Theoretical study of core ionized Cr(CO)6

JOURNAL OF ELECTRON SPECTROSCOPY and RelatedPhenomena

ELSEVIER

Journal of Electron Spectroscopyand Related Phenomena 70 (1995) 233-244

Theoretical study of core ionized

Cr(CO)6

J. B u s t a d a ' * ' l , C. E n k v i s t a, S. L u n e l l a, S. S v e n s s o n b aDepartment of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Upssala, Sweden bDepartment of Physics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden

First received 21 January 1994; in final form 7 June 1994

Abstract

The satellite structure of the Cls and Ols photoelectron spectra of chromium hexacarbonyl Cr(CO)6 has been calculated by the INDO/CI method and compared with available high resolution core level photoelectron spectra. A reassignment of some of the lines is made. It is found that the satellite structure in both cases is dominated by excitations from metal-ligand bonding to metal-ligand antibonding Me(3d)-Tr* orbitals, and that these shake-up excitations involve a significant charge transfer to the core ionized ligand from the rest of the molecule. Keywords: Chromium hexacarbonyl; Cr(CO)6; Shake up

I. Introduction

One of the most studied systems in surface science is CO adsorbed on transition metal surfaces. The interest is motivated by fundamental questions about the adsorbate-substrate interaction as well as technical applications (e.g. catalysis). The study of the transition metal carbonyls, where CO is coordinated to one or two metal atoms, has provided a detailed understanding of the C O - m e t a l interaction. It has been found that m a n y aspects of the adsorbate systems can be modelled by these molecules (see, for example, Refs. [1-3]). A striking resemblance between the satellite structures in core photoionization of transition metal carbonyls and CO adsorbed on transition l Also at the University College of Gfivle/Sandviken, Box 6052, S-80006 G~vle, Sweden. * Corresponding author.

metal surfaces has been observed [2,4]. The shake-up and shake-off satellites can be viewed as valence excitations taking place simultaneously with the ionization, and provide information about the valence region of the a d s o r b a t e substrate complex. However, the valence states are modified by the presence of the core hole. U p o n coordination to a metal atom or surface, the occupied (and unoccupied) valence states in the molecule and metal interact, forming new adsorbate states. This results in additional low-energy shake-up features in the core level spectra from coordinated, compared to free, CO molecules. In recent high resolution studies of Cr(CO)6 [5], it was possible to resolve a previously unobserved shakeup state about 2 eV above the main line in the C l s spectrum. In the energy region 5 - 6 eV above the main line, there seemed to be contributions from a number of states in each core level spectrum. N o n e of these states is observed in the gas phase spectra from CO, and they can consequently be

0368-2048/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02231-3

234

J. Bustadet al./Journalof ElectronSpectroscopyand RelatedPhenomena70 (1995) 233-244

attributed to excitations involving mixed states between the metal and the ligand molecule. The satellite intensity is related to the differences in charge distribution between the ground state and the core ionized state. If the ionization occurs in a free molecule, the molecular orbitals rearrange themselves in a way which is denoted internal screening. In a small molecule such as CO, the multielectron excitations contribute to about 30% of the total intensity [6], and each individual shakeup process has a relatively low intensity compared to the main line. In a t r a n s i t i o n metal complex, there is a new channel for internal screening, involving the central metal atom and the other ligand molecules. This leads to a larger gain in relaxation energy and consequently a larger satellite intensity. The same phenomenon can be observed for molecules adsorbed on surfaces, and in polar d o n o r acceptor aromatic molecules (see, for example, Refs. [7-9]). The response of the valence levels to the ionization depends on the position of the core hole in the ionized atom. In Cr(CO)6 the shake-up spectra associated with the Cls and Ols ionization are very different. By using the Z + 1 approximation, the modified valence states can also be modelled using semiempirical valence-electron calculations.

