Journal of Electrort Spectroscopy and Related Phenomena, 11 (1977) 371-382 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

VACUUM ULTRAVIOLET ABSORPTION SPECTRA OF SEVERAL GROUP IVA TETRABROMIDES

J. B. CLARK and B. R. RUSSELL

Chemistry Departmenf, North Texas Stafe Universify, Denton, Tex. 76203 (U.S.A.) (First received 12 October 1976; in final form 24 February 1977)

ABSTRACT

The vacuum UV absorption spectra of silicon, germanium and tin tetra- bromide are discussed. The spectra, taken in the vapor phase in the region from 33,000 cm-’ to 80,000 cm-l, are primarily due to intravalent transitions (e.g. a* + n, cJ* t o) with overlapping molecular Rydberg excitations in the higher- energy regions. Correlations between these data and previously reported studies of Group IVA halides are presented for the dual purposes of justification of the proposed spectral assignments and investigation of possible prc-dn interaction in these systems.

INTRODUCTION

The investigation of the unusual bonding properties of halosilanes has been the impetus for a large number of photoelectron studies. The work of Frost et al.’ is an excellent example of photoelectron spectroscopy in which the hypothesis of pn-dn stabilization is involved in the interpretation of the spectra of various halo- silanes. Since the pn-dn interactions are important in the ion states then these inter- actions are expected to be important in the excited electronic states. In fact, the absorption spectra of halosilanes have been analyzed utilizing the presence of prc-dn

interactions. The first electronic absorption spectrum of a halosilane, that of chloro- silane (SiH,Cl), was presented by Bell and Walsh’. The argument of pn-dn stabiliza- tion may account for the “disappearance” of the c* c n (Si-Cl) absorption (A band) in the SiH,Cl. Stabilization would occur if electron density originating on the chlorine lone-pairs were to be delocalized through a n-framework comprised of the empty 3d

atomic orbitals of silicon and the non-bonding p orbitals of the chlorine. If the non-bonding electrons are stabilized, the A band should be shifted to higher energies and possibly masked by various Rydberg absorptions. This explanation was given by Causley and Russell3 to account for the spectrum of dichlorosilane. The interpreta- tion of the spectrum of bromosilane (SiH,Br) likewise utilized this argument4. It is

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SiBr,

ABSORBANCE= 0.1

I 130 150 170 190 210 230 250 270

WAVELENGTH b-d

Figure 1. The vacuum UV absorption spectra of silicon tetrabromide (SiBra), germanium tetra- bromide (GeBr4) and tin tetrabromide (SnBrr).

the purpose of this study to investigate the possibility ofpn-&c interactions in silicon tetrabromide (SiBr,), germanium tetrabromide (GeBr,) and tin tetrabromide (SnBr,). The spectra of these compounds is given for the region 33,000-80,000 cm-’ (300-125 nm).

As can be seen in Fig. 1, the spectrum of each of the compounds is basically divided into two regions. Absorptions in the lowest-energy region are attributed to the intravalent CJ* t n type transitions, while molecuIar Rydberg transitions dominate the higher-energy region. For the most part, Rydberg transitions will be assigned on the basis of term values. In this presentation, term value refers to energies calculated from the equation

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TA3LE 1

PHOTOIONIZATION DATA FOR THE GROUP IVA TETRABROMIDESa

e

e

Ul

e

al + e

SiBr4 G&r4 SnBr4 Assignment (Td)

10.8

11.17

11.52

11.63

12.05

12.14

12.33

14.1”

10.75 11.0

10.85

11.11 11.25

11.23

11.89 11.75 le

13.81b 13.2 21-z

3h

* Data from ref. 5. b Splittings were observed in these ion states but the average is used in calculations.

where IP is the ionization potential and vobs the absorption maximum. Term values are given for intravalent transitions as this is a convenient manner of comparing the corresponding states of the different compounds. These term values do not have the theoretical implications of the term values for Rydberg transitions. The results of photoelectron spectral data, reported by Green et al.5, are presented in Table 1 and will be utilized extensively in the assignment of the various absorptions. When the splitting of levels in these ions is 0.1 eV or less, the average of the two values will be used in the calculation of the term values.

