shows that the composition of ionic states is quite similar to that in NiN2, so that the large difference in the spectral feature is essentially due to the difference in the ground state composition. In this case for the main line where the rt CT relaxation is dominating, the d population decreases but there is almost no change in the s population in comparison to the ground state. For the o

P. Decleva. M. Ohno / Core hole spectra of PdCO

shakeup satellite, there is a substantial increase in the s population in comparison to the ground state. This may be seen as the change from the d’s’ and d9s’, obtaing d’s’ and d9 as in the case of NiN2. However, because of much less d8s2 and d9s’ in the ground state in PdCO, there will not be a large contribution in comparison to NiN2. This analysis may provide a clue to the understanding of the appearance and disappearance of different kinds of shakeup satellites in the core hole spectra due to the changes of substrates and adsorbates. The core hole spectra of the metal carbonyls such as Ni (CO), show a close resemblence to the spectra of the adsorbates. The origin of the 5 eV giant satellite observed for Ni( CO ) _,may differ from that of NiCO because of a difference in the 3d population in the ground state. The 3d’O is much more dominating in Ni( CO), than in NiCO. In other words the giant satellite of Ni( CO), may be due to the K CT shakeup rather than o shakeup excitations (the core hole spectra of PdCO calculated for R = 4.0 au by the 3h2p CI method shows a strong n: CT shakeup satellite). As discussed in our recent work on NiCO and NiN2 [ 5,6], the theoretical total spectral intensity for the energy region where the metal-ligand CT excitations and the local metal excitations dominate, is approximately equal to the main line intensity of the free CO spectrum. For PdCO the corresponding total spectral intensity for carbon and oxygen (up to about 15 eV) obtained by the 3h2p CI method is 0.69 and 0.65, respectively. This intensity is quite independent of the bond length and at same time it is is close to the main line intensity of free CO (0.60 and 0.67, respectively). For a coordinated molecule the main line intensity of a free molecule is distributed to the main line of a (much) smaller intensity and several satellite lines by the metal-ligand CT excitations and local metal excitations. Thus the main line intensity of the adsorbate is (much) smaller than in the free molecule, in accord with experiment (this is also true for the valence photoemission spectra of the adsorbates and related systems [ 22 ] ). The question may be raised whether the single metal atom is suitable enough to describe the response of the metal substrate to the sudden creation of the core hole in the adsorbed molecule. A fairly successful description of the new high resolution XPS core hole spectra of CO on Ni( 100) and N2 on

359

Ni ( 100) by the present molecular many-body approach, using the single metal atom linear model molecule, implies that the response of the metal substrate to the creation of the core hole in the adsorbate seems to be very localized [ 5,6 1. This observation is supported by the fact that the core hole spectra of the adsorbates resemble very much those of the related metal carbonyls and also that the response due to the distant metal atoms neglected in the present approach, is most likely to occur on a time scale which is long in comparison to that required for the high energy photoelectron to be emitted [ 231. Recently it is argued that the deexcitation electron spectroscopy (DES) spectra of adsorbates cannot be interpreted in terms of the autoionization decay processes because the resonantly excited state relaxes to the XPS main line state before the core hole decay starts and the DES spectrum becomes identical to the normal Auger electron spectroscopy (AES) spectrum [ 24 1. In our recent work [ 5,6 ] we discussed a possible relaxation mechanism of the resonantly excited state to the XPS core hole state for the CO/Ni and NJNi systems. Here we discuss briefly the possibility that the DES spectrum and AES spectrum of adsorbates become identical (very similar) even when the 1s to 2n resonantly excited state does not relax to the XPS lowest energy state before the core hole decay starts. For the CO/Ni system, the Auger kinetic energy (KE) difference between the normal Auger decay (the final state is two holes i and j) and the spectator Auger decay (the final state is two holes 1 and j and the spectator 21t electron) is given by C’,*.a- U,, - U,,, (here U is the effective Coulomb interaction between the hole and the 2~ electron (a) ). Using the equivalent core approximation, the relevant inverse photoemission and valence hole photoemission data for the NO/Ni system (see references in ref. [4] ), we find that U,_ is almost zero. The effective Coulomb interaction between the 1s core hole and 2rc resonantly excited electron is negligible. This implies that the effective Coulomb interaction U,,, between the valence hole i and the 2rc electron may also be negligible. In other words, the presence of the 27t electron will be negligible for the spectator decay. If so, then the normal Auger decay KE and the spectator Auger decay KE become identical. The spectator and normal Auger decay rates become also very similar. Consequently, both spectra

360

P. Decleva, A4. Ohno / Core hole spectra of PdCO

look very similar. On the other hand, for the participant decay the same kind of analysis shows that it seems to be not possible that the participant Auger decay KE and the normal (so called backbonding) Auger decay KE becomes identical. Even if the 2~ resonantly excited electron still stays when the core hole decay starts, there is a possibility that the spectator Auger decay spectrum will be very similar to the normal Auger decay spectrum, but in the participant Auger decay energy region there will be changes in the spectral features with an increase of the excitation energy (see ref. [ 25 ] for further discussion). For the weakly coupled system (assuming that the XPS lowest energy state is the II CT 2h 1p shakedown state) the Auger KE difference between the spectator and normal Auger decay will be U,, , s- U,,, - U,,. For the participant decay, it will be Ud,,s- U,,,Ud,a. Here d is the q,, hole (M denotes the metal derived orbital). If the presence of the hole in the substrate in the screened core hole state is negligible (i.e. the resonantly excited state becomes identical to the XPS lowest energy core hole state), the Auger KE will become identical and both spectra become very similar.

4. Conclusion The results of the present ab initio CI calculations of the ligand core hole spectra of PdCO show that the spectral features are strongly dependent on the changes in the local metal configuration anticipated on the basis of different ground state configuration in the isolated atom, and can therefore afford a sensitive probe of the electronic structure of the adsorption site. The main line intensity is larger than that for NiCO and NiNz but still smaller than for free CO. The rr CT shakeup satellite which is observed around 2 eV for NiCO and NiNZ is also obtained, however, the intensity is very small and it will not be observable. There is a striking spectral feature for PdCO in comparison to NiCO and NiN2. The giant 5 eV o satellite is missing in the carbon spectrum of PdCO. It is shifted toward the higher energy (around 8 eV) in the oxygen spectrum and the intensity is much reduced. We expect only a weak x CT shakeup satellite around 9 eV. For NiCO, NiNz and PdCO the strong relaxation (appearance of different kind of shakeup satellites) seems to be associated more with a change

in the local metal configuration rather than with an effective CT, which, although significant, appears to be rather similar in both the main line and satellite lines. The absence of the o shakeup excitations is explained in terms of the do-s hybridization, s-do promotion and less s population in the ground state and preference of the s-do promotion in the ionized state by the o relaxation. We pointed out the possibility that for the CO/Ni system, the DES spectrum and the AES spectrum may become very similar in the spectator Auger decay energy region but in the participant Auger decay energy region the DES spectrum may differ from the AES spectrum.

Acknowledgement M. Ohno would like to thank Uppsala University and Swedish Natural Science Research Council for financial support.

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