In this approximation the final states containing a Cls or an Ols hole are described by molecular species where one of the ligands is replaced by an ion such as NO + or CF +, respectively. The exchange interaction between the unpaired core electron and the valence electrons cannot be included explicitly in this approximation. However, various approximate methods have been tried to estimate these effects [10]. In Ref. [5], the ligand core level spectra from Cr(CO)6 were measured and analysed. The analysis in Ref. [5] differs from previous interpretations (for example, that in Ref. [2]), by assigning the 5 eV peak to valence-Rydberg rather than valencevalence excitations and instead associating the valence-valence excitation with the 2 eV peak. In order to resolve this conflict we compare in this paper the experimental results of Ref. [5] with new theoretical calculations. The present results lend support to the earlier interpretations [2] but also suggest several new assignments.

2. Method

We have employed the intermediate neglect of differential overlap (INDO) model [11], adopted .................... l' i.:'i

........! i o~

_____.S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

/

/"

~

0

/

0

r................ ..............................................

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

~0

/'

,

If

i/

! .

/"i

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Fig. 1. The moleculeCr(CO)6, used as a model system,where the Cr(CO)5 moietyis modellingthe metal surfaceand the CO group the adsorbate. The bond lengths are indicated in the figure.

J. Bustad et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 233 244

Ols

ionized

Neutral

Cls

ionized

2t2u

0.0

~

9tlu LUMO

-5.0"

5bl 14e 22ai 13e

5b 1 14e 22a 1 LUMO

13e LUMO HOMO

- I 0 . O-

2t2g k 4b 2 12e

v

HOMO

8tlu itlq it2u

-15.0

HOMO ~k

4b 2 12e

5~

[3 lle 21a 1 la 2 10e

Z

• ~zq 8alg

b1 4bl -20.0- ~e 20a 1 8e 19a 1 3b 2

-25.0'

-30.0-

~

/ 4eg

•

~

6tlu

lle 21al la 2 lOe 4bl 9e 3b 1 20a 1 8e

7e 19a 3b12 2b1

2u 1 7e 17a I

18a I 17a 1

6e

6e

16a I

16a I

15a I 15a I

-35.0

Fig.2. Energyleveldiagramfortheneutral,theClsionized(right)andtheOlsionized(left)Cr(CO)6molecule.

235

236

J. Bustad et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 233-244

for spectroscopy including configuration interaction (CI). The wavefunctions were computed by means of the ZINDOprogram by Zemer and co-workers [12-15]. The molecular geometry (see Fig. 1) was obtained f r o m crystallographic data [16]. A p a r t from slight deviations, the total symmetry g r o u p is Oh with all six C O groups equivalent. In principle, the wavefunction o f the ionized molecule should also transform according to an irreducible representation o f the same symmetry group. It has, however, been shown (see, for example, Refs. [2,17]) that this requires a multi-determinantal description o f the core ionized molecule in order to give a satisfactory treatment o f b o t h relaxation and correlation effects, and that a one-determinantal description using a localized core hole in conjunction with a reduced s y m m e t r y (C4v in the present case) mimicks a full treatment very efficiently. In the present calculations, a localized description has been adopted, with an equivalent-core description o f the core ionized atom. The shake-up intensities were calculated using the p r o g r a m SHAKEINT [18]. F o r details o f the computational method, we refer to R e f [19]. The equivalent-core a p p r o x i m a t i o n implies that the only excited doublet states that obtain non-zero

intensity in the present calculations are those derived from singlet-coupled valence-shell excitations. F o r certain doublet excitations o f triplet parentage, however, a perturbation theoretical method [20] has been used to obtain an estimate o f the expected intensities in the shake-up spectrum (see below).

3. R e s u l t s a n d d i s c u s s i o n

The energy level diagrams presented in Fig. 2 allow a c o m p a r i s o n to be made between the levels in neutral Cr(CO)6 (see Ref. [21]) and the core ionized molecule with a C l s hole (right) or O l s hole (left). The C l s and O l s core holes are localized and therefore the shake-up intensities are calculated using a reduced symmetry. Symmetry designations in Fig. 2 are therefore given in the idealized Oh and Cav symmetries o f the neutral g r o u n d state and the core ionized states, respectively. One can observe strong similarities with the energy changes previously observed in related metal carbonyl c o m p o u n d s in core excitation experiments [22]. Table 1 shows the calculated intensities and shake energies o f the strongest C ls shake-up satellites

