THE
EMI$SION
BROMIDE
SPECTRUM OF BISMUTH
(BiBr)--"A" SYSTEM
BY S. SANKARANARAYANAN, M. M. PATEL* AND P. S. NARAYANAN
(From the Department of Physics, lndian Institute of Science, Bangalore. 12)
Roceived Septr
21, 1962
(Communicated by Prof. R. S. Krishnan, l*.A.sc,)
INTRODUCTION
As part of their extensive study of the emission spectra of the halides of the
elements of groups IV and V of the periodic table, Howell and Rochester
(1934) obtained the molecular spectrum of BiBr using high frequency discharge. Morgan (1936), in his detailed investigation of the absorption spectra
of Bismuth halides, obtained two systems in absorption, one in the region
Ah4595-5438, and the other in the region Ah 3862-4129, both attributable
to the molecule BiBr. On the other hand Sur and Maiumdar (1954) obtained
three systems in absorption for BiBr in the regions Ah2709-2968, Ah 2336-2450
and hA 2200-2350. They designated these systems as C, D and E respectively,
naming the other two long wavelength systems as A and B. No detailed
vibrational analysis of the A-system has been made in emission.
Our knowledge of the band spectra of the other bismuth halides is far
more extensive [Howell (1936), Saper (1931), Rochester (1937), P. T. Rao
(1949), Sur (1950), Ray (1942), P. Venkateswarlu and B. N. Khanna (1960),
B. N. Khanna (1961), K. C. Joshi (1961), G. D. Rochester (1961), Prasada
Rao and Rao, P. T. (1962)]. Consequently, a study of the A-system of BiBr
in emission was undertaken. In this, a transformer discharge may be expected
to yield a more complete spectrum and enable us improve our knowledge
of the vibrational constants of the states involved and also the isotope effect
in the band spectrum of bismuth monobromide.
EXPERIMENTAL
Extra pure anhydrous bismuth tribromide was taken in a smaU eavity
at the centre of a conventional type of discharge tube (fused silica) 30 era.
long and of about 1.1 cm. in diameter. Hollow nickel electrodes were
fitted coaxially inside the silica tube with their nearer ends separated by a
* Present Addross: profossor of Physics, J. & J. CoIlcge of Science, Nadiad, Gujarat State.
171
172
S. SANKARANARAYANANAND OTHERS
distance of about 6 cm. and placed symmetrically on either side of the cavity
containing the salt. Pyrex adopters through which aluminium rods of
diameter 2 mm. could be introduced were fitted to the ends of the discharge
tube. The nickel electrodes were connected to these aluminium rods by
means of a short piece of nichrome wire. To one end of the discharge tube
was attached a glass window through which the discharge could be observed
end on. The other end was provided with a side tube through which the
tube can be evacuated. Standard precautions were taken to avoid contami.
nation of the discharge and also to protect the pumping system from vapours.
Uncondensed transformer discharge (15,000 V, 30 mA) was used for excitation
and the voltage applied to the discharge tube could be varied by altering the
input voltage of the transformer by means of a variac. The substance was
vapourised by a careful and controlled heating of the substance and the discharge was deep green in colour. Fuess and El glass Littrow spectrographs
were employed to photograph the spectra. An exposure of about hall an
hour was found to be adequate to record the spectrum with Ilford special
rapid panchromatic plate in the case of the lower dispersion instrument while
an exposure of one hour was found necessary with the Littrow spectrograph
and Ilford rapid process panchromatic plates. The band heads were measured
against iron arc standards and the wave number values of the band heads
reported here ate accurate to ___2 cm. -a in general while on the long wavelength side it may be a little less on account of lower dispersion.
RESULTS
The band spectrum of BiBr obtained has been reproduced in Plate V.
Figure 1 represents the spectrogram obtained with Fuess while Fig. 2 shows
that re`
with the Littrow instrument. The wavelengths of the bands,
the corresponding wave numbers in vacuum, their visually estimated relative
intensities and the assignment of the vibrational quantum numbers llave been
given in Table I. The A-system is found to extend from A 4595-6063, while
some bands could be observed even slightly beyond 6063 A. All the bands
are red-degraded and are of the progression type. This system has been
previously observed only up to 5438 ,/~. In all, about 286 bands were recorded
in emission in the present study while Morgan reported only 61 in absorpfion.
The B-system of BiBr observed by Morgan was also obtained now in emission
in the same region (see Fig. l, Plate V). In this system, the first three members
of the (0, 0) sequence are red-degraded while the sequence itself is violetdegraded. The other bands are line-like. This system is notas fully developed
as in absorption. It is worth pointing out that this B-system bears some
resemblance to the corresponding system of BiI obtained by Rao (loc. cit.)
Emission Spectrum of Bismuth Bromide (BiBr)--"A " System
173
and an analogous system was not observed by Venkateswarlu and Khanna
in BICI.
TABLE I
BiBr bands
lnt.
