The photo-electron spectrum of XeF,

BURKHARD BREHM, MICHAEL MENZINGER,' AND CHRISTIAN ZORN Plzysikaliscl~es Ztzstitut, Urziuersity of Freibltrg i.Br., Gerrnany

Received May 4, 1970

Six electronic states of XeF, + have been found in the photo-electron spectrum of XeF,. The adiabatic ionization potentials, vertical ionization potentials, and relative ionization probabilities for these states are compared with theoretical predictions and an attempt is made to assign the states in terms of molecular orbitals.

Canadian Journal of Chemistry, 48, 3193 (1970)

Introduction

The discovery (1) of the xenon fluorides in 1962 was soon followed bv an extensive discussion of the electronic structure of these compounds (2, 3). Several different theoretical approaches were used out of which the linear combination of atomic orbitals and molecular orbitals (1.c.a.o.- m.0.) scheme (3-7) appears to be the most successful in explaining properties such as molec- ular geometry, thermochemical data, visible and ultraviolet-absorption spectra, and magnetic properties.

We report the photo-electron spectrum of XeF,, which gives direct information about some low lying electronic states of the XeF,' ion and within the approximation of Koopmanns' the- orem (8) about the binding energy and bonding properties of the molecular orbitals occupied in XeF,.

Experimental The spectrometer, which has been described previously

(9) employs a planar retarding field and an einzellens for collimation of the electron beam prior to the retarding field. The electrons produced by 584 A resonance radiation of a He discharge lamp are counted and regis- tered in the 400 channels of a n~ultichannel analyzer which is swept in the multiscaling mode synchronously with a sawtooth voltage applied to the retarding field grid and the einzellens. The XeF, gas emerged from a multi- channel jet of sintered glass. The resolving power, as measured by the 10 to 90% step width of electrons from Ar or Xe is 10 meV, although this resolution apparently has not been reached in this study, probably due to surface changes caused by the reactive gas under investigation.

Figure 1 shows a photo-electron spectrum of XeF,. Upon initial admission of the gas the two steps at 12.13 and 13.43 eV were ten times as large as in Fig. 1. By addition of some Xe gas it could be proven that these two steps are due to Xe-atoms formed by decomposition

'Present address: Department of Chemistry, University of Toronto, Toronto 181, Ontario.

of the xenon fluoride probably at the walls of the gas inlet system. After conditioning for I h these two steps had decreased to the size displayed in Fig. 1 where they serve as a convenient internal standard for the energy calibration.

Six steps corresponding to different electronic states of the XeF,+ ion are found and there seems to be a seventh structure at the high ionization energy end. I t occurs at the low electron energy end of the spectrum where the bad signal-to-noise ratio and an appreciable background of slow electrons make the investigation of the spectrum difficult. Table 1 summarizes the experi- mental findings.

Theory

The Results of Linear Combination of Atomic Orbitals and Molecular Orbitals

Before discussing the electronic state assign- ment in terms of the various molecular orbitals, a short summary of the theoretical predictions (6) may be appropriate. Taking only the three 5p orbitals of Xe and the six 2~ orbitals of the two F atoms as a basis set, the following molecular orbitals (m.0.) can be.formed: there is one o, orbital which is the difference of the two fluorine 2p, atomic orbitals (a.0.). Symmetry excludes any contribution of the Xe 5p orbitals. The same is true of the only n, orbital which is the difference of the two fluorine 2p, orbitals. These two orbitals should be non-bonding and close to- gether on the orbital energy scale. If the band intensities are assumed to be approximately proportional to the degeneracies of the vacated orbitals, an intensity ratio of 2:l in favor of the ng orbital is expected.

Two o, orbitals can be formed out of the sum of the two 2p, a.0. of fluorine and the Xe 5p, a.o., one of which should be strongly bonding (low energy) and the other strongly anti-bonding (high energy). The contribution of the Xe a.0. should be larger for the anti-bonding o, orbital because of the lower ionization energy of Xe.

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3194 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970

Finally two K, orbitals can be formed out of the sum of the two F 2pn a.0. and the Xe 5pn a.0. Again one should be bonding and the other anti-bonding, but the binding of these orbitals is expected to be small due to the small overlap. The intensity of the ionization process elimi- nating an electron from a K orbital is expected to be roughly twice as large as in the case of a o orbital, but deviations from this rule are not uncommon.