Table 1 Calculated INDO/CI results for the strongest C 1s shake-up satellites in Cr(CO)6 (only intensities larger than 1% are shown) State (assignment)

Before CI Energy/eV

12e --* 13e (S' = 1) (2% ---*27ra) 12e ~ 13e (S I = 0) (2% ---,27ra) 12e ~ 15e 7e ---, 13e (S' = 1) (lrco ~ It*CO) 7e ~ 13e (S' = O) (~rco ~ 7r'CO) 8e --~ 14e 2b I ~ 5bI 2b I ~ 5bI 18al ~ 22al 7e ~ 15e

After CI Intensity/%

Energy/eV 1.83

Experiment [5] Intensity/% 1.00 a

4.64

8.39

6.44

13.00

7.05

0.04

7.91

1.31

11.72

4.07

13.43

4.24

12.98 14.98 14.98 15.65 15.38

0.00 0.00 0.00 0.26 0.19

13.79 14.86 15.36 15.85 15.95

1.79 1.36 1.47 2.63 1.65

a Estimated by perturbation theory (see text).

Energy/eV

Peak number

2.3

1

4.6 5.6 7.1 8.9

2 3 4 5

15.0

6

17.0

7

J. Bustad et al./Journal o f Electron Spectroscopy and Related Phenomena 70 (1995) 233-244

237

Table 2 Energies and wavefunctions for the strongest C l s shake-up satellites in Cr(CO)6 using the I N D O / C I method and N i s h i m o t o - M a t a g a g a m m a integrals Energy/eV

Intensity/%

CI wavefunction

6.44

13.00

0.6363(12e---~13e)x - 0 . 6 3 6 3 ( 1 2 e ~ 13e)y + 0.1948(10e---~13e)x - 0 . 1 9 4 8 ( 1 0 e ~ 1 3 e ) y + ...

7.91

1.31

-0.4544(12e---~15e)x - 0 . 4 5 4 4 ( 1 2 e ~ 15e)y - 0.4519(lle--~13e)x - 0.4519(11e---, 13e)y +

13.43

4.24

0.5500(7e---~13e)x - 0.5500(7e---*13e)y - 0.2884(6e--,13e)x - 0.2884(6e-+13e)y + ...

13.79

1.79

0 . 3 7 8 2 ( 8 e ~ 14e)x+0.3782(8e--, 14e)y-0.3177(20al ~ 2 2 a l ) - 0 . 3 0 3 0 ( 4 b l ---+5 b l ) + . .

14.86

1.36

0.4539(2bl ---*5bl) + 0 . 4 1 4 0 ( 8 e ~ 15e)x - 0.4140(8e--,15e)y+0.3984(18al ---22al) + ..

15.36

1.47

0.5723(2bl ~ 5 b ~ ) - 0 . 4 7 3 8 ( 6 e ~ 13e)x - 0.4738(6e---~13e)y + 0 . 2 6 6 0 ( 7 e ~ 1 5 e ) + 0.2660(7e---*15e)y + ...

15.85

2.63

0.8220(18al---~22al) + 0 . 2 9 6 7 ( 2 b l ~ 5 b l ) + 0.2076(6e--*13e)x + 0 . 2 0 7 6 ( 6 e ~ 1 3 e ) y + ..

15.95

1.65

0 . 5 6 7 5 ( 7 e ~ 1 5 e ) x + 0 . 5 6 7 5 ( 7 e ~ 15e)y - 0.3227(17al ---~22al) - 0.2708(18al ~ 2 2 a l ) +

and Table 2 the corresponding CI expansions. Because the Cav axis is chosen as the z axis, we have indexed the two degenerate components of the e levels with an x and y, respectively. Tables 3 and 4 give the results for the O 1s shake-up satellites using the same nomenclature. In the Cls shake-up spectrum shown in Fig. 3, several distinct shake-up features can be observed. The experimental curve is based on the same raw data as in Ref. [5]. In the lower line peak 1 appeared at 2.3 eV shake energy. The dominant feature is

peak 3, centred at 5.6eV with an intensity of about 28%. Other peaks are found at 4.6eV, 7.1 eV and 8.9eV, with intensities about 7-10%. The rest of the lines have much lower intensity. The width of the main peak is 0.82 eV. In the Ols spectrum displayed in Fig. 4, also with raw data from Ref. [5], a number of distinct lines are found, including a small additional structure at about 2 eV, which in Ref. [5] was explained to be either an impurity or a triplet parent coupled doublet associated with one or more of the stronger