Observed
in A
v irl c m . -1
6062"2
6051.9
6045"2
6041.8
6031.2
16491
16519
16537
16546
16575
6029.0
6021.6
6020.2
6016.8
6009-2
6006.2
16581
16602
16606
16615
16636
16644
6001.1
5998"8
5987.9
5980.8
5978-7
5974.8
5971.5
3
16661
16665
16695
16715
16721
16732
16741
5970"1
5966.6
5964.3
5953-2
5949.6
5940-1
3
3
1
3
3
3
16745
16755
16761
16793
16803
16830
5937.8
5930.6
5915-7
5908.4
5873.6
5871.1
5867.0
5863.2
2
3
2
2
2
3
2
3
16836
16857
16899
16920
17020
17027
17039
17050
5843,7
2
17107
Assignment
v',
~Y'
4, 23 (81)
3, 22 (79)
12, 27 (79)
8, 25 (79)
12, 27 (81)
2, 21 (79)
5, 23 (79)
17, 28 (79)
13, 27 (79)
16, 28 (81)
1, 20 (79)
13, 27 (81)
4, 22 (79)
11, 26 (79)
7, 24 (81)
14, 27 (79)
6, 23 (79)
15, 27 (79)
12, 26 (79)
8, 24 (79)
10, 25 (79)
3, 21 (81)
7, 23 (81)
15, 27 (81)
12, 26 (81)
13, 26 (79)
18, 27 (79)
13,26(81)
11, 25 (79)
4, 21 (79)
l, 19 (81)
3, 20 (79)
8, 23 (79)
1, 18 (79)
4, 20 (79)
14, 25 (79)
1, 18 (81)
10, 23 (81)
8, 22 (79)
0, 17 (81)
Calculated v
16492
16519
16537
16546
16575
16577
16580
16603
16607
16616
16635
16645
16645
16643
16659
16668
16698
16718
16722
16732
16742
16741
16745
16758
16758
16792
16803
16828
16828
16835
16858
16898
16919
17018
17025
17037
17048
17047
17107
17106
Morgan's
value
174
S. SANKARANARAYANAN AND OTI-t~RS
TABLE I
(Contd.)
I
A
in A
lnt.
Observed
v in cm. -1
5840.6
5821.9
17116
17171
5816.3
5811.9
5810"2
5808.3
5798-7
17185
17201
17206
17211
17240
5793.7
5790.9
5786,3
5777"0
5767.8
5764-7
5759.3
5756.9
5748.4
5745.7
5739-7
5736.9
5725.6
5723.0
17255
17263
17277
17305
17332
17342
17358
17365
17391
17399
17417
17426
17460
17468
5716.1
5713.6
5711.4
5703.6
5701.4
5696;6
5693"7
5690"2
5681.4
17489
17497
17503
17527
17534
17549
17558
17569
17596
5673-1
5662.1
5659.8
5651.7
2
3
3
3
17622
17656
17663
17688
5646.5
5642.9
5638.9
5633-8
5631"4
1
1
3
3
2
17705
17716
17729
17745
17752
I
Assignment
t
~ tt
10, 23 (79)
18, 25 (79)
14,25(81)
7, 21 (79)
11, 23 (79)
9, 22 (79)
1, 17 (79)
9, 22 (81)
1, 17 (81)
14,24(81)
16,25(81)
3, 18 (79)
10, 22 (79)
10, 22 (81)
2, 17 (79)
18, 24 (79)
5, 19 (81)
11, 22 (79)
9, 21 (79)
11, 22 (81)
9, 21 (81)
6, 19 (79)
12, 22 (89)
17, 23 (81)
19, 23 (81)
12,22(81)
16, 23 (79)
17, 23 (79)
2, 16 (79)
19, 23 (79)
5, 18 (81)
7, 19 (79)
7, 19 (81)
1, 15 (79)
1, 15 (81)
12, 21 (79)
0, 14 (79)
12, 21 (81)
16, 22 (79)
8, 19 (81)
17, 22 (79)
2, 15 (79)
5, 17 (81)
17, 22 (81)
Calculated v
17115
17172
17173
17189
17202
17208
17210
17239
17240
17257
17265
17280
17303
17333
17343
17358
17366
17390
17398
17419
17426
17458
17468
17466
17490
17498
17501
17529
17536
17550
17556
17571
17596
17599
17624
17657
17660
17686
17689
17703
17717
17731
17747
17752
Morgan's
value
Emission Spectrum of Bismuth Bromide (BiBr)--"A " System
175
TAnLE I (Contd.)
A
in A
Int.
