Laser-excited fluorescence of BiBr

Specrrochumca Acta Vol Pnnted m Gnat

42A No

I pp 35-37,

1986 0

Bntam

058447539/&5 s3 00 + 0 00 1986 krgamon Press Ltd

Laser-excited fluorescence of BiBr P PATI&O, M CAETANO, D IACOCCAand M MARCANO Lab de Espectroscopfa Molecular y Plasmaqufmca, Escuela de Qufmmlca, Facultad de Clenclas, Umversldad Central de Venezuela, Apartado 47102, Caracas, Venezuela (Recetoed 13 Aprd 1985, mfinal form 26 July 1985) Abstract-Laser-exat@ fluorescence spectra of B1’gBrand Bl*‘Br were stuched by conventional grating spectroscopy The Ar+ 4765 and 488OA hnes exate extensive senes m the system AO+-X0+ Levels m the ground state were characterized m the range 0 < ud 35 and fits to several hundred hnes were made for the two molecules VibratIonal constants (m cm-‘) for the X0+ state of Bi’9Br are as follows o,=209705fOOlO

o,x,=O4758fOOOO8

o.y,=(485fO17)~10-~

AO+-X0+ system The upper level of one series, excited by the 4880 A line m B179Br, has not been fully ldentlfied The fluorescence pattern observed was that of the usual R(.l - 1) and P(J + 1) doublets of about equal intensity, and extended over wide ranges of U” In addition to the main doublets, weaker lines arising AJ’= fl, colhslon-induced changes from were also observed m the strongest series *2, The determmatlon of the vibrational and rotational numbering, using previously pubhshed values of the constants for the X0+ state [8], was straightforward However, due to the lack of an estimate of aeye, this procedure was limited to the range 0 i U”Q 10 For three series observed only at high v” levels, the assignments were done after more reliable vibrational constants had been obtamed from the other measured lines Details of the series are given m Table 1

INTRODUCTION The spectra of the heaviest members of Va-VIIa dtatorruc molecules are of interest Since m these species case c configurattons can be expected to be important

For BlBr, one of these radicals, the ermsslon spectrum was first observed by HOWELL and ROCHESTER[~] and the absorption spectrum by MORGAN[~] The vibrational analysis of nearly 280 bands lying in the

regon 4590-6063 A has been given m emlsslon [3,4] These bands have been assigned to the A-X system From the partial rotational analysis of seven bands m the A-X system of bismuth monobromlde, LAL et al [S] have proposed, by analogy with the similar system of BlCl[6], that the transition mvolved 1s a Hund’s case c % (O’)-‘Z- (O+) More recently, very accurate values of the rotational constants for the O+ ground state have been calculated by fitting the microwave absorption spectra measured m a number of vibrational levels [7] However, some discrepancy still remains between the vlbratlonal constants calculated by KUIJPERS and DYMANUS[~] and those obtamed from the optical ermsslon spectrum [3,4] We have therefore recorded laser-induced fluorescence spectra of BlBr at moderate resolution m order to improve the ground state constants, and possibly to observe weak AO+-Xl (i2 = 1) transitions

Table 1 Summary of the fluorescence Senes excited m BiBr*

Exatatlon

Excltlng line Ar II 4765 A

EXPERIMENTAL Ar II 4880A Bismuth monobromlde fluorescence was excited m a small quartz ampoule contammg about 0 2 mg of BIBr3and heated to ahout 1170 K The 4765 and 4880 A lines of a SpectraPhysics argon-Ion laser (Model 164-03) were used and the spectra were photographed on Kodak 103a-0 and 103a-F plates and on Ilford HP5 films m a Jarrell-Ash 3 4 m Ehert spectrograph equipped with a plane grating of 590 lines/mm The reciprocal dispersion tn the repon 4600-6800 A was approximately 2 4 A/mm m the second order Fe and Ne hollow cathode lmes were used as standards Fluorescence hnes were measured to a preclslon of *006 cm-’

Bl’ ’ Br P (62) P (87) P(119) R (67) P(132) R(179) P (79) Bl’ ‘Br

Ar II 4880 A Ar II 4765 A Ar II 4880 A

OBSERVATIONSAND ANALYSIS The two Ar+ laser lines excited 13 series eight in B179Br and five m Bl”Br All but one anse from the

R (208) Bls’ Br R (70) P (85) P(120) P(81) R (68)