Table 3 Calculated I N D O / C I results for the strongest O l s shake-up satellites in Cr(CO)6 (only intensities larger than 1% are shown) State (assignment)

Before CI Energy/eV

12e ~ 13e (S' = 0) (27rb ~ 2~ra) 1 le --* 13e 3b 1 ~ 5b I 9e ---* 13e 3bl ~ 5bl 9e ~ 14e 2bl ~ 5bl 18a I ---*22a I (S' = 1) (~rco ~ 7r'CO) 18al --~ 22al (S' = O) (lrco ~ 7r'CO) 17al ~ 22al

After CI Intensity/%

Energy/eV

Experiment [5] Intensity/%

Energy/eV

Peak number

5.65

6.93

6.90

9.81

5.0

1

8.61 11.34 10.88 11.34 12.34 14.90

0.24 0.00 1.29 0.00 0.00 0.00

8.89 10.27 11.13 11.50 12.46 14.47

1.95 1.47 2.12 3.49 1.54 1.17

6.2

2

9.5

3

15.8

4

14.96

0.13

14.63

4.30

16.77

0.13

16.36

1.26

238

J. B u s t a d et al./Journal o f Electron Spectroscopy and Related P h e n o m e n a 70 (1995) 2 3 3 - 2 4 4

Table 4 Energies and wavefunctions for the strongest O l s shake-up satellites in Cr(CO)6 Energy/eV

Intensity/%

CI wavefunctions

6.90

9.81

- 0 . 6 5 6 5 ( 1 2 e ~ 1 3 e ) x - 0 . 6 5 6 5 ( 1 2 e ~ 1 3 e ) y + 0.1621(12e---~15e)x - 0 . 1 6 2 1 ( 1 2 e ~ 1 5 e ) y + ...

8.89

1.95

0 . 4 6 1 1 ( 1 1 e ~ 13e)x - 0 . 4 6 1 1 ( 1 1 e ~ 13e)y + 0.4097(21al ~ 2 2 a l ) + 0 . 2 6 1 5 ( 1 2 e ~ 15e)x 0.2615(12e---*15e)y + ...

10.27

1.47

0.5219(3bl ---~5bl) - 0.3663(21al ~ 2 2 a l ) - 0.3633(20a I ~ 2 2 a j ) + 0 . 2 4 4 5 ( l l e ~ 0.2445(lle---~15e)y + ...

11.13

2.12

0.5253(9e~13e)x +0.5253(9e~13e)y +0.3694(lle~15e)x +0.3694(lle~15e)y +...

11.50

3.94

0.4517(3bl ---~5bl) - 0 . 3 7 6 5 ( 1 1 e ~ 15e)x - 0 . 3 7 6 5 ( l l e ~ 1 5 e ) y + 0 . 3 1 4 2 ( 9 e ~ 13e)~ + 0.3142(9e~13e)y +...

12.46

1.54

14.47

1.17

- 0 . 3 7 0 3 ( 9 e ~ 14e)x - 0 . 3 7 0 3 ( 9 e ~ 1 4 e ) y + 0.3308(10e--,15e)x + 0 . 3 3 0 8 ( 1 0 e ~ 15e)y 0.3296(20al ~ 2 2 a l ) + ... 0.5446(2bl---~5bl) + 0 . 4 5 3 6 ( 7 e ~ 1 3 e ) x + 0 . 4 5 3 6 ( 7 e ~ 1 3 e ) y + ...

14.63

4.30

0 . 4 7 6 5 ( 1 8 a l ~ 2 2 a l ) -- 0 . 4 5 4 7 ( 7 e ~ 13e)x -- 0 . 4 5 4 7 ( 7 e ~ 1 3 e ) y + 0 . 4 3 9 6 ( 2 b l ~ 5 b l ) + ...