5627"7
5624"6
5621-9
5618-6
5614"4
5611"1
5604"8
5602"0
5593"8
5585"1
5581"4
5577.1
5570"0
5563"0
5560-1
5557"1
5549"7
5545"2
5538"0
5535-1
5530"9
5528"4
5516"6
5510-1
5503"6
5499"0
5496"6
5492"6
5490"5
5484"2
5479"4
5477-4
2
1
2
2
1
2
1
0
1
2
1
3
5471"2
5462"7
5457"0
5451"6
5440"2
5437"3
5431-5
5429"4
5427"3
5419"9
5418"2
91
3
2
2
1
1
1
1
3
3
0
3
2
3
3
Observed
v in cm. 1
17764
17774
17782
17793
17806
17816
17836
17845
17871
17899
17911
17925
17948
17970
17980
17990
18014
18028
18052
18061
18075
18083
18122
18143
18164
18180
18188
18201
18208
18229
18245
18251
18272
18300
18320
18338
18376
18386
18406
18413
18420
18445
18451
Assignment
V~ V~
7, 18 (79)
19, 22 (81)
9, 19 (79)
4, 16 (79)
9, 19 (81)
4, 16 (81)
15, 21 (79)
12, 20 (79)
15, 21 (81)
10, 19 (81)
16, 21 (81)
2, 14 (79)
2, 14 (81)
9, 18 (79)
14, 20 (79)
1, 13 (79)
1, 13 (81)
15, 20 (79)
O, 12 (79)
8, 17 (79)
O, 12 (81)
8, 17 (81)
2, 13 (79)
2, 13 (81)
9, 17 (79)
4, 14 (79)
1, 12 (79)
4, 14 (81)
1, 12 (81)
6, 15 (79)
15, 19 (81)
6, 15 (81)
O, 11 (79)
O, 11 (81)
13, 18 (79)
2, 12 (79)
2, 12 (81)
9, 16 (81)
1, 11 (79)
l, ll (81)
15, 18 (79)
12, 17 (79)
12, 17 (81)
O, 10 (79)
Calculated v
17762
17777
17780
17794
17804
17817
17838
17847
17869
17898
17910
17927
17949
17970
17977
17991
18013
18028
18054
18062
18075
18084
18124
18144
18163
18183
18188
18204
18208
18229
18246
18250
18252
18272
18300
18321
18340
18377
18387
18406
18411
18423
18445
18451
Morgan's
value
18385.6
18404.6
18450.8
176
S. S A N K A R A N A R A Y A N A N AND OTHERS
TABLE I
(Contd.)
[
~t
in A
lnt.
5412.8
5405.6
5403.9
5400.2
5397.1
5393.1
5387.6
5381.7
5
2
3
3
5
3
3
3
Observed t
vin cm. 1
18469
18494
18500
18512
18523
18537
18555
18576
18586
18594
18605
18614
18619
18639
18651
18667
18685
18691
18696
18708
18718
18723
18730
18735
18764
18769
18786
18802
18842
18852
18860
18868
18882
5378.9
5376.5
5373.4
5370.9
5369.3
5363.4
5360-1
5355.3
5350.4
5348.4
5347.0
5343-7
5340.7
5339.4
5337.5
5336.1
5327.6
5326"2
5321.5
5317-1
5305.8
5303.0
5300.7
5298"5
5294"4
5291.5
5285.1
3
4
18892
18915
5281"2
5280"2
5275.6
5267.7
5265.7
5263"2
3
1
3
5
5
2
18929
18933
18949
18978
18985
18991
ssignment
y t Vv
Calculated v
0, 10 (81)
13, 17 (79)
19, 18 (79)
13, 17 (81)
5, 3 (81)
11, 16 (79)
14, 17 (79)
14, 17 (81)
4, 12 (79)
1, 10 (79)
4, 12 (81)
1, 10 (81)
12, 16 (79)
6, 13 (79)
6, 13 (81)
0, 9 (79)
0, 9 (81)
13, 16 (79)
19, 17 (79)
17, 17 (8t)
13, 16 (81)
2, 10 (79)
19, 17 (81)
11, 15 (79)
2, I0 (81)
9, 14 (81)
14, 16 (81)
1, 9 (79)
1,9 (81)
8, 13 (79)
0, 8 (79)
8, 13 (81)
0, 8 (81)
13, 15 (79)
18, 16 (79)
17, 16 (81)
19, 16 (81)
5, 11 (81)
11, 14 (79)
2, 9 (81)
7, 12 (81)
4, 10 (79)
1, 8 (79)
15, 15 (79)
18469
18493
18499
18515
18521
18538
18553
18576
18577
18586
18595
18603
18616
18621
18639
18652
18668
18686
18692
18699
18707
18718
18723
18732
18734
18764
18769
18786
18802
18842
18853
18858
18868
18881
18882
18891
18915
18913
18928
18933
18947
18975
18987
18991
I
Morgan's
value
18469.2
I
18585.0
18603.4
18652.2
18668.1
18786.8
I 18802.2
18852.6
[ 18867.5
Emission Spectrum of Bismuth Bromide (BiBr)--"A" System
TABeE I
A
in A
Int.
