Two emission systems of BS

JI)(:KI\'ALOFYOLECULAK SPECTROSCOPY

Two

63,521-526 (1976)

Emission

S. BELL

Systems

of BS

AND MEGAN M. L. MCLEAN

Two new systems of emission bands near 2100 and 31008 have ljeen produced by a micro\vave discharge in B& vapor. From the known XV and A%, states of BS, these systems have been assigned I:“+:

as E*S+ -+ X2x+ and I<%+ -+ .A%,. Constants

7‘0 = 47 929.3,

B, = 0.671

(1.c = 1.752 b),

in crn~’ for the new state are

01, = 0.008,

AG; = i81.0.

INTRODVCTION

In the previous work on BS (I, Z), electronic transitions among the states SW, _1211i, B*Z+, PII, and a “A; state, which for convenience we will label D, were photographed under high resolution. Thorough analyses have been made of the A-X system (5180-9035 A) (I), C-X system (241&2920 A) (I), B-A system (5000 A) (Z), and D-A system (3140 A) (Z). In the work reported here, we have observed a new emission system from 2050 to 2320 A, which is adjacent to the much stronger C-X system, and two bands at 3099 and 3131 A, which are close to the D-A system and of almost the same intensity. EXPERIMENTAL

DETAILS

In order to produce the BS emission spectrum, discharges were run in Pyrex tubes with an Evenson cavity using a microwave generator at 2450 MHz. The new bands were produced by BCl, with sulphur, and BC13 with SC& in flowing systems, but were produced with the fewest other molecular emissions using B& (Ventron Corporation) and argon in both static and flowing systems. In the region around 3000 A the B-S system of SZ (3) was observed and several fruitless attempts were made to reduce it b!. adjusting the ratio of boron to sulphur entering the discharge, but the D-A system and the new bands adjacent to it were never obtained without overlapping emission. Photographs of the emission spectrum were taken with a McPherson 3-m vacuum spectrograph on Ilford Q2 and N30 plates at a dispersion of 2.77 &mm. With a slit xvidth of 25 pm, the new bands between 2050 and 2320 A were obtained with esposures of 2 to 8 min, while the bands at 3100 A required 1 to 3 min, but the much stronger bands of the C-X system required only a few seconds. The bands were measured with reference to germanium lines (4) generated by a microwave discharge in GeC14 vapor at 2 Torr. Attempts to photograph the shorter wavelength bands at high resolution were unsuccessful because of low intensity. No attempt was made to photograph the bands at 3100 A at high resolution because of the overlapping &“emission. 521 Copyrigllt 0 1976 by Academic Press.Inc. Allrights of reproduction in any form reserved.

522

BELL AND MCLEAN

Wavenumbers

of

Bend

Heads

TABLE

I

oi

ElZ+ - X2X+

the

system Of

Bs

v v’ 1

0

0

1

3

2

4

4677'1.1

45630.6

44497.0

47933.6

46766.4

45612.0

44470.1

43340.7

llg92s

47927.4

46n8.0

45522.0

44341.0

43171.9

10$2$

48114.6

47547.5

46393.5

(45254.1)

44121.5

llg32s

llg34s

48735.8

10BX5

AN*kLYSIS

2100-A

System

Nine red-degraded bands of llB% have been observed in the system and a few heads of l”B3*S and 11B34Swere also recognized. From the isotope effect and the known groundstate vibrational constants, the lower state is X22+ and two v” progressions are assigned with ZJ’= 0 and 1. The wavenumbers of the bandheads of all the assigned bands are given in Table I. The (0, 3) band for the three isotopic species is shown in Fig. 1. In 1.4

0.3 llg3"s

t1B32S

444iO.l

Bo

“B32S

44142.2

E-A’I-T,,~

32265.8 FIG. 1. Emission bands of BS: (a) 0, 3 band of the E-S D-A and E-A systems.

cm-’

E-A’n,

31933.7

D’n-A’n

cm-’

system showing three isotopic species; (b) the

523

BS EMISSION TABLE

Constsnts

from Bma

Heeds

E-X Our

Fit

of

the

II

E2e+

!A;

-

X and A2ni

-

X Systems

- x

A’J!