The spin-orbit coupling can split only the two K, orbitals into K , , ~ , and KU3/, components, since the K, orbital is localized on the F atoms which are too small to cause measurable spin- orbit effects. Altogether, there are eight molecular orbitals (K, is doubly degenerate), the order of which with decreasing ionization energy from left to right is: (oub)2, (xu, (0J2, ( K , ~ / , ~ ) ~ , ( K , ~ , , ~ ) ~ , (oua), where "a" stands for

m ..- 3 0 0

20-

15-

10-

5-

1 -

anti-bonding and "by' for bonding. As there are only 16 valence electrons to be accommodated by the basis set chosen thus the o,;, orbital is left unoccupied for the ground state of XeF,. This leads to an atomic net charge distribution with about + 1 around the Xe atom and - 112 around each F atom (3, 6). The order of the o, and K,

orbitals is practically undetermined within this approximation. The ionic states in a photo- electron spectrum should correspond to the electronic states generated by removing one electron out of either one of the occupied orbitals.

3

=I; i

. XeF, 1 ..&,fa .*..hh'

t *... .*.p-i* 1 S" '

.*' 'b &#&.>.+-.

'1; 1 t"

x. l Z -' 'P-. 1 * "&&..*#

'4 1

.$ F.r.

x. I :-'

;J

--. eV

11 12 13 1L 15 16

Interpretation of the Results

A. Energetic Position and Relative Intensity Comparing these predictions with the mea-

surements (see Fig. l ) the first two steps a t 12.44 and 12.94 eV, essentially equal in size and shape,

17 1'8 19 20

FIG. 1. The photo-electron spectrum of XeF,. 584 A. The contents of consecutive channels of a multichannel analyzer are plotted as a function of the difference of the photon energy and the electron energy hv - E,,.

TABLE 1 Observed transitions and their assignments

-.

Ionization potential (eV) Relative step Width Molecular orbital

Adiabatic Vertical height (ev) classification

12.33+0.02 12.44 1 0 .2 nu,, R = 312 12.83+0.02 12.94 1 0 . 2 nu,, R = 112 13.58+0.05 13.68 0.43 0 .3 (?) 0, 14.06+0.05 14.36 1.3 0.7 Kg

15.4010.05 15.9 (?) 1 .6 1 .O nu,, R = 312, and 112 17.10+0.1 17.5 1 .1 1.0 Gub

20.0510.1 ? ? ? ?

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BREHM ET AL.: THE PHOTO-ELECTRON SPECTRUM OF XeF, 3195

can safely be assigned to the electronic states produced by ionization from the nu,l2, and the xul12, orbitals. Their small width also supports the prediction that these orbitals are only slightly anti-bonding.

Our value for the first ionization potential of XeF, 12.33 eV agrees with the photo-ionization result of Morrison et al. (lo), 12.28 eV. The 0.05 eV difference between the two numbers is caused by a difference in the interpretation of the curves. In both measurements the ionization starts with a small foot at 12.28 eV. We believe that this small foot is due to ionization processes of molecules in vibrationally excited states, because at 300 OK the majority of the molecules is in vibrationally excited states due to the relatively low vibrational frequencies (1 1). This assumption can be proven or disproven by mea- surements of the onset of the ionization at different gas temperatures. The apparent dis- agreement with the value 1 1.5 +_ 0.1 eV given by Wilson et al. (7) seems to indicate that these authors have overestimated the precision of their extrapolation procedure based on four bands out of a Rydberg series.

Of the two next steps which are expected to correspond to the two "gerade" orbitals the smaller one at 13.68 eV can only involve the transition to the (0,)-' state of the ion while the large step comes from (n,)-' state of the ion. The next broad step at 15.9 eV is assigned to the (nub)-' state and gives an indication of the strongly overlapping spin orbit doublet R = 112, R = 312. This leaves the step at 17.5 eV to the (cub)-' ionic state.

B. Bonding Character of the Orbitals While the energetic order and relative ioniza-

tion probabilities of the ion states seem to be in rough agreement with the expectations, an evaluation of the width of the observed structures in terms of bonding properties of the m.o.'s leads to some difficulties. The 1.c.a.o.-m.0. model predicts strong bonding only for the IS,, m.o., while the other orbitals should be non-bonding (o,, n,), or only slightly bonding (nub), or slightly anti-bonding (nu,), but only the first two steps identified as (nu,)-', R = 312 and (xu,)-', R = 112 are in agreement with this. The predic- tions of the bonding properties of the m.0. based on qualitative m.0. arguments have to be taken with caution, however, especially in molecules, where ionic binding makes an important con-

tribution to the stability of the molecule as in XeF, (6).

As has been mentioned before, the charge distribution in XeF, is believed to be + 1 at the Xe atom and - 112 at each F atom. Ionization of an electron out of an m.0. localized on the two F atoms will decrease the net charge of these atoms to zero eliminating the ionic contribution to the binding. So, within this simple model, the ionization of an electron from the o, or n, orbital will behave like the ionization of an electron from a bonding orbital in spite of the fact that these orbitals can have no interaction with any of the 5p a.0. of Xe. An application of this argument to the two nu electrons suffers from the lack of information about the relative contribution of the Xe a.0. and the F a.0. to these m.0. If one simply takes point charges, one finds that the attractive forces are increased as one removes an electron from an orbital where the contribution of the Xe a.0. is more than 63%; in the other case the forces are decreased. This is consistent with the experiment, if the nu, m.0. has about 60% Xe a.o., while the nub m.0. has about 40% Xe a.0. contribution.

