He(II) photoelectron spectra of the metal valence (d) shells of the group IIB dihalides

Journal of Electron Spectroscopy and Related Phenomena, 22 (1981) 247-259 Elsevler Sclentlfic Pubhshmg Company, Amsterdam - Prmted m The Netherlands

FINE STRUCTURE IN THE He(I)/He(II) PHOTOELECTRON SPECTRA OF THE METAL VALENCE (d) SHELLS OF THE GROUP IIB DIHALIDES

E P F LEE and A W POTTS Department of Physrcs, Kmg’s College, Strand, London WCSR 2LS (Gt Brrtarn) (First received 29 September 1980, m final form 24 November 1980)

ABSTRACT The He(I) and He(I1) photoelectron spectra of the group IIB dlhahdes (except fluorides) are reported Emphasis 1s laid on the metal valence d-shell lonlzatlons are discussed m terms of He(I)/He(II) relative mtenslty changes and a hgand-field It appears that the observed d-shell structure IS the result of a balance between field-effects and covalency

the dlThese model simple

INTRODUCTION

The He(I) and He(I1) photoelectron spectra of the Group IIB dlhahdes have been studied by a number of workers Eland [l] reported the He(I) spectra of HgX2(X = Cl, Br, I) together unth those of the correspondmg methyl mercury halides and dnnethyl and dlethyl mercury Cocksey et al. [2] and Boggess et al. [3] reported the He(I) spectra of ZnX, and CdXz (X = Cl, Br, I). Berkowitz [4] reported the He(I) (halogen-locahzed lomzatlons) and He(I1) (valence d-shell lomzatlons) spectra of all of the Group IIB (Zn, Cd and Hg) dlhahdes (except dtiuondes) These stules have gwen a general analysis of the observed spectra and a general understandmg of the electronic structure of the Group IIB dlhahdes based on conslderatlon of MO correlations, chemical shtits, electronegatlvlty scales and spm-orbrt effects. Orchard and Richardson [5] reported fine structure associated v&h the He(I) 3d-’ spectra of zmc dlhahdes The observed structure was mterpreted on the basis of weak crystal-field effects and it was concluded that there was no evidence for covalency on the part of the valence d electrons, m contrast [ 6,7] More recently, Egdell et al [ 7] to some mercury(I1) compounds stu&ed the He(I) and He(I1) spectra of HgC12 together vnth those of TlC14, VCL and WCl, However, m that study attention was psud to changes in the 0368-2048/81/0000-0000/$02

50 0 1981

Elsevler Sclentlflc Pubhshmg Company

248

band mtenslty patterns between the He(I) and He(H) spectra m relation to metal-hgand covalency It was concluded that Hg 5d orbit& were mvolved m chemical bondmg In addition to the dlhahdes, the He(I) and/or He(I1) photoelectron spectra of some Group IIB dlalkyls have been studied on several occasions [6,8131 In these studies 18-131 the sphttmg of the outermost d levels was interpreted as being due to electnc field gradients, and it was concluded that while m Hg compounds there was some 5d mvolvement m bonding, for the Zn 3d and Cd 4d spectra the spllttmg was due to an electrostatic hgand-perturbatlon rather than to bonding Such a conclusion seemed to be m agreement with the photoelectron studies on the dlhahdes by Orchard and Richardson [ 51 and Egdell et al [ 71 Recently we have reported the He(I) and He(I1) photoelectron spectra of ZnF, and CdFz [14] Although fme structure was not observed m the dshell metal-locahzed lomzatlon, a comparison of the valence halogen-localized romzatlons of the Group IIA and IIB dlhahdes showed that the dtiferences m the observed spectral features and the assigned MO ordenng could be explamed m terms of orbital murmg between the hgand and metal atom through 0 MO’s. Nevertheless, the metal d loruzatlon was found to be essentially atomic m ZnF, and CdF2 In the present work, we report the He(I) and He(I1) spectra of all the Group IIB dlhahdes (except dlfluondes, for which spectra have been published previously [ 141) wth a resolution apparently superior to those obtamed previously [l-4,7]. Fme structure m the d-l lomzatlon has been observed m the He(I1) spectra, and this, together with observation of intensity vanatlons between He(I) and He(I1) spectra, has enabled the degree of metal llgand covalency m the Group IIB dlhahdes to be better understood

