Volume 175, number 4
CHEMICAL PHYSICS LETTERS
14 December 1990
An investigation of the electronic structure of osmium tetroxide by photoelectron spectroscopy with variable photon energy Jennifer C. Green, Nicholas Kaltsoyannis, Kong H. Sze Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK
and MichaelA. MacDonald SERC Daresbury Laboratory, Daresbury, Warrington WA44AD, UK Received 30 July 1990; in final form 27 September 1990
Relative partial photoionization cross sections and photoelectron branching ratios have been obtained for the valence bands of osmium tetroxlde in the ionization energy range I2 to 18 eV. The photon energies used ranged between 24 and 100 eV. The ionization cross sections of the 21, and 2a, orbitals show evidence of substantial metal character establishing strong OS-O obonding. Very little evidence is found for n-bonding.
1. Introduction
-
3a,
/
The valence electronic structures of the do tetrahedral MO4 species are of the form
where the numbering scheme ignores orbitals correlating with the core orbitals of the constituent atoms A schematic molecular orbital (MO) diagram showing the possible atomic contributions to the various MO is given in fig. 1. The He I photoelectron (PE) spectra of 0~0, and RuO, have been studied previously by a number of authors [ l-41; live bands are observed, correlating with ionization from the top five orbital sets; the la, and ltz orbitals are principally oxygen 2s in character and they lie too low in energy to be ionized by He I radiation. X-ray PE experiments on 0~0, have located OS 4f, 4d and 4p as well as 0 1s ionizations [4]. In the most recent account, both He I and He II radiation are used for photoionization and the variation in band intensity with photon energy is discussed [ 5 1. Though during the course of these investigations, a variety of band assignments were proposed, the weight of the exper-
5d
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t2
2PO
-I
*a,-
f
2t2 -
al
12 112
,-
la,
,A
2s
0 s 04
OS
>
04
Fig. 1, Schematic molecular orbital diagram for
0009-26 14/90/$ 03.50 B@1990 - Elsevier Science Publishers B.V. (North-Holland
)
0 0~01. 359
imental
evidence
appeared
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CHEMICAL PHYSICS LETTERS
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to favour
the ion slate
=
hv 39eV
ordering
Both DVM-Xa [6 ] and MS-X& [7] calculations have been performed on 0s04, and these suggest an MO ordering It1 > 3t2> 2a, > le> 2t2 for the five highest-lying MOs, although the authors readily admit that the ordering of the energy levels is extremely sensitive to the choice of input parameters, namely the initial charge densities in the case of the DVMXa calculation and the sphere radii in the case ofthe MS-Xa calculation, In obtaining cross section and branching ratio data on OsO,, we hoped to clarify the band assignment, obtain information on covalency in this prototypical molecule and possibly observe resonances in the cross sections.
hv =52 5eV
2. Experimental hv
A sample of OsO, was obtained commercially from Johnson and Matthey. Its He I spectrum was compared with that reported previously [4], and the sample was judged to be sufficiently pure to use without further purification. The photoelectron spectra of 0s04 were obtained using the synchrotron source at the SERC’s Daresbury Laboratory. A full account of the angle-resolved photoelectron spectrometer employed and our experimental method has been given previously [ 81.
~66~4
IE IeV
3. Result and discussion
Fig. 2. Photoelectron spectra ofOs0, at (a) 39, (b) 52.5 and (c) 66 eV photon energy.
The PE spectra of OsO,, obtained at photon energies of 39, 52.5 and 66 eV are shown in fig. 2. We did not attempt to resolve the vibrational fine structure seen in previous He I studies [2,3]. Consequently, the bands had an asymmetric shape, and it proved necessary to use asymmetric Gaussian functions to tit the peaks, and thus obtain the band areas used in the relative-partial-photoionisation crosssection (RPPICS) calculations. The RPPICS for bands l-5 in the PE spectrum of OsOl are given in fig. 3 together with an expanded plot of that of band 4.