B

-1

Our Fit

zeeman

Our Fit

7.eemen

ye

48124.2

16207.78

16208.68

15875.13

15875.W

"

47933.6

15995.4

15996.34

15662.7

15662.73

00

we !1 uXef* we WXe'

1179.7

1180.31

1180.17

6.33

6.31

6.25

6.31

801.0

754.78

753.35

754.07

753.35

10.0

4.83

4.62

4.72

4.62

6.29

1179.7'7

1180.17

a search for the (2, 0) band at longer exposures, the only bands observed in the region were the (0, 3) and (1, 4) bands of the C-X system of BO (3) but no single-headed bands were observed. It is concluded that the P’ = 2 level is predissociated, and since it is not observed, we and wx, for the excited state are difficult to derive. A computer program that utilizes a nonlinear least-squares fitting procedure has been written to fit the wavenumbers of observed bands of the spectra of two isotopic species, in the case llBseS and ‘“BYi, and the vibrational constants of the two states are calculated, to w?;~if desired, and also an experimental value of the isotopic ratio, p, if desired. Unfortunately, the isotope shifts are not known accurately enough to determine ws, precisely, and throughout the range wx,’ = 5 to 15 cm-’ the standard deviation of the fit is 0.33 crnl, which is of the same order as the experimental error. The derived constants are given in Table II. The ground-state constants appear to differ significantly from those given by Zeeman and they will be discussed later. Unlike the A-X and C-X systems of BS, the bands of the 2100-A system are not in pairs and hence the excited state is probably %+. The rotational structure confirms this since the bands are single-headed and show only two branches. The excited state will be labeled EW for convenience. There is little sign of spin doubling and the bands are easily analyzed since B, and a, of the ground state are known. From graphical determinations of AB for a few bands, we have obtained Bo’ = 0.667 and BI’ = 0.659 cm-‘, and thus B, = 0.671 and CY~= 0.008 cm-‘. Hence for the ES+ state, we derive the bond length re = 1.752 f 0.002 A and the energy To = 47 929.3 f 0.4 cm-‘. 3100-i

System

To short wavelengths of the PA;-A211i band, which shows three heads with reddegraded structure, there are two bands with double heads which degrade to the violet (see Fig. 1). The bandhead measurements are

and the assignment

li?C+-A*lI$,

31 916.4, 31 933.7 cm-i;

E’B+-A21L 1,

32 249.3, 32 265.8 cm-‘;

IS justified as follows. The difference between the “Q” heads is 332.1

524

BELL AND MCLEAN

cm-r, which agrees with the spin-orbit splitting of the A state as obtained by McDonald and Innes. This assignment is confirmed and the excited state is shown to be the E%+ state by taking the differences between the electronic origins of the E-X and A-X systems. However, since one of the components did not quite agree within experimental error, it was necessary to refit the vibrational structure of the A-X transition using the “Q” heads measured by Zeeman. The II+ and IIt components were fitted separately and a comparison of the fits is made in Table II. It should be noticed that v,,(~) for the II+ component disagrees by 1 cm-’ from Zeeman’s fit although vow for the IIs component is in agreement, and hence ~~~~~~~~~~~~~ is 332.7 cm-’ in better agreement with the value of the spin-orbit constant used by McDonald and Innes. Since the standard deviation of our fit is 0.13 cm-‘, the vibrational constants of the excited state differ significantly from those derived by Zeeman. With the corrections to the origins of the components of the A-X system, the wavenumbers of the three electronic transitions, A-X, E-X, and E-A, agree within experimental error, but this may be fortuitous due to cancellation of the origin-to-head separations of the (0, 0) bands of the A-X and E-A transitions. Each of the two components of the 3100-A system shows a double head and degrades to the violet as expected, since B,,” = 0.620 cm-l for the A state (1) and B,,’ = 0.667 cm-’ for the Estate as already derived. A rotational analysis was not attempted because of overlapping Sz emission and insufficient resolution, but it is noticed that the branches on the short wavelength component @+-II+) appear to cut off at 32 340.8 cm-1 and on the long wavelength component (Y-~III) at 31 954.6 cm-l, which may be due to predissociation. However, there is no sign if a cut-off in rotational structure in the (0, 0) or (1, 0) bands of the E-X system.

