Effects of relativity in the He(I) photoelectron spectroscopy of the transient species TeCI2 and TeBr2

Chemical Physics 50 (1980) 11-20 0 North-Holland Publishing Company

EFFECTS OF RELATIVITY IN THE He(I) PHOTOELECTRON SPECTROSCOPY OF THE TRANSIENT SPECIES TeCI2 AND TeBr, G. JONKERS, CA. DE LANGE and J.G. SNIJDERS Deparrmenr of chemisrry. Free Univen-ity, 1081 HVAmsterdam,

l%e Netherlands

Received 24 March 1980

Valence ionization energies of the transient species TeC12and TeBr2, obtained with He(I) photoelectron (PE) spectiooscopy, are presented. The interpretation is based on the results of Hartree-Fock-Slater calculations, using STF basis sets of double zeta quality. Implementation of relativistic corrections to the ionization energies of TeBrz show that off-diagonal matrix-etements of the spin-orbit operator give rise ta a splitting in the non-relativistically almost degenerate bromine “lone pair” orbital% This splitting is observed experimentally. The assignments find additional support in a comparison with PE results of related oxygen-, sulphur- and seleniumdihalogenides, a simple LCBO model, sum rule considerations and a moditied Walsh empirical digmm for AB2 molecules with 20 valence electrons.

1. Introduction In recent years significant progress has been made in the study of transient species in the gas phase by means of UV photoeIectron (PEj spectroscopy [ 11. Because of the importance of short-lived species in various branches of gas-phase chemistry, and since physical information is often scarce, the study of transients by means of spectroscopic techniques does not lack incentive. The method of UV PE spectroscopy is well-suited fo; small molecules, because their spectra usually show well-resolved bands often with vibrational structure. Interpretation of these spectra is generally considerably facilitated if PE spectra of similar molecules are available. In a series of related small molecules several members may be unstable and therefore short-lived, thus providing a natural stimulus for the adaptation of the PE technique to the study of gas-phase intermediates. An illustration of a series of related small molecules which have been studied extensively with PE spectroscopy is provided by the AX2 (A = 0, S, Se; X= F, Cl, Br) compounds. Only three members of this series can be classified as stable and their PE data have been collected with conventional experimental methods, viz. OF2 [2], OClp [2] and SCIZ [3,4]. Application of PE spectroscopy to the remaining un-

stable members of the series has been met with varying degrees of experimental difficulty. No PE information has been obtained for OBrz until now, but attempts to observe SF2 [S], SBrz [6,7], SeF, [Xl, Se& [8,9], and SeBrz [7,8] have been successful. As an extension of the above series we present in this paper results obtained for the transient species TeCl, and TeBr2. Experimental investigations of physicochemical properties of the telluriumdihalogenides are scarce. Electrochemical and vapour density studies have shown that TeCi2 and TeBr, exist in the gas phase and liquid phase in equilibrium with other tellurium compounds [ IO,1 I]. Under normal conditions TeC12 and TeBrz are unstable in the solid phase [12,13] _Internuclear distances in TeC12 and TeBr2 have been derived from electron diffraction studies [14,15]. From gas-phase Raman and electronic absorption and emission spectra the vibrational frequencies have been determined for TeC12 and TeBr2 [ 16-191. To date no other properties of TeC12 and TeBr2 have been measured and no information about their ionic states is available. In this study the valence ionization energies (IE’s) of TeC12 and TeBr2 are determined. Hartree-FockSlater calculations are performed on these molecules as an aid in the interpretation of their spectra. For TeBr2 the importance of relativistic corrections ir~

12

G. Jonken

et a!_ 1Plmtoelectron spectroscopy of TeCl2and TeBr2

calculating the valence ionization energies is illustrated. Results are compared with PE data available for related dichlorides and dibromides.