The most pronounced feature is the “double hump” profile of the cross-section data for band 5 in the region 50-70 eV. This is a clear indication ofp+d giant resonant enhancement [ 91 of the cross section of this band, coinciding with the OS 5p subshell ionisation potentials (‘P 312=49 eV *P1,,2=61 eV) [lo]. This behaviour is expected from MOs having significant OS 5d character, and we agree with previous workers [l-5] in assigning band 5 to the strongly bonding 2t2 MOs. Between 24 and 37 eV photon energy, the cross section of band 5 passes through a maximum. This is a common feature of d ioniza-
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CHEMICALPHYSlCS LETTERS
(a) 60000
6
c f
m
BAND1
0
BAND2
A
BAND3
+
BAND4
C
BAND5
60000
2
z s s
40000
g !! ::
20000
2 0
0 10
30
53 PHOTON
70 ENERGY
90
110
(eV)
(b),
LI
s
16000 +
BAND4
z
&
0 10
30
50 PHOTON
70 ENERGY
90
110
(eb’)
Fig. 3. Relative partial photoionization cross sectionsfor 0~0~: (a) bands I-5, (b) band 4.
tions, the delayed maximum being due to centrifugal barriers preventing the continuum f-wave from occupying the inner-well region [ 111. Calculations on atomic ionization cross sections for OS 5d orbitals [ 121 predict the maximum to occur at a kinetic energy (KE) of 15.1 eV, which is similar to the kinetic energy of N 14 eV we find for the cross-section maximum of band 5. Both these features suggest that band 5 arises from ionization from an orbital with significant d character and confirm the assignment of band 5 to the 2tF’ ionization. Two possible origins have been suggested for the splitting of band 5 in the PE spectrum of 0~0~ [ 41, namely Jahn-Teller splitting and spin-orbit coupling. The intensity sequence is that expected for spin-orbit coupling, i.e. E” ( ZT,) > U’ (2T2); the 0.4
14December 1990
eV splitting suggests an OS 5d content of 60% [4]. In view of the size of the p+d resonance found for band 5, this level of OS 5d contribution to the 2t, orbital does not seem unreasonable. In the PE spectrum of RuO,, the corresponding band shows no splitting [ 31 reinforcing the view that the splitting found in the OS case has its origins in spin-orbit coupling. The RPPICS of band 4 maintains a fairly constant level between photon energies of 24 to 44 eV. Between 44 and 55 eV there is a strong intensity rise and fall with a maximum z 50 eV. Though this coincides with the region ofthe 5p ‘Psiz ionization, the absence of a second resonance between 55 and 70 eV leads us to discount a p+d resonance followed by super Coster-Kronig decay as its origin. At higher photon energies, the cross section falls off sharply. Band 4 results from ionization of the 2a, orbital and as such may be expected to show cross-section features characteristic of the OS 6s orbital. Calculations on atomic photoionization cross sections for OS 6s electrons [ 121 predict a Cooper minimum at photon energies of z 17 eV (KE 10 eV) and a subsequent maximum at h v zz40 eV ( KE 33 eV ) . The OS 6s cross section is calculated to have a fourfold increase between these two photon energies. The ionization energy of band 4 is 14.7 eV, so the KE of the cross-section maximum is 35.3 eV in agreement with the calculation in this respect. However, OS 6s cross sections are calculated to be between one and two orders of magnitude less than 0 2p cross sections at these photon energies, so on these grounds it is difficult to reconcile the 25% intensity increase in band 4 with an atomic effect due to OS 6s contribution to the associated orbital. Alternative possible causes are a shape resonance [ 131 or interchannel coupling [ 141, Molecular cross-section calculations are needed to resolve this dilemma. Overall, the very different cross-section behaviour of band 4 in comparison with bands 1 to 3 leads us to support the assignment of this band to the 2a;’ ionization, and assume that its cross-section features reflect a significant OS 6s contribution to the MO. Both theoretical studies assigned band 3 to the 2a;’ ionization [ 6,7]; we feel such a conclusion is not tenable. If the assignments of bands 4 and 5 are correct, bands l-3 must be assigned to the It,, 3t2 and le or361
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0.7 X
BAND1
0
BAND2
A
BAND3
06.