19001
19016
19024
19030
5261.3
5257.2
5255.0
5253.4
5246.7
5242.8
5240.3
5235.8
5232.8
5230"9
5229"2
5226.9
5225.2
5213-7
5205.3
5204.4
5201.7
5198-6
5197-1
5191"1
5188"1
5187"2
5185"6
5179'7
Observed
vincm. 1
10
10
1
1
5
2
8
3
9
7
3
3
3
4
4
1
2
19054
19068
19077
19093
19104
19111
19118
19126
19132
19174
19205
19209
19215
19230
19236
19258
19269
19272
19278
19300
5177.6
5174"1
5170"9
5166.9
5159.6
5158"0
5157-1
5154"7
5153"1
5149"1
5147"0
5142-0
5138.7
5137.0
19308
19321
19333
19348
19375
19381
19385
19394
19400
19415
19423
19442
19454
19461
5134"0
5130.3
5127.0
19472
19486
19499
177
(Contd.)
Assignment
I~', V"
Calculated v
1, 8 (81)
19001
19017
19024
19031
19031
19055
19068
19076
19094
19102
19111
19120
19124
19132
19175
19203
19207
19216
19228
19235
19258
19269
19272
19276
19298
19302
19309
19322
19333
19350
19376
19383
19387
19392
19402
19416
19423
19442
19452
19461
19462
19471
19485
19499
6, 11 (79)
12, 14 (81)
6, 11 (81)
16, 15 (79)
0, 7 (79)
0, 7 (81)
13, 14 (79)
13, 14 (81)
18, 15 (81)
5, 10 (81)
2, 8 (79)
11, 13 (79)
2, 8 (81)
4, 9 (79)
12, 13 (79)
15, 14 (81)
6, 10 (79)
6, 10(81)
10, 12 (79)
0, 6 (79)
0, 6 (81)
18, 14 (79)
19, 14 (79)
5, 9 (79)
19, 14 (81)
5, 9 (81)
2, 7 (79)
2, 7 (81)
14, 13 (81)
4, 8 (79)
15, 13 (79)
4, 8 (81)
1, 6 (79)
1, 6 (81)
6, 9 (79)
16, 13 (79)
16, 13 (81)
3, 7 (79)
3, 7 (81)
0, 5 (79)
o, 5 (81)
13, 12(81)
5, 8 (79)
Morgan's
value
19054.6
19067.9
19257.6
19268.6
19320.9
19330-5
19461-4
19470"5
178
S. SANKARANARAYANAN AND OTHER$
TABLE I
(Contd.)
in A
Observed
v in cm.-1
Assignment
V', ~2"
Calculated v
5125.2
5121-8
5119.7
5117.6
19506
19518
19526
19534
5114"5
5101-7
5099"4
5096"0
5093.7
5091.1
19546
19595
19604
19617
19626
19636
5088"2
5086"5
5084.3
5083.2
5081"3
5078.6
5075.2
5069"8
5067"4
19647
19654
19662
19667
19674
19684
19698
19719
19728
5, 8 (81)
11, 11 (79)
2, 6 (79)
2, 6 (81)
11, 11 (81)
14, 12 (81)
1, 5 (79)
1, 5 (81)
6, 8 (79)
6, 8 (81)
16, 12 (81)
10, 10 (79)
8, 9 (79)
8, 9 (81)
3, 6 (79)
3, 6 (81)
0, 4 (79)
0, 4 (81)
13, l l (81)
5, 7 (79)
1 l, 10 (79)
11, lO (81)
2, 5 (79)
2, 5 (81)
14, 11 (81)
l, 4 (79)
l, 4 (81)
6, 7 (79)
10, 9 (79)
10, 9 (81)
3, 5 (79)
3, 5 (81)
O, 3 (81)
11,9 (79)
2, 4 (79)
2, 4 (81)
9, 8 (81)
15, 10 (79)
4, 5 (79)
12, 9 (79)
12, 9 (81)
1, 3 (79)
1, 3 (81)
16,9 (79)
19509
19520
19525
19534
19532
19545
19596
19605
19618
19626
19638
19633
19637
19647
19655
19663
19666
19674
19681
19701
19719
19730
19729
19736
19742
19801
19808
19819
19833
19842
19859
19865
19878
19920
19933
19939
19947
19978
19985
19998
20007
20006
20012
20018
Int.
5065.6
5063.2
5048"5
5046"8
5044.8
5040.7
5037.9
5033.9
5032"0
5029"4
5018.6
5016"0
5014"4
5012.3
5005.0
5002"2
4999"2
4996-9
4996" 1
4994" 8
8
4
4
2
5
2
5
3
2
5
5
2
2
1
3
2
6
7
5
6
7
3
7
19735
19744
19802
19809
19816
19833
19844
19859
19867
19877
19920
19930
19937
19945
19974
19985
19998
20006
20010
20015
Morgan's
value
19596.3
19604.6
19666.8
19674.5
19801.3
19807.8
19932.7
19937.6
20007.6
20012.6
Emission Spectrum of Bismuth Bromide (BiBr)--"A '~ System
179
TABLE I (Contd.)
A
in A
Int.