3. Experimental

The PE spectra of the TeX, molecules are recorded on a home-buiit spectrometer especially designed for the study of transient species. A variety of design considerations has to be taken into account. High sensitivity, good resolution and a low background due to scattered electrons are desirable. In view of the different methods commonly used to produce transients (e.g. microwave discharge, high temperature pyrolysis, atom-molecule reactions) a versatile sample inlet system is needed. Because tran-

sients are usually generated outside the ionization region, their concentrations will be depleted during transportation to the ionization region due to both natural lifetime and secondary reactions. A fast pumping system consisting cfa 3-00 Q/s turbomoleculnr pump connected to the ionization chamber is therefore essential. Since serious contamination of the ionization chamber can often not be avoided, the system should be easy to dismantle and to clean. To maintain the necessary pressures and to avoid chemical attack on analyser and detector, the analyser is pumped seParately by a 4.50 Q/s turbomolecular pump. The vacuum UV light source used is a helium dc disc!rarge lamp, which produces mainly He IQ.58.4 nm (21.218 eV) radiation. Magnetic fields in the region of the analyser are compensated by means of three perpendicular pairs of Helmholtz coils. All critical surfaces close to the paths of the photoelectrons are coated with colloidal graphite (Acheson DAG 580). The photoelectrons are detected using a Mullard channeltron, connected to a standard pulse counting system. The performance of the spectrometer can be illustrated by results obtained for the *P,,a argon peak: 16 meV fwhm for a peak height of 700 c/s and 22 meV for 40000 c/s, with slits of 0.5 mm. The background is typically less than 10 c/s. The transportation time for 0 atoms, generated in a microwave discharge in molecular oxygen 15 cm upstream from the ionization region, is about 5 ms.

He(I) PE spectra of TeCla and TeBr;! molecules are detected by leading the corresponding molecular halogens over heated tellurium powder (T = 400°C). If the tellurium powder is not heated, no detectable reaction occurs. TeC!a and TeBra concentrations depend critically on the molecular halogen concentrations. If the halogen pressure is too high, bands

due to molecular chlorine and bromine interfere with TeCl, and TeBra ionization bands. No bands due to Te,Cl, or Te,Br, can be detected_ During the experiments the ionization chamber becomes contaminated leading to loss in intensity, but not in resolution (35 meV under conditions of the experiments). During the TeBra studies an ochreous deposit on the walls of the ionization chamber is formed which turns black on exposure to air. The deposit formed during the TeCla production is black and no color change occurs when exposed to air. The He(I) PE spectra of TeCIa and TeBra are calibrated against the known IE’s of HCI/HBr, C12/Br2, argon and nitric oxide. No vibrational structure can be detected on any of the bands of TeCla and TeBra.

3. Theoretical

In order to gain some insight into the bonding of the telluriumdihalogenides and to assist the interpretation of their PE spectra we have performed Hartree-Fock-Slater (HFS) calculations on these compounds. The HFS equation reads

where V, is the nuclear potential, V, the electronic Coulomb potential and Vx the exchange potential. The difference with the Hartree-Fock equation consists of the replacement of the non-local exchange operator by the local exchange potential proposed by Slater 1201 Yx = -3o[(3/&r)

p]“’

)

where p is the electronic densrty and (Yis a constant which will always be taken as Q = 0.7. The equations are solved by the method of Baerends et al. [21], which involves expansion of the orbit& in an accurate Slater-function basis together with a numerical scheme for the calculation of the matrix elements of the Fock operator. It should be

G. Jonkm

et al. /Photoelectron

emphasized that this method does not include unwarranted approximations of the potential like the muffin-tin approach_ IE’s are calculated by the Slater transition-state method [22] -which takes care of relaxation effects by removing half an electron from the orbital out of which ionization takes place. This scheme has been shown to afford reliable interpretations of PE spectra [5,23]. In view of the heavy nature of Te and to a lesser extent Br it seems important to take relativistic effects into account. Recently, Snijders et al. [24,25] developed a perturbational scheme to include relativistic effects in the HFS method, which was successfully applied to the PE spectra of I2 and HgII,. The method takes advantage of the fact that the largest influence of relativity is to be found among the core orbitals, which are very similar to their atomic counterparts_ The core orbitals are therefore kept frozen and are transferred from fully relativistic numerical Dirac-Fock-Slater calculations on the constituent atoms which can be routinely performed. The smaller relativistic effects on the valence orbitals can then be treated by perturbation theory to first order in the square of the fine-structure constant CX* (CY= e*/4ireJzc). We can distinguish four perturbations due to relativity: (a) The mass velocity correhion due to the relativistic increase of the mass of an electron with its velocity: h’Mv =