AO+-X0+ 40 61 42 5-3 63 7-3 8-5

Range of u” Observed O-26 20-27 &25 &26 O-32 29-33 O-35

7-x0+ 51

(r29

AO+-X0+ &O 6-l 4-2 8-5 i&6

O-26 O-32 O-25 31-35 ti

*A copy of the spectroscopy measurements has heen deposited with the edltonal office 35

P PAT&JO ef a/

36 RESULTS AND DISCUSSION

Table 4 RotatIonal 0

Molecular constants were determmed by slmultaneous fits to measured Imes, m which upper and lower state term values T” J = T,+o,(u+1/2)-o,x,(u+1/2)2+ +[S,-a&+1/2)+

]J(J+

T4 B, T, BS T6 B, D, x 109

2112242 0035428 21244 00352 213645 0 03469 c5 01

(5) (4) (4) (8) (4) (2)

0 0347 0 0342

0 03479 0 03423

ground-state quantum numbers 1s not m doubt and u = 5 m the upper state IS indicated by intensity dlstrlbutlon However, the minima of intensity are situated at completely different u” levels from those corresponding to the series excited by the same laser line m Bl”Br, R(67), 5-3 band of AO+-X0+ Addltlonally, the TI value pomts to u = 4 m AO+, whereas the B: IS close to that of u = 6 It may be thought that the upper level of this series 1s the u = 5 level of a state situated m the vlamty of the AO+ state, as has been observed for BlCl [ 10,l l] and BlI [ 123 The vlbratlonal constants reported in Refs [3] and [4] were obtained from band-head measurements at

(cm- ‘)

Umdentlfied 209705 (10) 04758 (8) 0485 (17) [0 04321526]* [0 132695]* [0 07682]* [7 341081’ 49 (14)

Number of hnes fitted 612 Weighted standard devtatlon of the fit 0 119 cmValues m parentheses are 20 [ ] indicates a fixed parameter *FIxed at the values reported m Ref [7]

T(v= 7) E”

1

from the fit to the BIs’Br senes (cm-‘) X0+ state

AO+ state T4 B4 T6 B6 D, x 109

0 03543 00352 0 03469

LAL er al [S]

*From

from the fit to the B179Br sews

o, o,x, w,y,x lo3 B, or,x103 ye x lo6 D, x lo9 8. x 10’2

Table 3 Constants

0 0353 00350 0 0348

4 6

X0+ state

AO+ state

Observed

Bls’Br

were represented by an appropriate number of adjustable parameters Constants derived by separately fittmg the series observed m Bl”Br and m BISIBr are given m Tables 2 and 3, respectively Rotatlonal constants for the ground state were fixed at the values reported m Ref [7] except for /3, of Bl”Br which was obtained from the fits Upper levels were ldentdied by rough calculation of the band orlgms Extrapolations here are not exact, mamly because of the high J levels of the transitions Rotational constants are m good agreement with those calculated by LAL et al [S], which confirms these assignments The comparison 1sgiven m Table 4 Identdicatlon of the series presented a problem m only one case, full details of which are given elsewhere [9] The series 1s excited by Ar II 4880 A m the hne R(208),u"= 1 of Bl”Br The assignment of

Table 2 Constants

2111757 0034793 213561 003423

C4 *I

B, for AO+

Calculated*

BI’~B~ 4 5 6

I)+

constants

(7) (4) (2) (3)

w, u&x, 0.y.x

B,

lo3

a,x 103 ye x lo6 D, x IO9 8. x lo’*

207 826 (14) 04678(11) 0484 (22) [004244071]* [0 129144-J* E ;;;;;.“I* c4 71

Number of hnes fitted 585 WeIghted standard deviation of the fit 0 146 cmValues m parentheses are 217 [ ] mdtcates a fixed parameter *Fixed at the values reported m Ref [7]

1

state 21141(l) 0 03474(3)

Fluorescence of BlBr low resolutton, whereas those reported in Ref [7] were calculated from the rotational constants by using standard relations [ 131 Having observed a relatively

large number of vlbratlonal levels m the ground state of both B179Br and Bi*‘Br (0 < u“ < 35), the vlbratlonal constants obtained for these molecules have a reasonable degree of confidence The fits to some 1200 lines for the two molecules have yielded more precise values for the vibrational frecuency w, and the anharmomclty constant WAX,, compared to those of Refs [3,4] and [7] For B179Br the followmg results have been obtained w,(cm-‘) w,x, (cm- ‘)