DISCUSSION

From the values of the rotational constant, the E*Z+ state is not one of the states involved in the %-JII; transition at 5100 A observed by McDonald and Innes. Since no emission is observed from the ZJ’= 2 level of the E state, it is probably predissociated and hence an upper limit to the dissociation energy of the ground state would be 49 500 cm-‘. If the possible cut-off of the rotational structure of the 3100-A bands is caused by predissociation, then the dissociation energy of the ground state would be nearer to 48 500 cm-‘. The breaking-off of emission at 40 000 cm-’ in the B-A system observed by McDonald and Innes was shown to be caused by perturbation rather than predissociation. Our estimation of the dissociation energy at 49 000 f500 cm-l is in good agreement with the mass spectrometric determination (6) of 48 200 f 770 cm-l. To consider the products of dissociation, combination of boron ‘P, and sulphur 3P, gives rise to the following doublet states (5) : ?Z:+,22z-, 2%, and 2A. The X%+ and A211 states can correlate to B2P and S3P atoms but the BQY state cannot and, from the extrapolated value of its dissociation energy, the C*II state cannot either. The D*A and E22+ states cannot correlate with B2P and S3P atoms, since these states lie so close to the probable dissociation energy of the ground state. The D and E states probably do not both belong to the electronic configuration (ZUP)~(xa) (UT) since they are separated by only 280 cm-’ (in fact, spin-orbit splitting causes the 2As component to lie only 104 cnl-’ from &YE+) which is much too small for the usual & interaction (7, 8). Also,

RS EMISSION

states of the same configuration usually have similar structure, but the bond lengths of the D and E states are quite different. Before making a summary of the known states of BS, some corrections to electronic energies are necessary. Firstly, the origin of the A-X system has not been observed but by fitting observed bandheads, the components have been calculated to have origins at 15 662.7 and 15 995.4 cm-l. A further correction is required to find the electronic energ! since the separations of the Q heads and the origins are 0.5 to 0.7 cm-’ for the (4, 0) and (5,O) bands analyzed by ‘Zeeman, and he employed corrections of 0.37 and 0.46 cm-‘. Hence the best value of To for the A state is 1.5 828.6 cm-‘. The B and D states have only been observed in emission to the A state and their electronic energies must be calculated from that of the A state. The values given by McDonald and Innes have been incorrectly calculated from ~0(I) of the A-.Y transition and To for the B and D states should be 36 017.5 and 47 649.2 cm-‘, respectively, as listed in Table III. Finally, a number of transitions among the states summarized in Table III could usefully be studied. The origin of the B-X system lies at 2i76 A but, as this is under the (2,4) band of the C-X system, it has not been possible to recognize it. The origin of the C*II,-A211i transition should lie around 4350 A, which should not be overiapped by other systems. The E-B transition would provide further information concerning the new E state, and perhaps further information on the B state: The origin should occur at 8400 A, which lies between bands of the A-X system. RECEIVED: Maj- 25, 1976 REFERENCES 1. I’. 1%ZEEMAX, 2. J. K.

MCDONALD

c‘amd. AND

J. Phys. 29, 336 (1951).

K. K.

INNES,J. Mol.

Speclrosc. 29, Z.il (196Y).

526 3. 4. 5. 6. 7. 8. 9.

B. V. G. 0. P. S. B.

BELL ROSEN (Ed.),

“Spectroscopic

Data

AND

MCLEAN

Relative

to Diatomic Molecules,” Pergamon, Oxford, 52, 1223 (1962). HERZBERG, “Spectra of Diatomic Molecules,” Van Nostrand, Princeton, 1950. M. UY AND J. DROWART, High Temp. Sci. 2, 293 (1970). R. SCOTT, J. RAFTERY, AND W. G. RICHARDS,J. Pltys. B 6, 881 (1973). BELL, T. L. NG, AND C. SUGGITT,J. Mol. Spectrosc. 44, 267 (1972). N. TAYLOR, W. H. PARKER, AND D. N. LANGENBERG,Rev. Mod. Pkys. 41,375 (1969). KAUFMAN AND K. L. ANDREW, J. Opt. Sot. Amer.

1970