+*p4

= -Q$ v4 ,

where p is the linear momentum operator. (b) The Darwin correction_ Tnis term has no classical relativistic analogue and is due to the small scale irregular motion of an electron about its mean position, the so called “Zitterbewegun$ (see e.g. ref. [261)1,’ =1&?2p D a

T>

where v”T is the total (non-relativistic) potential. The main contribution to this operator comes from the nuclear potential

is almost a sum of Dirac 6-functions on the nuclei. It is therefore only important for orbitals that have a non-zero value at the nuclei i.e. orbitals that contain atomic s-orbital contributions. (c) Spin-orbit coupling. This is the well-known coupling of the electron spin to the magnetic field produced by its own orbital motion, h&,=$a2i-(~V~X~),

where s is the spin-angular momentum operator. (d) The change in electronic potential induced by relativity. The core contribution to this term can be written as just the difference of the relativistic and non-relativistic atomic core potentials. For the valence contribution we need the f=st order change in the orbitals &, which is calculated by perturbation theory, to construe the first order change in the valence density & h&T = c +J&


G%,J

drz + exchange

terms _

Since h’ = h’MV +hb fJ& +h.& hence the @i which are determined bation equation

depends on p’ and through the pertur.

(h’ - E;) $I; = (E; - h;) 0; , which in turn contains !I.*_,one has to solve this equation iterativeiy until self-consistency is reached. The correction to the orbital energies and thus in the transition-state ionization energies can now be calculated from ef = (@ Ih’ I@ = &

+ EL f &

+ ehoT _

Apart from computational advantages, one of the important assets of a perturbational approach is the clear-cut separation of the relativistic corrections of different physical origin (f&V, fzb, h&, hh=), which allows a more illuminating interpretation of the results.

4. Assignments so

13

spectroscopy of TeCZ2 and Te5r2

and discussion

The He(I) PE spectra of TeC12 and TeBrl are presented in fig. 1. Assignments of the bands in the

14

15

13

1.4

-

12 IONIZATION

Fig. 1. He(l) photoelectron

spectra are strongly based.on HFS calculations including relativistic effects (table i)_ Further support for the assignments is derived from a comparison with results obtained for related AX2 molecules with twenty valence electrons, and from a Walsh type empirical orbital scheme for these compounds. A simple LCBO model and sum rule considerations provide additional contirrnation for the assignments of the r-type IE’s.. 4.1. Results and interpretation We have performed both non-relativistic and relativistic HFS-calculations on TeCi, and TeBr,. The basis sets used are of double zeta STF quality with the geometries r(Te-Ci) = 236 A ] 141, an estimated bond angle L(CI-Te-Cl) = 100°, r(Te-Br) = 2.5 1 A and L(Br-Te-Br) = 100” [ 151. In the following discussion the molecules are assumed to lie in the xz plane with the z-axis as the symmetry axis. The results of these calculations are presented in

11 ENERGY spectra

10

9

(eV)

ofTeClt

and TeBrz.

tabIe I_ Concentrating first on the non-relativistic IE’s of TeBr,, we show in fig. 2 how the level ordering can be understood in terms of a simple molecular orbital (MO) scheme involving only the interaction of Te Sp and Br 4p orbit& (a very similar diagram can be drawn for TeCl,). We consider the various symmetry species separately: a,. From the Br 4p orbitals in the molecular plane we can construct two combinations of a, symmetry, one in which the 4p orbitals are directed along the Te-Br bond axes and one oriented at right angles to these axes. The first combination strongly interacts with’the 5p, orbital giving rise to a bonding and an antibonding MO_ A Mulliken population analysis shows that the bonding la, orbital consists of almost equal Te and Br p-contributions without any appreciable s-admixture_ The Br 4p combination at right angles to the bond axes hardly interacts at all and forms an essentially (95% Br 4p) nonbonding MO. bl _Here again we find a Br 4p combination along the bond that strong!y interacts, this time with Te

15

G. Jobnkers et al. /Photoelectron spectroscopy of TeCl2 and TeBr2 Table 1 Summary of experimental band positions and of non-relativistic and relativistic HFS-calcolations

for TeClz and TeBr2 (in ev) .EXp.

Rel.