An absorption attributed to a oh t c transition is observed in the SnBr, spectrum. This is indeed a rarity for vapor-phase spectra, in which Rydberg transitions generally predominate. An occurrence of this type offers an excellent opportunity for a comparison of absorption and photoelectron data that in turn may give a qualitative view of the resultant orbital-energy rearrangement which takes place during an electronic transition_ The information derived from the SnBr, spectrum, the spectrum of SiH,Br and the appropriate photoelectron data will be correlated in an attempt to discern the location of the C* c 0 transitions in SiBr, and GeBr,.

EXPERIMENTAL

The spectra were obtained using a McPherson RS-225 l-meter vacuum UV

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spectrophotometer equipped with a 1200 line mm-’ grating with a dispersion of 8.3 A mm-l. The basic instrumental techniques are described elsewhere3. All samples were obtained from the Ventron Corporation and vacuum distilled before obtaining the spectra. The samples were run in the vapor phase at pressures from 0.02 to 0.16 torr. Flow techniques were necessary for SiBr, in order to obtain a spectrum without observable degradation products. Scans of individual bands in SiBr, at fixed pressures were used to estimate the sample pressures for the spectrum involving flow techniques. This was necessary for calculation of oscillator strengths. This estimation falls within the 10 % error usually associated with the calculation of oscillator strengths. The spectra of GeBr, and SnBr, contained no evidence of degradation.

RESULTS

Table 2 is a compilation of band maxima, assignments and oscillator strengths for the c* t n transitions in the three compounds, as well as the literature values for CBr, (ref. 6). Tables 3-5 contain the band maxima, assignments, and term values for the spectra of SiBr,, GeBr, and SnBr,, respectively.

The remainder of this section will be divided into an analysis of the A band region, followed by a separate analysis for the higher-energy region of each compound.

A Band (a* t n) Absorptions occurring in the lower-energy regions of the electronic spectra

of SiBr,, GeBr, and SnBr, are assigned to the Q* + n transitions, with the electrons involved in the transitions arising from the non-bonding electrons of the bromine

TABLE 2

SPECTRAL DATA FOR THE R BANDS ( a* c n) OF THE GROUP IVB TETRABRbMIDES

Maximum (nm) Transition Uscif!ator Total osciffator strength strength

CBrda 252.8 224.8 200.0

SiBr4 195.0 175.0

GeBrr 245.0 215.0

SnBr4 263.0 231.0

al+ e 0.010 aI+ e 0.073 0.095 alt a1 0.012 aIt e 0.05 0.28 al+-- e 0.23 al + 02 0.05 0.27 aI+ tz + e 0.22 al t tl + tz 0.05 0.29 al + tz + e 0.24

a Data from ref. 6.

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TABLE 3

BAND MAXIMA, TRANSITION ASSIGNMENTS AND TERM VALUES FOR SiBrc

Maximum (nm) Transition Term vale (cm-1)

195.0 u* t n(e) 36,200 175.0 u* t n(e) 36,200 155.0(s) 5s t n(e) 29,300 147.3 5p + n(e) 19,600 142.5(s) 5p + n(u2) 19,900 140.2 5d + n(e) 16,200 137.2 5~ + n(e) 20,oQo 135.3 5~ + n(e) 19,900 133.8 5d + n&a) 15,400 131.1 5d + n(e) 16,600 126.6 6d t n(e) 8,450

TABLE 4

BAND MAXIMA, TRANSITION ASSIGNMENTS AND TERM VALUES FOR GeBrr

Maximum (nm) Transition Term vahe (cm-l)

245 .O u* t n(h) 49,800 215.0 u* t n(t2 -j- e) 49,300 150.0 5p t n(t1) 20,900 140.7 59 + n(t2) 19,500 139.2 5d t n(t1) 15,300 137.3 5d t n(t1) 16,800 126.7 5dt n(t2 + e) 17,cKxl

TABLE 5

BAND MAXIMA, TRANSITION ASSIGNMENTS AND TERM VALUES FOR SnBrlr

Maximum (nm) Transition Term value (cm-l)

263.0 o* + n(t1 + ta) 52,700 231 .O IP t n(G2 + e) 51,500 179.5 6* t a(tz) 50,800 151.1 5p t n(e) 22,500 143.3 5p t n(t1 + ta> 21,000 137.6 5~ t n(lz + e) 22,100 127.8 6~ t n(b) 10,500