EXPERIMENTAL

The photoelectron spectrometer and the molecular beam arrangement used m this study have been described elsewhere [ 15,161 Briefly, the sample 1s mtroduced into the lomzatlon reaon as a pseudo-molecular beam produced by a tubular alumma furnace The He(I)/He(II) radiation 1s produced by a hollow-cathode discharge at 90’ to the molecular beam Photoelectrons are energy-analyzed usmg a 150” spherical--segment electrostatic analyzer Commercially avrulable samples were used For zmc and cadmium dtialldes, the samples were heated under vacuum to Just below their melting pomts for several hours before use. Before heatmg and at relatively low temperature (1 e , HgXz spectra), the resolution for argon with He(I) radlatlon was 30 meV (FWHM) At relatively higher temperatures (1.e., ZnX, and CdXz spectra), the resolution was slightly reduced probably due to sample contammatlon m the lomzatlon

249

chamber The measured vertical lomzatlon potent&s (Tables l-3) were cahbrated with respect to N2 and He spectra on various He(U) hnes The measured relative band mtensltles were corrected for transmlsslon and normalized with respect to the hgand o, band which, from symmetry conslderatlons, should not mix with the metal d orbltals. Although the d-’ lomzatlon was observed with He(I) radatlon for HgXz , the relative mtensltles are not rehable because of the character&c low sensltlvlty of the type of energy analyzer used towards electrons of low kinetic energy The measured relative mtensltles of the d-’ bands are therefore gven only for He(U) spectra (Table 4) For the same reason, the correspondmg d-l lomzatlons by the He(I) radlatlon were too weak to be observed for ZnX, and CdX, with our spectrometer

RESULTS

AND DISCUSSION

In general, the observed He(I) and He(I1) spectra are m good agreement with those reported previously [l-5,7] Therefore only those spectra which have not been reported or which show addltlonal fme structure are reproduced here (Figs l-5) Although the halogen-localized lomzatlons due to He(H) radlatlon for ZnX, and CdX, have not previously been reported, then observed spectral structure 1s slmllar to that for HgX2 (see Table 4) Therefore the full He(I1) (and He(I)) spectra only of HgCl, (Fig 3) and HgIz (Fig 5) are shown here, as representative spectra The d-l lomzatlons due to He I

IP(PV)

Fig 1 d-’ Metal-locahzed lomzatlons m %I2 (top, He(I) Richardson [ 51, bottom, He(I1) spectra from this work)

spectra

from

Orchard

and

250

IP(eW

Fig 2 d-’ Metal-locahzed lonlzatlons in CdXz (X = Cl, Br and I) by He(U) radiation

3

II

13

IP(eV)

15

L 17

F-

Fig 3 He(I) (top) and He( II) (bottom) photoelectron spectra of HgCl2 (structure m the He( II) spectrum with apparent IP’s of 9 13, 9 47 and 1106 eV corresponds to Hg d-’ lonlzatlon on the He(II@) lme)

He(I1) radiation for ZnIz , CdX2 and HgXz are shown m Figs 1, 2 and 4 (and 5) respectively For ZnClz and ZnBr 2, fme structure was not resolved in the He(H) spectra (only slight shoulders m ZnBrz spectra, see Table 1) The measured vertical lornzatlon potent& for the halogen-localized ionizations obtained m this work are m good agreement, wlthm expernnental error, v&h those published previously [l-5,7] and therefore ~11 not be reproduced here However, the measured vertical lonlzatlon potentials and assignments for the d-l lomzatlons are shown m Tables 1, 2 and 3 for ZnXz , HgXz and CdX2 respectively, together with previously published values The measured relative mtensltles of spectral bands m both the He(I) and He(U) spectra obtamed in this work are shown m Table 4 Changes m the relative intensities of bands between He(I) and He(I1) are used m the proceeding dlscusslon to infer covalent character for particular orbit& It 1s known that the photolonlzatlon cross-section of the valence p