C. Spin-Orbit Coupling The observed spacing between the two com-

ponents of the (nu,)-' state of XeF,' (0.5 eV) can be compared with the splitting of the two 'P states of Xe' (1.3 eV). Since the expectation value of the spin orbit coupling operator is large only around the nucleus and the inner shells of the Xe atom, the part of the wave function around the Xe nucleus determines the spin-orbit splitting. Within the "almost closed shell" approximation widely used for atoms and within the one-electron m.0. approximation, the nu, m.0. must be com- pared with the 5p a.0. of Xe in the vicinity of the Xe nucleus. They should be very similar, except for a normalization factor h2 giving the relative contribution of the Xe a.0. to the m.o., because the binding energy of the two electrons compared is fortunately the same while the difference in net charges influences mainly the outer shells and not the inner shells of Xe, where the spin-orbit coupling takes place. If one furthermore assumes that in the molecule the orbital angular momen- tum is more strongly coupled to the molecular axis than to the spin (Hund's case a), the ratio of the molecular spin-orbit splitting to atomic spin-orbit splitting comes out to be $ h2. This leads to h2 = 0.58 or 58% if one takes the

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3196 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48. 1970

experimental numbers given above. This value of 58% Xe a.0. contribution to the IT,, m.0. is in amazing agreement with the other simple model presented above to rationalize the bonding characteristics of the m.o.'s.

If one accepts that the IT,, m.0. has 60% contribution of the Xe a.o., the IT,,, m.0. must be about 40% a.0. due to the orthogonality require- ment. From this one could predict a spin-orbit splitting of 0.37 eV, but the interaction of the Q = 112 component of the IT,, m.0. with the close, by (1.7 eV) o,, m.0. induced by the spin- orbit coupling and the effect of the different energy of the IT,, m.0. as compared to the Xe 5p a.0. on the wave function in the vicinity of the Xe nucleus cannot easily be taken into account. As a result the value of 0.37 eV for the spin-orbit splitting of the IT,, orbital can only be taken as a rough guess, but together with the argument about the "bonding properties" of this orbital the possibility to clearly resolve the two spin-orbit components of this state appears to be plausible.

Even though both theoretical models are quite crude they seem to lead to a consistent inter- pretation of all experimental results.

D. Contribution of Other Atoinic Orbitals Aside from the structure observed at 20 eV,

the energetic order of the os and IT^ orbital seems to make the consideration of the interaction of the orbitals discussed so far with other atomic orbitals worthwhile, even though they are either 10 eV or more below (Xe 5s, Xe 4d, F 2s) or 10 eV or more above (Xe 6s, Xe 4f, F 3s) the orbital energies used in the present basis set. Calculations indicate (6) that the F 2s orbitals show the largest interactions. These F 2s orbitals generate one a, m.0. and one o, m.0. which would have the effect of a small upward shift of the o, and o, m.o.'s discussed here. This may be the reason why the o, orbital has a lower ionization potential than the IT^ orbital.

It is not easy to see which of the additional orbitals can be taken to explain the structure observed at 20 eV. This question must be left open until better calculations or photo-electron spectra produced with more energetic radiation give enough information about the lower molec- ular orbitals of XeF,.

NOTE ADDED IN PROOF: After submission of the manuscript an article by Brundle, Robin, and Jones (12) about the photo-electron spectrum of XeF, appeared. These authors found some vibrational structure around the ionization threshold and detected the S-0 splitting of the IT, level at 15.8 eV and the structure at 20 eV using the 304 A radiation of He+. Their results are in very good agreement with the results given in this article; some small deviations in the numbers given for the 2., 5. and 6. adiabatic ionization potential (i.p.) appear to be due to differences in the evaluation procedures. Sub- traction of estimated contributions from hot bands resulted in the somewhat higher values given in this article for the 5. and 6. adiabatic i.p. The structure at the 2. i.p., after subtraction of the hot band contribution, seems to be broadened either by unresolved vibrational structure or by pre-dissociation, as Brundle et al. (12) suggest. Both effects would, however, tend to shift the peak of the ionization probability away from the adiabatic i.p. toward higher energies. In addition the definition of an "adiabatic ionization poten- tial" itself loses precision, if broadening due to pre-dissociation is assumed.

We thank Dr. J. Heitz and Dr. J. P. Adloff of the Centre des Recherches Nucleaires in Strasbourg, France, for providing us with a XeF, sample. The award of an Alexander von Hurnboldt Fellowship and financial support through the National Research Council of Canada to one of us (M.M.) is gratefully acknowledged.

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