0.5 1 0.4 -
0.3 -
:
0.2 -
0.1 -
0.0-I 10
I
30
50 PHOTON
ENERGY
I
I
70
90
I
110
(eV)
Fig.4. Photoelectronbranchingratiosfor bands 1-3 of Os04. bital ionizations. These three orbitals are considered to be primarily 0 2px in character, and whereas the 3t: and le orbitals may also have a contribution from the OS Sd orbitals, any metal contribution to the 1t, orbital is symmetry forbidden. The classic view of these orbitals is that 1t, is the highest lying and nonbonding, and 3t, and le are lower lying and x-bonding. Band 1 has rather different vibrational structure from bands 2 and 3 [ l-41, which have the O-O transition as by far the most intense indicating very little geometry change on ionization. Thus, the evidence from the vibrational structure is that two of these orbitals (those associated with bands 2 and 3) are nonbonding; the third (giving rise to band 1) may have more bonding (or anti-bonding) characteristics but paradoxically is the easiest orbital from which to ionize an electron. There is a conflict, therefore, between the expectations from simple MO theory and the interpretation of the vibrational structure of the first three PE bands. The RPPICS of these three bands is given in fig. 2 and their branching ratios in fig. 4. It is evident that the cross-section behaviour of band 1 is rather different from that of bands 2 and 3, whose RPPICS resemble one another. Overall the intensity patterns tend to be 1 > 2 z 3, which suggests that band 3 may be assigned to lee’ ionization. Though all three bands show p+d resonance structure in the same region as band 5, it is weaker; out of the three bands, the p+d resonance is strongest in band 1. Such p+d resonances have been observed previously for ioni-
14 December 1990
zation bands from orbitals which, by symmetry, may have no metal d character [ 131 but they are normally weaker than from those associated with orbitals in which d character is symmetry allowed. These considerations lead to the hypothesis that band 1, which shows the strongest resonance of the three, is associated with ionization from the triply degenerate orbital with d character, namely 3t,, and that band 2 must be assigned to 1t,. These assignments are also consistent with the generally greater intensity of band 1 than 2 as the OS 5d cross section is generally greater than that of 0 2p orbitals in the region of interest [ 121. Both the vibrational data and the cross sections suggest little x-bonding, but strong 0 2p/Os 5d mixing in the t2 set of orbitals, leading to very strongly bound 2t, electrons and the 3t, electrons being the easiest to remove. There is, however, a body of evidence (for example cited by Burroughs et al. [ 41) for the reverse assignment. The chief plank of the counter argument is that the non-bonding 1t, orbital is expected to have the lower ionization energy as is found extensively in analogous tetrahedral halides. It is difficult to reconcile this assignment with the results of our study, and we feel that the weight of the evidence favours the ion state ordering
Further theoretical investigation of OsO, is called for; calculations to date have neither treated relaxation effects fully nor included relativistic effects which are expected to be significant for a third-row transition metal. The overall picture of the bonding in 0~0, which emerges from this study is of the primary covalent interaction being localized in the 2tz and 2a, orbitals, that is, strong o-bonding. The 1e orbitals appear to have very little metal d-character and to be nonbonding, as must the lti orbital be for symmetry reasons. The 3t2 orbital appears to have some metal 5d character but its electrons have lower binding energies than the It, and le electrons. x-bonding therefore appears minimal.
Acknowledgement We thank the Johnson Mathey Chemical Com-
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CHEMICALPHYSICS LETTERS
pany for the loan of the Os04> and the SERC for financial support.
References [ I] E. Diemann and A. Miiller, Chem. Phys. Letters 19 (1973) 538. [ 21 S. Foster, S. Phelps, I.C. Cusachs and S.P. McGlynn, J. Am. Chem. Sot. 95 (1973) 5.521. [ 31S. Evans. A. Hamnett and A.F. Orchard, J. Am. Chem. Sot. 96 (1974) 6221. [ 41 P. Burroughs,S. Evans, A. Hamnett, A.F. Orchard and N.V. Richardson, J. Chem. Sot. Faraday Trans. II 70 ( 1974) 1895. [ 51R.G. Egdell, Ph.D. Thesis, Oxford (1977).
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[6] A. Rauk, T. Ziegler and D.E. Ellis, Theoret. Chim. Acta 34 (1974) 49. [7] J. Weber,Chem. Phys. Letters45 (1977) 261. [8] G. Cooper, J.C. Green, M.P. Payne, B.R. Dobson and I.H. Hillier, J. Am. Chem. Sot. 109 (1987) 3836. [9] J.L. Dehmer. A.F. Starace, U. Fano, .I. Sugar and J.W. Cooper, Phys. Rev. Letters 26 ( 1971) 1521. [IO] D. Brigs, ed., Handbook of X-ray and ultraviolet photoelectron spectroscopy (Heyden, London, 1977). [ 1I] R.D. Cowan, The theory of atomic structure and spectra (Chemical Abstracts Service, Columbus, OH, I98 1). [ 121J.J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32 (1985) I. [ 131J.-H. Fockand E.E. Koch, Chem. Phys. 96 (1985) 125. [ 141S.H. Southworth, A.C. Parr, J.E. Hardis and J.L. Dehmer, Phys. Rev. A 33 (1986) 1020. [ 151G. Cooper, J.C. Green and M.P. Payne, Mol. Phys. 63 (1988) 1031.
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