251

HgCl, 512

16

18 IP(eV)

Fig 4 d-’ Metal-locahzed lomzatlons m HgCl* and HgBrz by He(Ia) (bottom), (middle) and He( IIP) (top) radlatlon

HePa)

HgI,

Fig 5 He(J) (top) and He(H) (bottom) photoelectron spectra of HgIz (structuf; m the ionizHe(I1) spectrum with apparent IF% of 8 39 and 10 33 eV corresponds to Hg d ation on the He(I@) hfie)

IONIZATION

(19 46) (19 52)

2A g1/2

19 22

19 13 +o 02

18 82 f0 04

18 71a

ZnBrz

TABLE 2

&l/2

18 62 +O 03 18 75 f0 04

17 24 +O 04 (18 65,18 (18 65,18

(17 27,17 52) 52)

13)

a This work, average from He(Ia), (IIa) and (IIP) spectra b Eland [l] c Egdell et al [7]

*bw*

*b/z

2

03 18 30 f0 02 18 48 f0 03

1683+0

16 66 38 *O +O 03 Ola

93)

(16 71b, 16 60’) (17 05,16

17 03 f0 04 1669+002a

sw*

(19 26)

(19 13)

(18 87)

(18 34)

(16 83)

(16 69)

(16 40b)

(eV) OF THE d-’ METAL-LOCALIZED

~~5~2

POTENTIALS

HgBr2

IONIZATION

WC12

VERTICAL

Ionic state

MEASURED Cl, Br, I)

1

(18 75)b

(eV) OF THE d-’ METAL-LOCALIZED

a This work, from He(I1) spectra b Estimated from He(I) spectra of Orchard and Richardson [ 5 ]

19 52 f0 05

*&l/2

(19 Oqb

POTENTIALS

(19 17)

19 18fO05’

ZnClz

VERTICAL

5?zs/2

*rI g3l2

*&l/2

Ionic state

MEASURED Cl, Br, I)

TABLE 1

17 89 f0 01

16 23 +O 03

16 15 95 11 +O f0 02a 02

W2

IONIZATIONS

18 64 *O 04

18 58 f0 05

18 39 +O 04

18 31

18 04 +O 05a

ZnIz

IONIZATIONS

(17 91)

(16 17)

(15 99y

OF HgX2 (X =

(18 67)

(18 54)

(18 36)

(18 29)

(18 Ol)b

OF ZnXo (X =

253 TABLE

3

MEASURED LOCALIZED

Ionm

state

VERTICAL IONIZATION POTENTIALS IONIZATIONS OF CdXz (X = Cl, Br, I)

(eV)”

OF THE

d-’

CdClz

CdBr2

Cd12

19 37 +o 03

1907+004

-18

1954*004

19 30 *o 03

19 68

1

METAL-

sb

18 92 +O 04

2bg1/2

2009+003

19 79 +o 04

19 33 +o 05

243/2

2023*003

20 00 +o 03

19 62 *O 04

* Thu work, from He( II) spectra b Estimated from correlation (Fig

6)) see text

orb&& of the halogens Cl, Br and I decreases substantially mth respect to that of metal d orbltals between the He(I) and He(I1) frequencies Thus m a spectral regon associated largely with halogen-locahzed orbltals an increase m the relative mtensrty of a particular band m the He(II) spectrum can be associated with d character, while m a regon associated largely with metallocalized d orbltals a decrease m relative intensity m the He(I1) spectrum can be associated with halogen character [7] ZnX2 Although parts of the He(II@ spectra due to the halogen-localized lornzatlons are masked by the d-’ lomzatlon on the He(IIP) line (reflected by the uncertamty m the measured relative mtensltles m Table 4), it 1s clear that the band intensity patterns m the He(I1) spectra are different from those m the He(I) spectra (see Table 4, cf Figs 3 and 5) For the halogen-localized lonlzatlons, the maJor increase m the relative mtenslty on changmg from He(I) to He( II) radiation occurs for the 2E:r 1,2 lonlc state Although we were not able to observe the d-’ lomzatlon of ZnX, at the He(I) frequency, comparison between the He(I) spectra of Orchard and Richardson [5] and the present He(I1) spectra for the d-’ lonlzatlon m ZnI, (Fig 1) shows that the first peak, and to a smaller extent the fourth peak, decreases m mtenslty uflth respect to the other d-l peaks on changmg from He(I) to He(I1) radiation Since symmetry allows metal d-shelllhgand murmg only m the Q, and 7cg MO’s, and spin-orbit coupling can allow murmg between ionic states havmg the same 52, the above observations for the d-’ lomzatlon confirm the asslgnment by Orchard and RIchardson [ 51 that the observed lomc-state sequence 1s *x:Bl/2 I *r&l 312 *45/2 ,*Q? l/2 ,24312 9 m order of increasing lonlzatlon potential It appears that there IS httle murmg between metal and hgand n, orbit& but that the fourth state possesses 2x:g character as a result of a second-order spin-orbit effect Thus both the first- and fourth-band mten-