Observed
vin cm. 1
Assignment
ly', lfl'
Calculated v
6, 6 (81)
8, 7 (79)
17, 10 (79)
17, 10 (81)
19, 10 (79)
13,9 (79)
O, 2 (81)
5, 5 (79)
20028
20040
20047
20062
20067
20068
20083
20108
20112
20121
20128
20136
20139
20143
20179
20190
20199
20213
20236
20241
20243
20263
20269
20313
20323
20328
20339
20346
20380
20422
20476
20519
20545
20554
20603
20637
20660
20684
20753
20810
20848
20938
4991.1
4988.6
4987.1
4983"5
4981.9
1
2
2
2
2
20030
20040
20046
20060
20067
4977.7
4972.2
4970"5
4968.2
4966.6
4965.1
4964"0
4962.9
4954.0
4951.8
4950.0
4946.6
4940.9
4939.3
4938"5
4933"2
4931"8
4922.0
4918.9
4917.9
4916.0
4912.9
4905.9
*4894"8
4882"5
4872"8
4866-7
4864"5
4852.7
4844.1
4839"2
4833.6
4816"8
4804.3
4794.7
4774"2
3
2
2
3
4
5
20084
20106
20113
20122
20128
20134
20139
20143
20180
20189
20196
20214
20233
20240
20243
20265
20270
20311
20324
20328
20336
20348
20377
20424
20475
20516
20542
20551
20601
20637
20658
20682
20754
20809
20850
20940
3
3
3
4
5
5
5
7
6
4
4
4
6
6
2
6
7
4
7
5
7
6
5
6
I6
4
5
5, 5 (81)
11, 8
11, 8
7, 6
2, 3
(79)
(81)
(79)
(79)
2, 3 (81)
15,9
15, 9
12,8
1, 2
10, 7
10, 7
8, 6
18,9
3, 3
5, 4,
11, 7
I1, 7
7, 5
2, 2
15,8
(79)
(81)
(79)
(79)
(79)
(81)
(79)
(79)
(79)
(79)
(79)
(81)
(79)
(79)
(79)
1, 1 (81)
3, 2
5, 3
7, 4
2, 1
4, 2
6, 3
17, 7
3, 1
9, 4
4, 1
10, 4
14, 5
(79)
(79)
(79)
(79)
(79)
(79)
(81)
(79)
(79)
(79)
(79)
(79)
*The following bands were moasured on Fuoss Spectrogram.
t Maskod by Br lino 4816.7.
Morgan's
value
20079" 1
20140'0
20143"4
20268"8
20347"8
20476"7
20555"3
20600"9
20683"3
20809"9
180
S. SANKARANARAYANAN AND OTHERS
TABL~ I
Int.
in A
4757.5
4749.8
4729.9
4703.5
4702.3
4678.5
4655.3
4633.6
4612.1
4595.1
Observed
vin cm.
3
2~
6
4~
2
0
21013
21047
21136
21254
21260
21368
21474
21575
21676
21756
1
(Cont£
Assignment
Vt~ V ~
4, 0
6, 1
5, 0
6, 0
6, 0
7, 0
8, 0
9, 0
10,0
li,0
(81)
(81)
(81)
(81)
(79)
(81)
(81)
(81)
(79)
(81)
Calculated v
Morgan's
value
21015
21047
21137
21254
21260
21367
21474
21575
21676
21756
21015.9
21047.6
21135.6
21254-1
21258.O
21365.7
21473-1
21574.2
21678.1
21756.0
VIBRATIONAL ANALYS1S
As the two isotopes of bromine (Br 7~ and Br 81) occur in the ratio of
50"57: 49.43, one may expect the intensities of the corresponding bands
to be nearly the same. In fact, in BiBr two band systems have been observed
even earlier for each of the two isotopes o f bromine (Morgan). In the shorter
wavelength region of the spectrum, this isotope effect gives the bands a double
headed appearance. An anatysis by the usual procedure (Herzberg, G., 1950)
shows that all the band heads observed could be fairly well represented within
3 cm. -1 by means of the foUowing formul~e:
Bi 2~ Br 79 vL,e,a = 20532 + [135"91 (v' + 89 -- 0"534 (v' § 89 __ 0"103
(v' § 89 __ [209"50 (v" § 89 -- 0"466 (v" "-k 89
Bi 209 Br 81Vhead = 20532 § [134"69 (V' q- 89 -- 0"524 ( V ' + 89 __ 0"1002
(V' + 89 __ [207"61 (V" -t- 89 -- 0"458 (V" + 89
It is interesting to note that these formul~e are exactly identical with those
given by Morgan and no need was necessary to alter the vibrational constants
for explaining in a satisfactory manner all the observed bands. As has been
pointed out by Morgan earlier, the coefficient of the cubic term is large for
the upper level and this is necessary to explain the rapid convergence o f the
band heads. The isotope effect was observed in about 87 bands, and their
vibrational quantum numbers, observed shift and the theoretically expected
shift on the basis of the band head formul~e given earlier are listed in Table II.
The expression for BiBr 81 is related to that of BiBr 79 by means of the known
formula (Herzberg, G., 1950).