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chromophore. The contour of the absorption band is structureless and approximately the same for each compound. This intravalent band consists of two components, with the lesser component appearing as a shoulder ca. 5,500 cm-l to the red of the first distinct band maximum. The oscillator strengths for the A bands are the same within experimental error (& 0.02) for each of the compounds investigated. Using the three-dimensional oscillator strength expression

j- = 4.32 x 1O-9 s Edv

the oscillator strengthsf = ca. 0.05 andf = ca. 0.23 were obtained for the first and second components, respectively. The total oscillator strength for this band system is at least three times larger than that for CBr, (ref. 6). This apparent anomaly in intensity is not unique to the tetrabromides but is also observed for the Group LVA tetrachlorides’. The rationale for the intensity variation is attributed to the electronic distribution for the series of compounds, arising from differences in electronegativities of the central atoms (C = 2.5; Si, Ge, Sn = ca. 1.9). The highest occupied bonding level in the carbon compound contains less bromine character than the corresponding levels in silicon, germanium and tin tetrabromides. Then the lowest energy anti- bonding level (3a,) possesses greater central atom character in the SiBr,, GeBr, and SnBr, compounds. The cr* +- n transition would correspond more to “charge transfer” for CBr, when compared to silicon, germanium and tin tetrabromides. While retaining the same contour and strength, the entire band system shifts to longer wavelengths with increasing size of the central atom. This effect is expected if the bonding levels are destabilized5 with increasing bond length’ and the anti-bonding levels are concomitantly stabilized. By examining the MBr, series of compounds and varying the central atom from Sn to C it is expected that the CT* t 12 term value for SnBr, would be the largest (52,100 cm-l) and the term value for CBr, (44,800 cm)6 would be the smallest. However, the value for SiBr, is only 36,200 cm-l.

Of the three compounds under investigation the splitting of the non-bonding pairs is greatest in SiBr,, as is evident in the photoelectron spectrum5. Upon excitation the effective symmetry is reduced from Td by spin-orbit coupling5 and Jahn-Teller splitting (as reported for CBr,)6 to yield eight distinct ionization potentials for the non-bonding electrons. The electron pairs of t, symmetry are reduced in the C,, point group to a pair designated e and one of a 2 symmetry. Likewise, those of t2 symmetry are reduced to an additional pair of e symmetry and one al. It is the electrons of these first and second sets of e symmetry which are believed to be involved in the two absorptions at 195.0 nm and 175.0 nm. For these absorptions, which have been assigned to the c* c n transitions, the O-l-eV splitting in each of the sets of e symmetry cannot be resolved. Therefore, term values for the A bands are determined using the averaged ionization potentials. The term value for the absorption at 195.0 nm using the first two ionization potentials (10.8 and 10.9 eV, e) is ca. 36,200 cm-r. The absorption at 175.0 nm using the second set of ionization potentials of e sym- metry (11.52 and 11.63 eV) also has a term value of ca. 36,200 cm-‘. This is in good

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agreement with the value of 36,600 cm-l reported for the corresponding transition in SiH,Br (ref. 4). The A bands for the LQ, a,, and the set e are not observed. The transition arising from electrons of a _, origin is symmetry forbidden and those transitions arising from aI and remaining e electrons would occur in the region of the first 5s t n Rydberg transitions for SiBr,.

Replacement of Si by Ge, an atom approximately equal in size and electro- negativity, drastically reduces the splitting of the non-bonding pairs and results in a slight destabilization of both the non-bonding and bonding energy levels. The electrons associated with the first three ionization potentials are thought to be tl in nature, the fourth assigned as tz and those of the fifth ionization potential containing both b, and e character_ The two ion states assigned as f, are those believed to be involved in the C* t n transitions at 245.0 nm and 215.0 nm. Electrons from orbitals of t, symmetry are the only ones “allowed” to participate in an CP t rz transition in Td symmetry. The first absorption at 245.0 nm has a term value of 49,800 cm-’ with respect to the fourth ionization potential (1 I.32 eV, t2). If the electrons associated with the fifth ionization potential (11.89, t2 + e) are involved in the 215.0-nm ab- sorption, a term value of 49,300 cm-’ results. A comparison of the bond energies of SiBr,, GeBr, and SnBr,, together with their ground-state energy levels, leads one to expect this increase in term value for the o* c n for GeBr,. Assuming that these comparisons are valid, a greater term value should be observed for SnBr,.