TABLE 4

a b c d

I

g 112 l,

i

(5)

(10)

(7)

(10)

(35?

lll(40)

172 (62d)

-30

-10

50 (40)

75 (60d)

13

-10

I -50

20

67 (40)

103 (62d)

17

-10

-13

15

He( II)

TRANSMISSION)

(-913)

(-34)

(Bb)

-

3 -

(3)

9,7,8(9,8,8) 10 (10)

10

-

3 (3) -

8 (9) 10 (10)

8 (9)

12 (11)

11 (lob)

-

-

4 (3,4)

10 (10,lO)

15 (18,20)

20 ( lgb ,19”)

He(I)

Hg

108

194

22

10

-31

13

93

165

20

10

10

11

-17

16

208 (40)

38 334 (64d)

10

21

39

He(I1)

(40)

(72d)

(40)

(70d)

(282)

(48) (376)

(10)

(22)

WC)

OF MX2 (M = Zn, Cd, Hg, X = Cl,

to ‘LD”3/2 branchmg ratios, “D” 3/2 1s the band mn%%s~tynormalized to 40. this work

-

7 -

12

(9)

57 (40)

10

10

_2;

(10)

(37?

43 (5sd)

(7) -

5 -

12 31(40)

10

31

10

1

14

-

-

6

10

(10)

-40

12

91 (64d)

.I/2

(37?

36 (40)

48 ( 53d)

14

“Dg2

,2b/2

-

6 (10) -

10

17

10 (10)

21

He(I)

Cd

FOR ENERGY

24 ( lga) 26

He( II)

(CORRECTED

22 (19)

He(I)

Zn

INTENSITIES

“D;,2

g 112

u2

2L

2z

l/2

2ng

gz2

&2

h/2

21-1

“2

2iz ,,2

2&L112

2Kw2

2L/2

2rI g1/2

2%/2

D&2 rr2 II O3/2

“2

2z B

2zl

Iomc state

RELATIVE

Cocksey et al [2] Eland [l] Egdelletal [7] In order to show “D”g2

Br

Cl

X

MEASURED Br, I)

255 slty changes reflect mlxmg between the Zn 3do and halogen a orbltals Thxs strong ug muring 1s also reflected m the mtenslty changes for the halogenlocalized spectra (Table 4) Cd& Both the He(I) and He(D) spectra due to the halogen-localized lomzatlons m CdXz are slmllar to those for ZnX, The maJor intensity change between the He(I) and He(II) spectra again occurs for the ‘E9 1,2 lonlc state (Table 4) However, the de1 lonlzatlons m CdXz at the He(I1) frequency (Fig 2) were observed with fine structure dlffermg slightly from that for ZnXz (Fig 1, cf ref 5) Since the He(I) spectra due to the d-l lonlzatlons of CdX2 were not observed here and have not previously been reported, no mformatlon on the He(I)/He(II) mtenslty varlatlon 1s avalable Nevertheless a correlation of the fine structure associated mth the d-’ lonlzatlon m CdX2 (X = F, Cl, Br and I) as shown in Fig 6 allows a fanly definite assignment (Table 3) to be made The present correlation for CdX2 suggests that the 2x:, 1,2 ionic state of Cd12 is at an ionization potential of -18 5 eV However, presumably because of orbital muring with the hgand, it 1s too weak to be observed m the He(U) spectrum This is m agreement wth a slml1a.r correlation m ZnX2 (cf Frg 4 m ref 14) and with the observation that for Zn12 the correspondmg 2L:B1,2 lonlc state decreases in relative intensity (Fig 1) on changmg from He(I) to He(I1) radlatlon. For Cd12, it seems possible that there 1s greater orbital-mlxmg between the metal d-shell and the hgand valence uB MO’s than