Refs [3,4] 209 50 045

Ref [7] 209 62 0 52

This work 209 705 0 4758

Moreover, for the same molecule, the value w,y, = 4 85 x 10-4cm-1 has been calculated from our measurements for the first time Constants for Bi*‘Br together with the values calculated by using the constants for B179Br and the usual lsotoplc relations [ 133 are given m Table 5 For both molecules, the measured lines can be reproduced within If:0 15 cm- ’ by using the constants reported m this work As has been mentioned, the microwave absorption spectra of B179Br and BIEIBr were measured by KUIJPERSand DYMANUS[7] In their study they did not observe transitions m the 1 (i2 = 1) component of the ground state Based on their observation of rotatlonal transltlons m relatively high vlbratlonal levels of X0+, they estimated that the separation between the O+ and 1 components IS larger than 2000 cm-’ The fluorescence excited by the Ar+ 4880 A line of a Coherent Radiation CR12 laser (5 W) was focussed on the entrance slit of a 2 m Sopra scannmg monochromator A search out to 8000 A failed to reveal any lines which could be attributed to a AO+ - 1 (Q = 1) transition Two possible interpretations for these results are (1) the AR # AA forbidden

Table

5 Vibrational constants BislBr

for

X0+,

31

transition 1stoo weak to be observed m our condltlons or (2) the separation between the two components of the X ‘Z- (case c) state 1s larger than 8000 cm- 1 In going from Hund’s case 6 to case c, spmarblt mteractlon occurs This IS, for example, the situation for homonuclear VI-VI dlatomlc molecules The case b scheme ISa good description for the 32- ground state of 0, with & = 1 985 cm-‘, but with mcreasmg atomic number, the spm-couphng constant increases m magnitude For the heavier molecules the case c descrlptron 1s more appropriate, the R = &l sphttmg m Te, bemg 197497cm-‘, which approaches the order of atomic spu+rblt coupling constants The splittings for the X3x- states of SbF and SbCl are 796 3 and 816 3 cm-’ respectlvely[8] From semlemplrlcal relations for the spu-rblt couphng constant [ 141 it follows that the atomic integral in, could be used to estimate the Q = O-l sphttmg m the X 3x_ state of BlBr This should be approximately 800 (
are grateful to Dr R F BARROWat Oxford Umverslty for kmdly lettmg us take the plates m his laboratory One of us (P P) thanks the CDCH, Umversldad Central de Venezuela, for the award of a fellowshp The comments of the referee are gratefully acknowledged REFERENCES [l] H G HOWELLand G D ROCHESTER,Linru Durham Phd Sot Proc 9, 126 (1934) [2] F MORGAN,Phys Ret, 49, 41 (1936) [3] S SANKARANARAYANAN, M M PATEL and P S NARAYANAN, Proc Indlan Acad SCI 56, 171 (1962) [4] S P SINGH,I&an J pure appl Phys 6, 299 (1968)

[5] K M LAL,K A MOHAMEDand B N KHANNA,I&an J pure appi Phys 13, 53 (1975) [6] B N KHANNA,J molec Spectrosc 6, 319 (1961) [7] P KIJLJPERS~~~ A DYMANUS,~~~~Phys Lptt 39,217 (1976) [8] K P HUBERand G HERZBERG, Molecular Spectra and Molecular Structure, Vol IV, Constants of Dlatonuc Molecules Van Nostrand Remhold, New York (1979) [9] M CAETANO, P PATINO, D IACOCCAand M MARCANO,Acta Clent Venezolana, m press [lo] Y K S CH BABUand P T RAO, Indran J pure appl Phys 5, 19 (1967) R YAMDAGNI, J molec Spectrosc 35, 149 (1970) [12] R YAMDAGNI, Spectrochlm Acta 26A, 1071 (1970) [13] G HERZBERG, Molecular Spectra and Molecular Structure, Vol I, Spectra of Dtatonuc Molecules Van

[ll]

Calculated from BI’ 9Br constants

Observed

207 818 04673 47x1o-4

207 826 04678 48~10-~

Nostrand Remhod New ‘iork (1950) [14] E ISHIGUROand M KOBORI,J Phys Sot Japan 22, 263 (1967)