The photoelectron spectrum of SnBr, indicates that the non-bonding electrons are split into three distinct groups. The first ionization potential (11 .O eV) corresponds to the electrons of orbitals t, in r, symmetry. The second ionization potential (11.25 eV) corresponds to a mixture of t, and t2, while the third ionization potential is a composite of tZ and e. If the first electronic absorption at 263.0 nm of SnBr, corre- sponds to the excitation of the least stable electrons of tz symmetry, a term value of 52,700 cm-l is obtained with respect to the second ionization potential. The larger of the two cr* t II absorptions is then attributed to electrons associated with the third ionization potential, also partially t, in character. A term value for the 213.0-nm band with respect to the third ionization potential is ca. 51,500 cm-‘.

The CT* t n transitions involving the 0 * level of b, symmetry have been neglected in this study. These transitions are expected to be of sufficient energy to occur in the p-d Rydberg region. Although the location of these transitions is un- certain, they are expected to add intensity to the p-d Rydberg region.

The distinct absorptions of SiBr, in the region from 150.0 nm to 125.0 nm are attributed principally to molecular Rydberg transitions. The absorption at 155.0 nm is assigned as a 5s c n Rydberg transition’ which corresponds to electrons associated with the fifth ionization potential (11.63 eV, e) and has a term value of ca. 29,300 cm- ‘. Other possible 5s t n Rydberg transitions would add underlying intensity to the blue side of the U* t n absorption or to the p and d Rydberg region.

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The major absorptions in the region 150.0 nm to 130.0 nm are assigned as 5p c n and 5d t n molecular Rydberg transitions. The average term value for the 5p is 19,800 cm-‘, and 16,100 cm-’ for the 5d. These term values for the 5p and 5d Rydberg transitions of SiBr, are in agreement with those for SiH,Br (ref. 4). It should be noted the apparent degeneracy of the p-d Rydberg transitions for the chloro- silanes3’ ’ is split in the bromine compound. This splitting is rationalized if the Rydberg states of the chlorosilanes are more sily14 while bromosilanes exhibit more bromine character. As previously stated, the term values of the rr* t n and Rydberg transitions are virtually the same in SiBr, and SiH,Br. Based on this information, the b* t a absorption is thought to be in the region of 135.0 nm, providing uncler- lying intensity to the p-d Rydberg band system. The lowest energy 6s c n and 6p t n

Rydberg transitions contribute a portion of the intensity from 135.0 nm to 130.0 nm while the 126.6 nm absorption is primarily due to the lowest energy 6d t n transition with a term value of 8,450 cm-‘.

GeBr, The largest number of transitions in the vacuum UV spectra of GeBr, lie

below 150.0 nm. Absorptions from 150.0 nm to 130.0 nm are assigned to 5p t n and 5d t n molecular Rydberg transitions. The average term values are 20,100 cm-l and 16,200 cm-l, respectively. These term values are slightly larger than those found for SiBr,. Although the absorption at 150.0 nm is thought to be the 5p Rydberg with respect to the average of the first two ionization potentials, it also has a term value of 29,200 cm-’ with respect to the fifth ionization potential. The fourth and fifth ioniza- tion potentials of GeBr, have been assigned as t, and tz + e in characte$, leading one to expect at least two 5s + n Rydberg transitions. If the term value of 29,200 cm-l is appropriate for the 5s Rydberg transition, one would expect a 5s t n Rydberg transition at ca. 163.0 nm. However, this corresponds to the origin of the 5p5d band system.

The absorption at 126.7 nm is assigned to be primarily 5d and 6d Rydberg. The maximum has a term value of 16,900 cm-’ with respect to the fifth ionization potential, which is slightly high for a d term value but within reason. A term value of 8,200 cm-’ is calculated using the first “averaged” ionization potential (10.80 eV) and corresponds to a 6d t n term value.

With respect to this complex band, the lower-energy shoulder is assigned to a 6s + np Rydberg for the 11.23 eV (t2) ionization potential. The higher-energy shoulder is assigned to the 6p t n transition with respect to the third ionization potential (11.11 eV) and has a term value of ca. 9,900 cm-‘.