16.36

1.26

0 . 5 7 1 6 ( 1 7 a l ~ 2 2 a l ) + 0 . 5 2 8 1 ( 6 e ~ 1 3 e ) x -- 0 . 5 2 8 1 ( 6 e ~ 1 3 e ) y - - 0.2155(18al ~ 2 2 a j ) + . . .

shake-up lines• The most intense shake-up line appears at 5.0eV, here also with approximately 28% of the main line intensity. Relevant additional lines are also seen at 6.2 eV and 9.5 eV. The width of the main peak here is 1.23 eV. In Ref. [5] the peaks in the Cls spectrum were predominantly assigned to a series of Rydberg transitions (states 2-4), except for line 1, which

"•20

15e)x +

was attributed to a 2t2g-3t2g (270 shake-up process. The first two pronounced shake-up peaks in the O ls spectrum, which appear at energies of 5.0 eV and 6.0 eV, were assigned to Rydberg transitions. Line 3 was explained by transitions to higher Rydberg states. In this paper the above assignments are reconsidered, based on the results of INDO/CI calculations, which leads to a partial reinterpretation of both the C 1s and O 1s spectra. ©

g~

c~

i

i

i

i

I

i

i

i

i

I

r

i

i

i

I

i

~

i

i

¢0

c~

o

2 .......

....

," •

Experimental

"~- I 0 "

.

.

.

.

:

• ......

:

.

.

.

.

........... ~

£

.

1

" 2O

. 2

:

0

.

:

.,

?"

,

10

0

2~

-

E n e r g y (eV)

Fig. 3. Experimental (lower) and calculated (upper) C l s spectra for Cr(CO)6. The theoretical spectrum is obtained using a convolution with a Gaussian of 1.0 eV F W H M . This convolution does not attempt to mimic any physical effect but rather serves as a guide to the eye. The experimental spectrum is based on the same raw data as in Ref. [5].

I

-20

-15

I

.

.

.

.

-10

Energy

I

-5

. . . .

(eV)

Fig. 4. Experimental (lower) and calculated (upper) O l s spectra for Cr(CO)6. The theoretical spectrum is obtained using a convolution with a Gaussian of 1.0 eV F W H M . The experimental spectrum is based on the same raw data as in Ref. [5].

J. Bustad et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 233-244

We will first discuss the states which are derived from the H O M O and L U M O orbitals in the neutral molecule. In a standard description of CO on transition metal surfaces, it is generally assumed that the molecular 7r and metal d states interact upon coordination and form hybrid orbitals, which can be bonding or antibonding with respect to the metal-ligand bond. Upon core ionization, the valence levels are pulled down in energy (see Fig. 2), and their charge distribution is modified. In addition, the triply-degenerate levels are split into several levels. For example, the 2t2g level is split into one doubly-degenerate (12e) and one non-degenerate (4b2) level. That shape of the 12e orbital is shown schematically in Fig. 5(a) for the Cls ionized case and in Fig. 6(a) for the O ls ionized case. In standard ligand field theory this is commonly denoted as a backbonding orbital, because it is a bonding combination of a metal d orbital and the originally empty 7r* (270 orbital of CO. In order to enable a comparison to be made with the notation for adsorbates on surfaces we also use here the notation 2% for this orbital, where the subscript b conventionally denotes bonding character between adsorbate and surface. In future discussions we shall also encounter its antibonding counterpart, denoted 27ra, where the subscript a indicates antibonding character between the metal d and the adsorbate 7r* orbitals. As we have mentioned earlier, Cr(CO)6 is used here as a model system to discuss CO adsorption on metal surfaces. The system is thus naturally divided into a Cr(CO)5 moiety modelling the metal surface, and a CO group (Fig. 1). Using this division into subgroups and analyzing the Mulliken populations, we find that in the unionized molecule, the 2tzg orbitals are mainly formed from the metal 3d (population = 0.45) and the 27r (Tr*) orbitals of the CO ligands in the Cr(CO)5 moiety (0.46). The total population of the Cr(CO)5 moiety is thus 0.91, leaving 0.09 for the "adsorbed" CO 7r* character. The amount of 7r* population in the 14e orbitals is 0.23 upon Cls ionization and 0.25 upon Ols ionization, respectively, which illustrates the charge transfer screening from the rest of the molecule to the ligand. These populations can be compared with the