LdF, 18

20

19

2

IP
Fig 6 Correlation of the fine structure associated with the d-’ metal-locahzed lowatlons m CdX2 (X = F, Cl, Br and I) (length of bar represents the relative peak height)

256

m ZnI, This 1s apparently reflected by the increased enhancement of the halogen-locahzed *& band m the He(I1) spectrum of CdI, when compared to Zn12 (Table 4). This difference does not, however, appear to exist for the chlorides or bromides

The He(I) and He(H) photoelectron spectra of HgX2 obtamed m this work (Figs 3-5) are slmllar to those reported previously [ 1,4,7] although our spectra appear to be slightly better resolved The fme structure assoclated with the d-l romzatlon was observed at the He(Ia), He(IIa) and He(II@) frequencies (Figs 4 and 5) In contrast to the He(I1) spectra of HgCl, obtamed by Egdell et al [ 71, where the spin-orbit sphttmg m the (n&l hal o ge n -1oc al lze d lomzatlon and the 211g3,2/2L:g1,2 and 2Ag3,2/21’Ig1,2 splitting m the d-l metal-localized lomzatlon were not resolved, they are well resolved m our spectra (Figs 3 and 4) Comparmg the mtenslty patterns between the He(I) and He(I1) spectra of HgX2 m the low-lomzatlon-potenteal reson, it 1s observed that the maJor intensity change agam occurs m the *I: B1,2 ionic state (Figs 3 and 5, Table 4) However, there 1s also an increase m the relative intensity of the *II, 3,2 and *I& 1,2 loruc states with respect to the *&, and *x:, states on changmg from He(I) to He(H) frequency, partlcularly for HgC12 Such an mtenslty vanatlon has been observed by Egdell et al [7] for HgCl2, although It seems possible that Egdell et al overestunated the relative mtenslty of the *III, 3,2, 1,2 features m the He(I1) spectra because of the overlap mth the “D”3,2 features from the He( II@ line, see Fig 3 and Table 4 Regardmg the fine structure associated mth the d-l lomzatlon, mtenslty changes are clearly observed m the 2E91,2 lomc state of HgC12 and HgBr, between the He(I) and He(I1) spectra (Fig 4) In addition, there are signs mdlcatmg that a similar decrease m the relative mtenslty occurs between the He(I) and He(I1) spectra for the *Eg 1,2 state (shoulder of peak, at 16 23 eV, see Fig 5) of Hg12 and, perhaps to a smaller extent, for the *I& 1,2 states of HgC12 and HgBr2 (Fig 4) In the case of Hg12, the 211p1,2 state seems to comclde with the *A, 3,2 state (see Table 2)

METAL-LIGAND

COVALENCY

IN THE GROUP IIB DIHALIDES

From the above observations and conslderatlon of the He(I)/He(II) mtensity vmatlons, it seems mth little doubt that there 1s a certm amount of metal-hgand covalency m the Group IIB dlhahdes Although all of the Group IIB dlhahde molecules can be considered as havmg lomc character, especially the dlfluondes, orbltal admixture between the valence halogen and d-shell metal AO’s 1s defmltely reflected m the intensity changes between the He(I) and He(II) spectra This conclusion seems to be at valclance mth that of Bancroft et al [8--131 m their studies of some Group IIB dlalkyls, where they