N1el

*POT

*MV

*DAR

la2

8.47 11.25 11.22 11.20

-0.60 -0.31 -0.33 -0.25

0.51 0.08 0.43 0.08

lb,

11.78

-0.35 -0.48 -0.49

0.00 0.00 -0.18 0.00 -0.01

8.38 11.02 11.14 11.03 11.61

-0.01 -0.01 -0.01 0.00

0.69 0.37

-0.21 -0.04

1256 13.51

0.00 0.01

-0.01 004 -0.10 -0.02 -0.02 -0.24 -0.14

8~01 9.83 10.06 10.29 10.78 11.86 12.93

TeCl*

2% 2bl a1

Ial lb1

1256 13.67

TeBrz 2b2

0.19

2bl

10.33

-0.83 -0.87

0.60 0.33

a1

10.55 10.43 11.07 12.07 13.14

-0.94 -0.49 -0.68 -0.89 -0.78

055 0.37 0.41 0.92 0.71

la2 lbz 131

lb1

8.25

TQ Br2

0.03

0.10 0.04 0.20 0.19 0.22 -0.11 -0.09

MO-Scheme

8.41 11.01 11.13 11.02 11.61

8.99 11.75 11.95 11.99 12.76

1236 1352

13.43 + 0.03 14.21 + 0.03

8.11 9.87 10.26 10.48 11.00 1197 13.02

8:76 10.80 11.14 11.19 11.98 12.73 13.42

f t t r + f 2

0.03 0.1 0.03 0.03 0.04

0.02 0.02 0.02 0.02 0.02 0.02 0.02

HFS -Calculations

__--

---

aI

* r f f t

a2

Fig. 2. Ionization energies for TeBr2 obtained from non_relstivistic md rekth’ihe

HFS-xkuhtions.

_-

__-

2b,

lb,

16

G. Jonkers et al. / Pizotoeteciron specrrorcopy of TeCI, and TeBr;!

Sp,, while the other combination remains an essentially noninteracting nonbonding MO 2bI. The bonding lb1 lies deeper than the la1 due to a more favourable overlap with the corresponding Te orbitals. The calculation shows that lb1 is more skewed (67% Br-33% Te) towards Br than lal_ a~_ Since there are no low-lying orbit& of a2 symmetry on Te, the a, combination of the out of plane Br 4p forms a nonbonding MO. bL_ Finally the bz combination of the out of plane Br 4p orbitals has a n--interaction with Te 5p,, giving rise to a bonding lb? and an antibonding 2b, MO both of which are filled in these molecules. As expected the lb2 a-orbital is less bonding than the Because of the greater electroIal and lb, a-orbitnls. negativity of Br compared to Te (Br 4p deeper than Te 5p) the bonding 1bz is skewed towards Br (69% Br-3 1% Te), whi!e the opposite is true for the antibonding 2b2 (3 1% Br-69% Te). It must be pointed out that the dotted Br 4p and Te 5p levels in fig. 2 arc estimates for their position in the final selfconsistent molecular field. The Fredicted ordering from these non-relativistic calculationsis thus2b2, (3-a,, 2b,, la,), lb*, la,, lb,,where the nonbonding 2al, 2bl and I+ are almost degenerate. For TeCI, we find the same ordering while the levels are displaced to higher 1E’s which is to be expected since the Cl 3p levels lie deeper than the corresponding Br IeveIs (Cl more electronegative than Br) and which is in agreement with experiment. Correspondingly the bonding 1b,, 1a, and 1b, MO have a higher Cl character. Next we consider relativistic corrections. Referring to table 1 we first note that relativistic corrections are quite large (up to 1 eV), but that the different contributions tend to cancel each other. In particular the mass velocity term always increases the IE (cf. the classical expression -p4/SmW is negative definite), while due to relativistic contraction of the orbit& the electronic repulsion tends to rise, causing a decrease in IE. Comparing TeC& and TeBr? we see that relativity is more important in the bromide than in the chloride as expected, Br being heavier than Cl. This is particularly evident in the orbitals that are mostly centred on the halide ligands (Za,, 2b,, la,). The Darwin correction is only important for orbitals which have some s-character (see above), so it only contributes to those orbitals that have a slight admix-