239

Table 5 Summary of ground state orbital populations according to Mulliken (total charges); the Cr(CO)6 molecule is split into a Cr(CO)5 moiety and an "adsorbed" CO group Cr(CO)5

Neutral molecule Carbon ionized Oxygen ionized

(CO)*

Cr

5CO

Carbon

Oxygen

3.37

52.19

3.99

6.45

3.47

51.52

4.88

6.13

3.24

51.67

3.95

7.14

corresponding results for Mo(CO)6 [3], where the calculated population for adsorbed CO 7r* character was 0.11 before ionization and where the 7r* contribution to these orbitals increased to 0.27 and 0.21 after Cls and Ols ionization, respectively. The total valence populations of the different groups/atoms are given in Table 5 for the neutral, Cls and Ols ionized Cr(CO)6 molecules, respectively. As is seen from this table, a charge of approximately 0.6e is transferred from the Cr(CO)5 moiety to the ionized ligand, both upon Cls and upon Ols ionization. It can be noted that most of this charge comes from the five CO ligands and that only minor changes (of the order of 0.1 eV) occur on the chromium atom. One can also observe that the local screening of the C ls hole is more efficient than that of the Ols hole, as was found before for the Mo(CO)6 molecule [3]. The difference in direction of the change in charge on the chromium atom in the case of C 1s and O 1s ionization, respectively, provides an interesting illustration of the oscillatory behaviour observed for the charges in a molecule when creating a localized core hole (see, for example, Ref. [23]). In Fig. 3, a comparison is shown between the calculated C 1s spectrum of Cr(CO)6 and the associated experimental spectrum. The corresponding (theoretical and experimental) spectra for Ols ionization are shown in Fig. 4. The intensity scales are normalized to the main Cls and Ols lines, respectively. As mentioned above, the dominant feature of the C ls shake-up spectrum is a strong structure at 5.6eV, having about 30% of the main line

240

J. Bustad et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 233-244

(a)

C

dxy - 0.625 py 0.446

Fig. 5. Schematic representation of the d-Tr* (a) bonding (27rb or 12e) and (b) antibonding (27ra or 13e) orbitals in the presence o f a C l s hole. The sizes of the contributing AOs are proportional to their weight in the MO. The largest MO coefficients are given in the figure.

intensity. From the calculations, it is evident that the dominant contribution to this structure comes from the 27rb ~ 21ra excitation, which is placed at 6.4eV, with a calculated intensity of 13% (see Tables 1, 2 and Fig. 3). The corresponding doublet of triplet parentage (S' = l) is obtained at 1.83 eV shake energy. The latter cannot obtain any intensity in the equivalent core model. The obtained intensity (about 1% of the main peak) shown in Fig. 3, has been estimated using a perturbative treatment analogous to the one given by Cory and Zerner [20] for metalligand exchange coupling in transition-metal complexes. The energies of excited triplets are generally

calculated too low by the INDO/CI method, which is a well known consequence (for example, Refs. [14,24] of the Nishimoto-Mataga parametrization [25]. These two states correspond to peaks 1 and 3 in the experimental spectrum. This interpretation is different from the one given in Ref. [5] of the Cr(CO)6 shake-up spectrum, where the first peak was attributed to the 27%-2~ra orbital excitation, but with singlet parentage, and the third peak to transitions to Rydberg-derived states. The assignment of peak 3, however, agrees with the one previously given by Freund and coworkers [2,6]. Upon Ols ionization, the 2~"b and 2~-a states obtain quite different character than in the case

J. Bustad et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 233-244

(b)

241

Y

0

py 0,667

O

D Fig. 5. Continued. of C ls ionization. In the presence of an oxygen core hole, the charge in both the 27rb and 27ra orbitals will be more heavily concentrated on the carbon end of the ionized ligand (see Fig. 4). This is understood from a comparison with the Z ÷ 1 molecules, NO + and CF ÷. In the NO molecule, both the 17r and 27r orbitals are rather evenly distributed over the two nuclei. In CF, in contrast, the C - F bond contains very little double bond character and the 17r orbital is essentially a lone pair orbital on F. For orthogonality reasons, the 27r orbital will hence be localized almost entirely on the C atom. The S t = 0 component of the 27rb-27ra intergroup excitation is positioned at 6.9 eV according to the calculations, which is somewhat too high.