The first distinct maximum at higher energies to the A band is assigned as the cr* t cr transition. This maximum at 179.5 nm has term values of 33,000 cm-l, 35,000 cm-’ and 39,000 cm-l with respect to the first three ionization potentials of

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the non-bonding electrons of SnBr,. These term values are low for a U* t 12 transition for this compound, but also seem high for a 5s t n Rydberg transition.

While a Rydberg assignment is possible for this transition, the Q* t c assignment appears to be the most plausible. The intensity of this allowed intravalent transition appears small but the extinction coefficient of the cr* t n and c* t c compare quite well.

An interesting aspect of this assignment is noted when the difference in energy for the maxima of D* t n and the o* t 0 absorptions is compared to the spacings of the non-bonding and bonding levels from photoelectron studies. The energy difference between the most intense Q* t n component (43,300 cm-l) and the c* t Q maxima (55,700 cmmi) is ca. 1.54 eV. The corresponding difference in the reported ionization potentials (11.75 and 13.20 eV) is 1.45 eV. The 0.09-eV difference is within the O-l-eV experimental error of photoelectron data.

The three maxima at 151.1, 143.3 and 137.6 nm are assigned as primarily 5p t n Rydberg transitions with the greatest intensity at 143.3 nm associated with the three non-bonding ionization potentials (11.0, 11.25, and 11.75 eV). The average term value for these absorptions is 22,000 cm-‘. This is higher than the value of 20,900 cm-’ reported for SnCl, (refs. 7, 10). Since the higher-energy portions for the spectra of the previous compounds exhibited absorptions which are attributed to p and d Rydberg transitions, the absence of discrete maxima corresponding to 5d t n transitions in this spectrum is surprising. Rationalization of this anomaly and the unusually large absorption at 143.3 nm are one and the same. An increase in the p term value would specify an increase in the corresponding d term value. A term value of 17,000 cm-’ is then reasonable and suggests that the first 56 + n transition would appear at 140.0 nm. This would in effect yield underlying intensity to the 143.3 nm band.

The absorption maximum at 127.8 nm is Rydberg in nature exhibiting term values of 10,500 cm-’ and 12,500 cm-’ when compared to the first and second ionization potentials, respectively. A term value of 10,500 cm-l would correspond to a 4p t np Rydberg transition. Although no absorptions attributed to the 5s * np

transitions were observed, the 6s t np transition should have a term value of ca. 12,500 cm-‘.

CONCLUSIONS

The compilation of experimental and published data found in the correlation diagram of Fig. 2 summarizes this study. The interpretation of this diagram leads to the final observations and conclusions in this analysis of the Group IVA tetra- bromides. The term values of SiBr, support the assignments in the spectra of SiH,Br (ref. 4) and suggests the presence of pn-dn interactions in the bonding scheme of SiBr,. Figure 2 illustrates the trends in the bonding (2t,), non-bonding (e, 3t,, tl) and first anti-bonding (3~2,) levels of the Group IVA tetrabromides. A cursory

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CONTINUUM

-5d -5P -5s -DO

-”

- 5d _____5d -5P -5l.J _____.5s ______5s

l -O-

-C-*

-n -”

-o- -o-

C Br4 SiBr4 Gel%4 SnBr4

Figure 2. A correlation diagram for carbon tetrabromide (CBra), silicon tetrabromide (SiBre), germanium tetrabromide (GeBr4) and tin tetrabromide (SnBrs) derived from photoionization data and transition energies. See refs. 4 and 5. Note: the a*(al) levels are given qualitatively for illustrative purposes only.