ture of Te 5s (lal, 2al), while in TegrZ the Br 4s starts to have a small effect on the lb,. It invariably destabilizes those orbitals decreasing their IE. In these molecules with CZv symmetry there is no direct spinorbit splitting, the double group C;, having only one irreducible representation (e,&_ The sum of all these contributions is found in table 1 under the column AsuM. It is seen that the destabilizing AmT + ADAR always outweighs the stabilizing effect of AM”, causing a general shift to lower IE which is however not very large especially in TeCl*. An interesting phenomenon occurs among the nonbonding, halide centred 2a,, 2’0~ and la, levels. Both in the TeC12 and TeBr2 these are non-relativistically virtually degenerate. Relativity does not change this very much in TeC12 but in TeBr2 these levels start to separate. There is a sizeable destabilization ANT: which is not completely compensated for by AMv- That Apo~ is larger for 2al and 2b, than for Ial? can be explained by the fact that the first has some amplitude on Te, where core potential effects are largest, while la2 is completely localized on the halides (a similar effect can be seen in the r-pair lbl, 2b2, where the latter, being more Te-like, has the largest ApoT)_ Apart from this splitting due to the direct relativistic effect, there is an additional effect that has to be taken into account, namely the spin-orbit interaction_ Although as mentioned earlier there is no direct spin-orbit splitting in CZv symmetry, all the orbitaIs are of the same e,,* C;, double group symmetry and can hence interact with each other through the spin-orbit operator, i.e. there are non-zero spinorbit matrix elements between them. This off-diagonal indirect spin-orbit effect is especially important far IeveIs that are close to each other (see ref. [25] for an example of this in the Hg d-shell of HgI& Diagonalizing this spin-orbit interaction provides an additional correction to the IE’s; which is shown in table 1 under the heading A@. It is seen that while this correction is negligible in T&12 this is not so in TeBrz, where it causes the least stable of the nonbonding levels to split of from the other two by about 0.4 eV in total $. Since all these non-bonding $

Due to the spin--orbit mixing of the MO’s of different singk goup symmewy, we c3n no longer assign a unique sin& &oup symbol to these levels, they all belong to the double group IR eya-

G. Jonkem er al. j Phoioelecnon

orbitals are essentially Br p-like, this spiitting can be compared to the *P,,, -‘Pi ,* pattern in the Br atom, while the magnitude is comparable to the 211s,2-211,,a splitting in the Br; molecule (-035 eV) [27]. Since the spin-orbit splitting in Cl; is much smaller(;=0.08 eV) [27j it is not unexpected that we do not find a corresponding effect in TeCl*. Comparing the final relativistic results with experiment, we see that we find essential agreement. Although all IE’s are about O-7-0.8 eV too low their separations are much better. In particular the splitting off of the highest nonbonding level in TeBr*, but not in TeCla is in complete agreement with experiment. The general pattern of IE’s shown to the right in fig. 2 is very similar to the experimental spectrum and we feel confident that with the help of these calculations we have given a reliable interpretation. 4.2. Compmison

with related molecules

Our assignment of the PE spectra of TeClz and TeBrz given in table 1 is compared with the results obtained for the related ACla and ABr, molecules with twenty valence electrons. From this comparison a consistent picture arises for the entire series of compounds. In all A&, molecules the first ionization band is assigned to the occupied antibonding 2ba MO. The next three ionization phenomena, which are relatively close together are ascribed to the three chlorine “lone pair” combinations with symmetry 2b,, 2ar and Ia*. The sharpest band in this region is consistently assigned to 1a, [2-4,8,9] _ The combined results for this series suggest that for SCla the assignment of Colton and Rabalais [4] with one band at 12.2 eV and two coinciding ionization phenomena at 12.5 eV has to be preferred over the assignment of Solouki et al. [3], who propose the reverse. &fortunately, the work of Na&Felsobuki and Peel [9] adds to the confusion because both papers on SCla j3,4] are misquoted on this point. In view of their proximity, for the order of the 2bl and 2a, orbitals a strong case cannot be made. Our HFS calculations on AXZ molecules commonly locate 2ar below 2br, but VEOMP calculations on SeClz [9j come up with the reverse order. There is general agreement on the order of the next three ionization bands (Ibn. lal, Ibr). In the work of Colton and Rabalais [4j lai is located below 1bl, but an assignment based on semifmpirical calcula-