257

concluded that the 3d and 4d orbltals of Zn and Cd respectively had little or no partlclpatlon m bonding However, if one examines their He(I) and He(I1) spectra due to the d-l lomzatlons of some Group IIB dlalkyls [ 8,11--131, intensity changes slmllar to those reported here for Group IIB dlhahdes can be observed At the same time It would be expected that the dlhahdes would be more ionic m their bondmg than the dlalkyl counterparts At this point, based on the present expenmental observations on the Group IIB dlhahdes, we can only conclude that the outermost d-orbit& of Zn, Cd and Hg are mvolved in chemical bondmg m their dlhahdes (perhaps mth the exception of the dlfluondes, where no dvect mtenslty-evidence has been observed [ 14]), and that metal--hgand covalency seems to be larger m HgX2 than m ZnX, and CdX, This follows from He(I)/He(II) intensity data, which indicate that there 1s orbital mlxmg not only m the ug but also m the rITgMO’s of HgXz No obvious relative mtenslty changes m the (~~)-l lomzatlon between He(I) and He(I1) radiation are observed for ZnX, and CdX2, mdlcatmg neghgble mixing in these cases LIGAND-FIELD MODEL

In previous dlscusslons of ZnX, de1 photoelectron spectra, fine structure in the d- ’ band has been interpreted by fitting a simple hgand-field model to the observed structure [ 5,141 A similar model was used m the dlscusslon of fine structure observed m the Tl d-’ spectra of TlX [17,18] Usmg the model outlined m ref 17 we have denved hgand-field parameters to fit the CdX2 and HgXz spectra (Table 5). Values for ZnX, are those derived by Orchard and Richardson [ 51 These parameters indicate that, as already conTABLE 5 SUMMARY OF LIGAND-FIELD DIHALIDES MXP X

Cl Br I

atomic b

SPLITTING PARAMETERa

ZnC

Cd

A,d,

a,:

i-3d

+017 +015 $034

+oo1 +002 tO08

0135 0136 0128 0 135

FITS FOR GROUP IIB

Htz

A on

A ns

+033 +04 i-06

-005 -0 11 -008

bd

026 026 0 26 0 276

A an

A nS

r Sd

-0 -0 -0

-024 -015 -0 14

060 065 068 0 744

62 60 27

a IneV b Atomic k from Suzer et al, J Electron Spectrosc Relat Phenom , 12 (1977) 357 ’ From Orchard and RIchardson, J Electron Spectrosc Relat Phenom , 6 (1975) 61 d Aor represents the shift of the *z state relative to the unperturbed ‘II state e &i represents the shift of the 2A state relatwe to the unperturbed 2n state

258

eluded, it 1s mamly the 21=, states of ZnX, and CdX, which are perturbed, while for HgX, both the III, and ZI, states are affected A further pomt of interest that arises from this analysis 1s that there 1s a sign change m the A parameters (see Table 5) between ZnX, and HgX2, mdlcatmg a change from electrostatic perturbation, A posltlve, to covalent perturbation, A negative A comphcatlon which arises from the use of this model IS the apparent reduction m Aar for ZnX, and CdX2 along the spectrochemlcal series Thus for ZnF2 a value of Aon < 0 05 eV has been estimated [ 141 This 1s the reverse of the trend that would normally be expected on the basis of a crystalfield effect This reduction might be interpreted m terms of a balance between opposmg electrostatic and covalent effects. Thus for ZnF, , covalent bonding would balance the crystal-field effect, gvmg rise to an effectively atomic d-’ spectrum. For ZnIz the crystal field dommates, while for HgX, the covalent effect dommates This mixture of overlap and electrostatic effects would explam the frequency-dependent effects observed throughout this group of molecules and which can only be explamed m terms of overlap, 1 e., a degree of covalent bondmg taking place m all cases.

CONCLUSIONS

The fine structure observed m the metal valence d-l photoelectron spectra of the Group IIB dlhahdes has been assigned and discussed m terms of changes m relative mtenslty occurrmg between the He(I) and He(U) spectra. Covalent mteractlon appears to occur between the metal dog and halogen ug orbit& m all cases. Llgand-field conslderatlons show the observed patterns to be the result of a balance between snnple held-sphttmg and covalency.

ACKNOWLEDGMENT

We should like to thank the S R C. for financial assistance

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259 7

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