The same tendency to overestimate Ols shake-up energies has previously been observed in a series of calculations on carbonyl compounds, using the same computational method [26]. The corresponding triplet is estimated to have virtually no intensity in the case of an O core hole. This agrees fairly well with the experimental spectrum, where essentially no satellite intensity is obtained below 5 eV shake energy. One can, in this connection, note that a recent core excitation study of a series of carbonyl compounds [21] shows an analogous difference between the C ls and O ls spectra, namely that a state is seen around 2eV in the Cls but not in the O 1s spectra. The antibonding 13e orbital (27ra) is dominated

242

3". Bustad et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 233-244

(a)

i,rZ v,vv~

z 0.630

Fig. 6. Orbital picture of the d-Tr* (a) bonding (2% or 12e)and (b) antibonding (2~ra or 13e)orbitals in the presenceof an Ols hole. by the molecular CO 27r in both the neutral and ionized states (see Fig. 6(b)). Quantitatively, the weight of the "adsorbed" ~r* (CO) in the 13e orbital is 0.68 in the Cls ionized and 0.63 in the Ols ionized system. These numbers should be compared with the values 0.23 and 0.25, respectively, for the ~r* population in the ground states of the corresponding core ionized systems. The 27rb-27ra excitation will hence in both cases imply a significant charge transfer (0.4e) from the rest of the molecule to the core-ionized ligand, and can therefore be characterized as an "inter-group" shake-up excitation. The structure centred at about 9eV in the

Cls spectrum (state 5) can be understood by a comparison with previous calculations of the free CO molecule. The doublet of triplet parentage derived from the 7r--~ ~r* excitation in the Cls shake-up spectrum of free CO falls at an energy of 9.1 eV, while the lowest singlet coupled doublet is found around 17 eV [27]. As already mentioned, in the present calculations doublets of triplet parentage are not included in the CI expansions, as a consequence of the equivalent core approximation, which explains their absence in the calculated spectrum. We therefore assign line 5 to the internal CO ~r-~r* excitation. The corresponding doublet of singlet parentage is found in the calculated

J. Bustad et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 233-244

243

(b)

0

!5~

C Pz 0.756

0

C Fig. 6. Continued. spectrum around 13 eV (approximately the 7e ~ 13e excitation; see Table 2), where it is mixed with a number of other excitations. In the experimental spectrum [5], line 6 is extremely broad due to the interaction with the underlying shake-off continuum (see Fig. 3). We have not attempted to estimate the contribution from the triplet coupled 7r-Tr* excitations in the 9 eV region, since the simple perturbational method used to estimate the intensity of peak 1 does not work in the presence of strong configurational interaction. In the calculated Ols spectrum the intramolecular CO ~--zr* transitions are spread out over a multitude of states. Our calculations indicate that the singlet-coupled (S t = 0) zr-zr* shake-up peak is

around 14eV (see Table 4). This is in line with the findings for free CO, where the doublet of singlet parentage was found to be so strongly mixed with the (r-or* Rydberg type transitions that it was virtually smeared out. Such states cannot be described properly by the present I N D O procedure because Rydberg type orbitals are not included in the atomic basis. For the carbonyl compounds Rydberg states can be expected to appear about 2 eV below the first ionization potential. Because Rydberg states carry little or no intensity of their own, they will not affect the main features of the calculated spectra. They may be expected to borrow intensity through configurational mixing with more intense singly excited valence states [15].