Emission Spectrum of Bismuth Bromide (BiBr)~"A" System
TABLE II
Vibrational isotope effect in BiBr bands
Assignment
'O', 'O"
12, 27
13, 27
15, 27
12, 26
13, 26
1, 18
10, 23
14, 15
9, 22
1, 17
10, 22
11, 22
9, 21
12, 22
17, 23
19, 23
7, 19
1, 15
12, 21
17, 22
9, 19
4, 16
15, 21
2, 14
1, 13
0, 12
8, 17
2, 13
4, 14
1, 12
6, 15
0,11
2, 12
1, 11
12, 17
0,10
13, 17
14, 17
4, 12
1, 10
6, 13
0,9
A2
Observed Shift
in cm.-~
38
38
4O
40
37
30
66
132
34
29
27
28
27
29
57
60
27
26
32
36
24
23
35
23
24
23
22
21
21
2O
22
21
18
2O
25
18
18
21
18
19
2O
16
Theoretical
Shift in cm.-a
38
38
4O
36
36
30
68
136
31
30
3O
29
28
30
63
60
25
25
29
35
24
23
31
22
22
21
22
20
21
20
21
20
19
19
22
18
22
23
18
17
18
16
181
182
S. SANKARANARAYANAN AND OTHER$
TABLE II
Assignment
V'~ V '~
13, 16
19, 17
2, 10
1,9
8,13
0, 8
1,8
6, 11
0,7
13, 14
2, 8
6, 10
0,6
19, 14
5,9
2, 7
4, 8
1,6
16, 13
3,7
0, 5
5,8
11, 11
2, 6
1,5
6,8
8,9
3,6
0, 4
I1, 10
2, 5
1,4
10,9
3, 5
2,4
12, 9
1,3
17, 10
5, 5
11,8
2,3
15, 9
10, 7
11,7
6,0
(Contd.)
Observed Shift
in cm.-1
Theoretical
Shift in cm.-x
23
32
17
16
18
16
16
14
14
16
14
15
11
22
8
12
10
6
19
7
11
7
16
8
9
9
11
8
7
9
7
7
11
8
7
8
4
14
7
6
4
9
7
4
6
21
31
16
16
16
15
14
14
13
18
12
12
11
26
11
11
I1
10
19
9
9
10
12
9
9
8
I0
8
8
11
7
7
9
6
6
9
6
15
4
7
4
11
5
5
6
Emission Spectrum of Bismuth Bromide (BiBr)--"A" System
183
G (v) = pw e (v -~ 89 -- p2w e Xe (v @ 89 + p3eoe Ye (v -+- 89
where p2 is the ratio of the reduced rnasses of Bi~~ ~9 and Bi2~ si. But,
some of the bands attributed to Bie~ 79 may easily well be due to Bi2~ si
of different v' and v" values and both possibilities have been indicated in
Table I. This view is further supported by the fact that the intensities of
such bands are slightly higher than the isotopic counterpart of each of them
separ~.tely. The intensity distribution observed in emission is in agreement
with what is to be expected from the absorption spectrum reported by
Morgan and the most intense bands lŸ on a wide open Condon parabola.
Thus the close agreement of the observed and calculated isotopic displacements on the one hand and the coincidence of the absorption bands found
by Morgan for BiBr with the corresponding emission bands on the other
convince us that the band system observed now is the A---~X system of BiBr.
Also the fact that the B--->X system, though feebly developed, occurs in the
same region ~s recorded by Morgan in absorption shows that the present
system can be safely attributed to the BiBr molecule.
DISSOCIATION ENERGY AND DISSOCIATION PRCDUCTS
A spectroscopic determinafion of the heat of dissociation of a molecule
obviously requires a precise knowledge of the energy of the dissociation limit
above the ground state as weU as the products of dissociatior~ at this limit.
Often the systems do not converge rapidly enough for an accurate determination of the dissociation limit and extrapolation methods have to be resorted
to. Fortunately, the convergence of the excited levels of the A-system of
BiBr is sufficiently rapid and Morgan calculated from band head formula
the l i m i t a s about 4520 A. In view of the larger number of bands observed
in emission now, the dissociation energy for the upper state was evaluated
more accurately by plotting a graph between /kG and v and the irnproved
value came out as 0.19 ev. Ir we add to this, the electronic excitaª
energycorresponding to 20495 cm.-I (voo), viz., 2.54 e.v. we find the dissociaª
energy of the upper state D'e to be 2.74 ev. (including zero-point energy)
and the corresponding convergence l i m i t a t ,~4513-6 is close to the value
estimated by Morgan. On the other hartd, the dissociation energy of the
excited state of the B-system of BiBr (Morgan, loc. cit.) when corrected by
using the current conversion factor (1 ev. = 8068.3 cm-a ; Herzberg, 1950)
comes out to be 4.12 ev. Since the convergence is rapid here also this value
can be taken to be close to the true value. Thus the difference between the
dissociation energies of the excited states of the A and B systems is 1.38 ev.