examination of this diagram points out several anomalies. The silicon-containing compound exhibits a destabilized anti-bonding level with a slight stabilization in the non-bonding levels, while the carbon analog features a slight stabilization of the bonding (2t2) levels and a destabilization of the non-bonding levels. These perturbing effects in CBr, may be considered as a simple repulsion of states of like symmetry. If CBr, is distorted to C,, symmetry during excitation, as proposed by earlier in- vestigations6, the 0 bonding levels (a, + e) are of the correct symmetry to interact with the largest portion of the non-bonding levels (e + a2, e + a,, and e). The resulting stabilization of the o bonding levels and destabilization of the non-bonding levels prohibit valid comparisons between C and Si analogs with respect to pn-drc stabilization of the non-bonding levels. Although, pn-dn interaction theory is not entirely satisfactory for the interpretation of the non-bonding data, the authors propose d interaction to be responsible for the destabilization of the anti-bonding orbital (30,) of SiBr,. This mixing is believed to occur upon excitation as the molecule distorts from Td to C,, symmetry. The 3d orbitals transform from e + t, in 7” symmetry to e and e + a, in C3V symmetry. The first unoccupied anti-bonding orbital is 3~2, in Td and likewise in C 3V symmetry. The a, component of the 36 orbital is therefore available for mixing. The non-bonding levels are largely e and a, in C,, symmetry and are consequently available for mixing with the empty 3d orbitals. Although as stated above, the non-bonding data do not imply pr-dx stabilization, the presence of d interaction in the IT * levels suggests that px-drc stabilization of the non-bonding levels is possible.

381

An unusual trend is observed in the p Rydberg term values. As the size of the central atom is increased, a corresponding increase in the p term value results. A decrease in term value would be expected based on the atomic size versus term value relationship observed in rare gas correlations 10-12. This trend would indicate a greater penetration of the electron to the molecular core as the size of the central atom increases. If one refers to the previous argument concerning electronegativities, the halide removes more electron density from the central atom as the size of the central atom increases. This decreased electron density on the central atom or center of the molecular core would give rise to increased penetration as manifest in the p term values,

The observation of absorptions attributed to the 6* t n and 6* t d transi- tions in SnBr, is important. The data enable one to check the validity of the technique” in which the CT* t n transition energy and photoelectron data are used to predict the location of the o* t 0 transition. This method appears to be valid, within experimen- tal error (0.1 eV) for the photoelectron data. Based on this information, the first D* t 0 transition of GeBr, is predicted to occur in the region of 160.0 nm.

The initial purpose of this study was the search for experimental evidence for pn-&c interaction in a series of Group IVA tetrabromides. Although no substantial evidence for this type of interaction was observed in the non-bonding levels of these compounds, d interaction seems to be extremely important in the excited states of SiBr,.

ACKNOWLEDGEMENTS

Support of this investigation by the Robert A. Welch Foundation Research Corporation and the North Texas State University Faculty Research Fund is grate- fully acknowledged. The authors wish to thank Gary C. Causley and Melvin B. Robin for many helpful suggestions.

REFERENCES

1 D. C. Frost, F. G. Herring, A. Katrib, R. A. N. McLean, J. E. Drake and N. P. C. Westwood, Chem. Phys. L&t., 10 (1971) 347; Can. J. Chem., 49 {1971) 4033.

2 S. Bell and A. D. Walsh, Trans. Faraday Sot., 62 (1966) 3005. 3 G. C. Causley and B. R. Russell, J. Electron Spectrcwc. Relat. Phenom., 8 (1976) 71. 4 G. C. Causley, J. B. Clark and B. R. Russell, Chem. Phys. L&t., 3X (1976) 602. 5 J. C. Green, M. L. H. Green, P. J. Joachim, A. F. Orchard and D. W. Turner, Philos. Trans.

Roy. Sot. London, A., 268 (1970) 111. 6 G. C. Causley and B. R. Russell, J. Chem. Phys.. 62 (1975) 848. 7 G. C. Causley and B. R. RusseIl, J. Electron Spectrosc. Relat. Phenom., 11 (1977) 383. 8 F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry, Interscience, New

York, 3rd edn., 1972, and refs. therein. 9 For consistant notation the authors use quantum numbers associated with the bromine atom

in the description of Rydberg transitions.

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10 M. B. Robin, Higher Excited States of Polyatomic Molecules, Vol. I, Academic Press, New York, 1974.

11 John D. Scott, G. C. Causley and B. R. Russell, J. C&m. PIzp., 59 (1973) 6577. 12 John D. Scott, G. C. Causley and B. R. Russell, in C. Sandorfy, P. J. Ausloos and M. B. Robin

(Eds.), Chemical Spectroscopy and Photochemistry in the Vacuum Ultraviolet, D. Reidel, Dord- recht, 1974.