244

J. Bustad et al./Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 233-244

The artificially high intensity obtained for the peaks around 15 eV in the calculated Cls and Ols spectra (see Figs. 3 and 4) reflect the absence of such a configurational mixing together with the general effects of truncating the CI space. The latter is well known to cause an artificial enhancement of the shake-up intensity of the highest roots of the truncated expansion.

Acknowledgements We thank the authors of Ref. [5] for putting their raw data at our disposal. This work was supported by the Swedish Natural Science Research Council (NFR) and the National Board for Industrial and Technical Development (NUTEK).

References [1] E.W. Plummer, W.R. Salaneck and J.S. Miller, Phys. Rev. B, 18 (1978) 1673. [2] H.-J. Freund and E.W. Plummer, Phys. Rev. B, 23 (1981) 4859. [3] J. Bustad, C. Enkvist, S. Lunell, H. Tillborg, A. Nilsson, S. Osborne, A. Sandell, N. Mhrtensson and S. Svensson, Chem. Phys., 179 (1994) 303. [4] A. Nilsson and N. M~rtensson, Phys. Rev. B, 40 (1989) 10249. [5] A. Nilsson, N. M~rtensson, S. Svensson, L. Karlsson, D. Nordfors, U. Gelius and H. ,~gren, J. Chem. Phys., 96 (1992) 8770. [6] H.J. Freund, E.W. Plummer, W.R. Salaneck and R.W. Bigelow, J. Chem. Phys., 75 (1981) 4275. [7] H.J. Freund and R.W. Bigelow, Chem. Phys., 55 (1981) 407.

[8] B. Sj6gren, A. Naves de Brito, N. Correia, S. Lunell, B. Wannberg, U. Gelius and S. Svensson, J. Electron Spectrosc. Relat. Phenom., 59 (1992) 161. [9] H. Tillborg, A. Nilsson and N. M~rtensson, J. Electron Spectrosc. Relat. Phenom., 62 (1993) 73. [10] R.W. Bigelow and H.-J. Freund, J. Chem. Phys., 77 (1982) 5552. [11] J.A. Pople, D.L. Beveridge and P.A. Dobosh, J. Chem. Phys., 47 (1967) 2026. [12] A.D. Bacon and M.C. Zerner, Theor. Chim. Acta, 53 (1979) 21. [13] J. Ridley and M.C. Zerner, Theor. Chim. Acta, 32 (1973) 11. [14] J. Ridley and M.C. Zerner, Theor. Chim. Acta, 42 (1976) 223. [15] M.C. Zerner, G.H. Loew, R.F. Kirchner and W.T. Mueller-Westerhoff, J. Am. Chem. Sot., 102 (1980) 589. [16] B. Rees and A. Mitschler, J. Am. Chem. Soc., 98 (1976) 7918. [17] P.S. Bagus and H.F. Schaefer III, J. Chem. Phys., 56 (1972) 224; L.S. Cederbaum and W. Domcke, J. Chem. Phys., 66 (1977) 5084. [18] S. Lunell, Tech. Rep. 892, Dept. Quantum Chemistry, Uppsala University, 1987. [19] S. Lunell, M.P. Keane and S. Svensson, J. Chem. Phys., 90 (1989) 4341. [20] M.G. Cory and M.C. Zerner, Chem. Rev., 91 (1991) 813. [21] G. Cooper, K.H. Sze and C.E. Brion, J. Am. Chem. Soc., 112 (1990) 4121. [22] A.T. Wen, E. Riihl and A.P. Hitchcock, Organometallics, 11 (1992) 2559. [23] S. Canuto, J.-L. Calais and O. Goscinski, J. Phys. B, 14 (1981) 1409. [24] R.W. Bigelow, J. Chem. Phys., 66 (1977) 4241. [25] K. Nishimoto and N. Mataga, Z. Phys. Chem., 12 (1957) 335. [26] M.P. Keane, S. Lunell, A. Naves de Brito, M. CarlssonG6the, S. Svensson, B. Wannberg and L. Karlsson, J. Electron Spectrosc. Relat. Phenom., 56 (1991) 313. [27] J. Schirmer, G. Angonoa, S. Svensson, D. Nordfors and U. Gelius, J. Phys. B, 20 (1987) 6031.