Since both these systems have the same lower state, namely the ground state
184
S. S A N K A R A N A R A Y A N A N A N D OTHERS
X, this difference in energy most probably corresponds to the difference in
the excitation energies of the dissociation products of both these states. In
Table III the probable products and the corresponding energies referred to
the ground levels of Bi and Br atoms have been given. A scrutiny suggests
that in the excited state A, BiBr possibly dissociates into the normal atoms,
viz., Bi (4Sz/2) and Br (~P3/2) and in the state B with Bi in the excited state
TABLE I I I
Atomic excitation
Component atoms and levels
Energy in ev.*
Bi (2D~/2~ + Br (~Pa,2~
..
.
1" 4
Bi (4Sz/2~ q- Br (~Pv2~
..
.i
0.45
Bi (4S3/z ~ + Br (~Pa/~ ~
..
0
* Values calculated from data on Atomic Energy States, Bacher, R. F. and Goudsmit, •.,
McGraw Hill Book Co., Inc., 1932.
Thus the situation appears to be very similar to what has been observed in
the case of the upper state of the well-known visible iodine absorption bands
and analogous bands of bromine. The low value of the dissociation energy
in the upper state of the A--+X system, namely 0.19 ev., indicates that the
influence of the electric field along the axis on the electron orbitals is not large
enough to break the coupling between L and S of the component atoms and
it appears therefore quite likely that the appropriate coupling is of Hund's
case (c) type. This is supported also by a calculation of the ratio of the
re values in the two states using the relation due to Birge and Mecke, Le.,
r2e coe = constant. From the known oae' and oae" values one may expect the
re' value to be about 25~ more than the ground state. In BiCI where Hund's
case (c) coupling has been established (K_hanna, loc. cit.), the observed increase
in the upper state is only about 7~. The fact that the (0, 0) band is not excited
in the A-system of BiBr, both in emission and in absorption and that the
bands themselves ate aU red-degraded, show that the re value in the upper
state is much larger than in the ground state and that the coupling is in all
probability of Hund's case (c) type. In the light of the Franck-Condon
principle, the observation of the extension of the band system in emission
only can be understood.
Emission Spectrum of Bismath Bromide (BiBr)--"A" System
185
Another interesting feature to which attention may be drawn is the faet
that the linear extrapolation method yields a value of 2.91 ev. as the dissociation energy o f the lower state, which is also the ground state of the molecule.
It is true that such an extrapolatio• is justified only for non-poplar molecules
and may also lead to wrong values in the case o f the excited states of molecules
containing an atom with high electron affinity such a s a halogen (Gaydon,
A. G., 1947). But in the ground state, the linear extrapolation may probably
give a value 25% larger than the true value (Herzberg, G.) and so the value of
2.91 ev. should be regarded as an upper limit. The lower limit naturally
may be taken to be the energy of the highest vibrational level observed in that
state; in this case it is 0.68 ev. (v" : 28). Ir, however, the value of 2.91 ev,
turns out to be close to the true dissociation energy of the ground state, then
one is forced to conclude that the excess for the state X over A, in view of
the energy difference of 0.45 ev. between the 2P1/2 and 2P3/2 levels of bromine,
may be due to the molecule dissociating in the lower state into Bi (4S3/2) and
Br (2P1/2) atoms.
ELECTRONIC TRANSITION INVOLVED
In considering the molecular states that can arise from a combination
of the atoms, it is often adequate to consider the outermost electrons of the
constituent atoms. The ground state electronie configuration of the two
atoms in the present case are
Bi (KLMNOspa) 6s 2 6p 3 (~$3/2)
Br (KLM) 4s 2 4p 5 (~P3/2)
where Ospa represents the O shell in which only the s, p, and d sub-shells are
filled. The electronic terms that can be derived from the ~S and the ~P states
are 3 27-, aH, 5 l - and 5/-/. But to decide as to which of these are low lying,
one may, as has been done by Khanna, B. N. (loc. cit.), consider the levels
of the molecule NF. N and F belong to the same groups in the periodic
tableas Bi and Br respectively and so their electronic terms will be similar.
In the 'united a t o m ' model, sulphur with its 16 electrons corresponds to
the " u n i t e d atom " for the NF molecule and the states derivable from a
sulphur atom in its low lying states ~P2,1, o are 327-and 3/7. Hence
these two states may be expected to lie lowest for BiBr molecule and we may
therefore leave out of further consideration the higher multiplicity terms.