spectroscopy of TeCI, and TeBr2

17

tions only should be viewed with caution. Our TeBrz assignments can be compared with results obtained for SBr, [6,7] and SeBrz [7,8] _ Again the fust ionization band is consistently‘ assigned to the antibonding 2b2. The next three bands (2b,, 2a,, laz) correspond again to the bromine “lone pair” orbitals with laa assigned to the sharpest band in the region. For 2bi and 2a, the situation is similar to the dichlorides: HFS calculations locate 2ar below 2br [6,8] but the VEOMP method favours the reverse order [7]. Concerning the order of the three “lone pair” orbitals it should be pointed out that Na,T-Felsobuki and Peel [7j seriously misquote the work of de Leeuw et al. [S]. The agreement on the assignment of the remaining three bands (IbZ, la,, lbr) is complete. In view of the importance of relativistic corrections in our TeBr, assignment, it should be stressed that the normal Clv designations are no longer valid but that the bands belong to the same irreducible representation (er,a) of the double group C;,. In fig. 3 the trends found in the He(I) PE spectra of ACla and ABra molecules studied to date are presented. For OCll and Set& band positions and assignments given in refs. [2,8] are used. For SCla the assignment of ref. [4] but with an interchange of la, and lbr is taken. For SBra recent experimental data [7] are used, but an assignment mainly based on HFS calculations [6] is followed. For SeBr, results from ref. [S] are taken. 4.3. LCBO model and mm nde The three n-type valence IE’s of TeXa molecules can be approximated by a simple model involving the linear combination of bond orbitals [28]. If this model is applied to a tellurlumdlhhalogenide, the irtype IE’s are described by two Coulomb parameters o,, and ox and a resonance parameter &&x-The interaction between the halogen atoms described by’ flxx is neglected. Assuming the validity of Koopmans’ approximation [29] the three x-type IE’s are completely determined by ore, ax and &x-As noted by Haink et al. [30] these parameters should show an approximately linear dependence on the electronegativity and hence on the first IE of the central atom. Linear extrapolation of the LCBO parameters obtained for the A& series [S] to TeClz and calcu-

18

G. Jonkers et al. /Photoelectron

spemoscopy

of TeC12 and TeBrz

ACI, ; A=Te, Se. S and 0 Ta

50

A0r,;

A=TQ.SQ

and

S

5

8-

lO-

i ‘4

18.

8

12 I.E.

14

8

ATOM

A (eY)

L3.

12

Fig. 3. Correlation diagram of the observed vertiul ionization energies of OCIz, SC&. .SeC12, TeC12 and SBr2, SeBr2, TeBrz versus the fust atomic ionization energies of their centraI atoms.

lating the 1r-1E’s for the latter compound Ieads to 9.02,12.23 and 13.13 eV for 2b2, la* and lb*_ These predicted va!ues show reasonable agreement with the results in table l_ A similar approach for the ABrz series predicts 8.X6,11.30 and 12.33 eV for 2bz, la, and i bl in TeBrt thus providing additional support for our assignment in table ! _ According to ;he sum rule [31] thk sum of the three n-type IE’s should equal the sum of the first atomic IE’s of the atoms which constitute the AX* molecule [5]. This rule is based on the assumptions of the validity of both the use of a minimal basis set in describing the electronic structure and of Koopmans’ approximation_ For TeC12 and TeBr, the n-sums calculated from the fust atomic IE’s are 34.95 and 32.63 eV, compared to experimental

values of 33.74 and 31.93 eV. Somewhat better agreement is obtained if a version of the n-sum which incorporates trends between related molecules is used. From experimentai n-sums of Se& and SeBr, [X] the TeXz n-sum can be obtained from n(TeXz) = n(TeY*) f n(SeX.J

- a(SeY,)

_

If this procedure is followed, calculated s-sums of 31.23 and 3 1.44 eV are obtained for TeClz and TeBrz. 4.4. Walsh

diagram

In previous papers [3,5,8] the connection between the PE spectra of AX2 molecules with twenty valence electrons and Walsh’s empirical orbital scheme [32]

C. Jonkerset al. /Photoelectron spectroscopyof TeC12and T&r=

salt (loo01 30,

.

for the present series the ns orbit& orrthe central atom are located so much deeper than its np orbitais, that a new Walsh diagram can be constructed, based only on p orbitals on the cent& atom and the halogens. In fig. 4 we present the orbital energies (note:

Linear

-\ .I

19

..

not IE’s) for TeBr, from nonrelativistic HFS cakulations fcr the bent (IOOO) and hypothetical linear molecule. Some differences with the original Walsh diagram are apparent, leading to new correlations between MO’s belonging to the bent and linear structures_ In view of the satisfactory agreement between the PE spectroscopic results for the AX2 series and the modified Walsh picture, the need to explain a discrepancy between preferred assignments and the original Walsh diagram has disappeared_ The suggestion, in particular, that correlation energies and relaxation contributions need not be the same for orbitals of different symmetries, is superfluous in this context.