By analogy with the molecule BiCI and in view of the much larger re' value
in the excited state, here also the reasonable coupling appears to be of Hund's
case (c). The appropriate O values for Bi (4S3/2) § Br (2P3/2) are therefore
= 3, 2 (2), 1 (3) and 0 (4), where the numbers in parenthesis indicate the
186
S. SANKARANARAYANANAND OTHERS
number of states of the type given. For .Q = 0 the different states are 0+,
0 +, 0-, 0-. On the other hand, the higher dissociation energy for the lower
state would seem to indicate in that state either Hund's case (a) or case (b)
or any intermediate coupling. A correlation of the states by standard methods
(Herzberg, loc. cito shows that 0 + state corresponds to a l - state. Similarly
the atoms Bi (483/2) and Br (2p) can give rise to an upper 3/-/(0 +) state. In
molecular orbital notation, the transition involved in the A--~ X system
under study may therefore be represented by
(zo) 2 (ya) 2 (xa) ~ (orrr)' (vzr) (uo) : 8H (0+)--+(za) 2 (ya) e (xo) 2 (o~~')' (wr)e: 81where the symbols have their usual significance (Mulliken, R.S., 1932;
Coulson, C.A., 1959). vzr represents an antibonding orbital of the type
(6p~'~3,- 4p'rBr) and ua an antibonding orbital of the type (6paBl--4apBl).
The reasons for not considering other possible states and allowed transitions
have been discussed in detail by B. N. Khanna (loc. eit.) for BiCI and wiU
not be given here as those arguments can equally well be used for BiBr. The
bond order is seen to be 2 in both states of BiBr (A-system) and so they may
be expected to have dissociation energies of the same order of magnitude.
But our knowledge of the ordering of the energies of molecular orbitals arising
from high quantum atomic orbitals is still meagre; if one assumes that ua
is above wr as in the molecular orbitals arising from atomic 2 quantum orbitals
then there is some rough theoretical justification for the linear extrapolated
value of higher dissociation energy for the, ground state of BiBr. This simple
argument will then fail to account for the converse observation in BiC1.
However, both facts can be reconciled ir we suppose, as suggested earlier,
that in BiBr in the ground state dissociation takes place with Br in the higher
multiplet level 2P1/2. Since in the correlation of the atomic and the molecular
states, multiplet splittings are ignored, i t i s
not possible to establish
unambiguously if that is so. A more precise experimental determination
of the dissociation energy in the ground state of BiBr wiU therefore be of
interest.
Ir is with great pleasure that we acknowledge and thank Professor
R. S. Krishnan for his kind interest in our work and valuable support and
encouragement he gave throughout the investigation.
SUMMARY
The emission spectrum of bismuth monobromide has been investigated
and a vibrational analysis of the A--+X system has been made. About 286
bands were recorded in the region ~A 4595-6063 and the isotope cffect due to
Br ~9 and Br 8~ was observed in about 87 bands. A value of 2.74 ev. for the
Emission Spectrum of Bismuth Bromide (BiBr)--"A" System
187
dissociation energy of the excited state has been obtained artd arguments
have been given to show that the dissociation products in the excited state
are Bi (4S8/2)and Br (~P3/z) and that those of the grourtd state are most probably
Bi(4S8/2) and Br(~Pt/2) atoms.
REFERENCES
Vatence, Oxford at the C b i e n d o n PIe~s, 1959.
(2oulson, C. A.
Gaydon, A. G.
o .
Dissociation Energies and Spectta of Diatomic Molecules,
Chapman and Hall Ltd., London, 1947.
Molecular Spectra and Molecular Structure (1 Spectra of Diatornic
Molecules), D. van Nostrand Co., Inc., N.Y., 1950.
Unir. Durham. Phil. Soc. Proc., 1934, 9, 126.
t-/orzberg, G.
Howell, II. (3. and Rochester,
G,D.
How~ll, H . G .
..
Joshi, K. (2.
..
Ptoc. Roy. Soc. Lond., 1936, 155A, 141.
Proc. Phy~. Soc. Lond., 1961, 78, 610.
Khanna, B . N .
..
Morgan, F.
..
Jou•n. MoL Specttoscopy, 1961, 6, 319.
Phys. Rey., 1936, 49, 47.
Mulliken, R . S .
..
Rey. Mod. Phys., 1932, 4, 1.
lnd. 3ourn. Phys., 1962, 36, 85,
Ptasada Rao, T. A. and Rao,
P.T.
Rao, P . T .
..
lbid., 1949, :23, 379.
Ray, S . K .
..
lbid., 1942, 16, 35.
Proc. Phys. Soe. Lond., 1961, 78, 614.
Rochoster, G . D .
..
fiaper, P . G .
..
Sur, P . K .
..
and Majumdar, K.
Vcnkatoswadu, P. and Khanna,
B.N.
..
PhyJ. Rey., 1931, 37, 1710.
Proe. Natl. lnst. Sci. lnd., 1950, 19, 16.
lbid., 1954, 20, 235.
Proe. lnd. Aead. Sr
1960, 51, 14.
FIG. 2
FIG. 1
(b)
B Su
(a)
o.o
4,115
0,14
2,16 9 , 2 1 I O , 2 2
0,7 0 , 8
FxG. 2. Ej Glass Littrow Spectrogram
F]G. 1. (a, b) Fuess Spectrogram
=J -',lq.
Z,3 1,4
2,3
(BiBr emission bands)
o,a
13, 2 7
SYSTEM
fSTEM
iYSTEM
r
P