2

k

m 8

Acknowledgement 15i

rpss(o,~-__________________

_-___

::

____-----+

Br45(a,)

;;

CALCULATED

5

%

____---------

WALSH

DIAGRAM

(T&r,

1

Fig. 4. Walsh diagram for TeBrz obtnined from c&uhted orbital energies in the bent (lOO”) and hypothetical linear contigumtion.

The authors are grateful to Professor Nevllle Jonathan and Drs. John Dyke and Alan Morris from Southampton University for their invaluable advice concerning the design of the spectrometer, to Paul van Dieren, Hans Martensen, Jaap Musch and Gijs Verlaan for their various skills in constructing it, and to Roe1 Mooyman and Harm Muller for experimental assistance in the course of this work. One of the authors (GJ.) is grateful to the Netherlands Organization for the Advancement of Pure Research (Z.W.O.) for fuancial support.

References has been pointed out. However, the preferred order of the fifth and sixth band in the PE spectra (la, below lba) appeared to be at variance with the diagram originally published by Walsh. Various explanations for this situation have been suggested [3,5,8]. In the present paper we want to point out that the original Walsh diagram is not completely applicable to the series of AX2 (A = 0, S, Se, Te; X = F, Cl, Br) molecules. In the original Walsh picture MO’s are formed from ns and np atomic orbitals on the central atom and p atomic orbit& on the ligands. However,

[l] J.hf. Dyke, N. Jonathan and A. hforris, in: Electron spectroscopy, theory, techniques and applications, VoL 3 (Academic Press, New York, 1979) p. :89. [2] A.B. Cornford, D.C. Frost, F.G. Herring and CA. McDowell, J. Cbem. Phys. 55 (1971) 2820. [3] B. SoIouki, P. Rosmus and H. Bock, Chem. Phys. Letters 26 (1974) 20. [4] R.J. Colton and J.W. Rabalais, J. Electron Spectry. R&t. Phenom. 3 (1974) 345. [5] D.M. de Leeuw, R. Mooyman and C.A. de Lange, Chem. Phys. 34 (1978) 281. [6] D.M. de Leeuw, R. Mooyman and C.A. de Lange, Chem. Phys. Letters 61 (1979) 191.

20

G. Jonkers et al. /Photoelectron

[71 E. Nagy-FeIsobukJ and J.B. Peel, Chem. Phys. 45 (1980) 189. [S] D.M. de Leeuw. R Mooyman and CA. de Lange, Cbem. Phyr 38 (1979) 21. [9] E. Nagy-Felsobuki and J.B. Peel, J. Chcm. Sac. Faraday Trans. II 76 (1980) 148. [IO] F-G. Bodewigand J.A. Plambeck, J. Electrochem. Sot. 117 (1970) 618. [ 11) H. Opperman, G. StSver and E. Wolf, 2. Anorg. A&. Chem. 410 (1974) 179. [ 121 A. Robenau md H. Rau, Z. Anorg. Allg. Chem. 395 (1973) 273. [ 131 C-C. Chriirensen and J. Al&d, Radiochem. Radioawl. Letters 13 (1973) 227. [ 141 W. Grether, Ann. der Phys. 26 (1936) 1_ [ 151 h1.T. Rogers and R.A. Spurr. J. Am. Chem. Sot. 69 (1947) 2102. [ 161 W’. Spinnler, Helv. Phys. Act. 18 (1945) 297. [i7] LR. Beattie and R.O. Perry, J. Chem. Sot. A (1970) 24 29. 1181 J-P. Marteel. B. Vidal and P. Goudmnnd. Compt. Rend. C 276 (1973) 731. [ 19: J.P. Marteel, B. Vi&J and P. Coudmand, Compt. Rend. C277 (1973) 361.

spectroscopy

of TeCIz and TeBr2

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