The laser-induced fluorescence of IBr79

JOURNALOF

MOLECULAR SPECTROSCOPY 61,395~402 (1976)

The

Laser-Induced ELLIOT

I)epartmentof Chemislry,

Four lluorescence

Fluorescence

IBr”

M. WEINSTOCK

lT&evsiiy (If Massaclrltsetls/Boston, I)orchester, Massadwsetts 0212.i

progressions

of

of the BO+-X2+

IIarbor

hand of IBrTg excited

Cam@s,

hy a single-mode

dye-laser have been observed and analyzed. Discrepancies in the reported rotational of the Q+ state have been resolved and improved vibrational constants determined. method of discriminating between IBr and interfering IS fluorescence is described.

cw

constants .4 simple

INTRODUCTION

The potentials of diatomic interhalogens have recently been the subject of a comprehensive theoretical study (1). The RKR method, often used to generate potential curves under investigation, requires accurate vibrational and rotational constants. The determination of these constants for the ground state of a molecule utilizing high-resolution absorption spectroscopy can be severely inhibited by insufficient population of all but the lowest vibrational levels. In the case of IBr79, the three most recent publications have reported absorption by the first seven vibrational levels of the X12+ state (Z-4). With the knowledge that IBr has a dissociation energy of 14 660 cm-’ (5), the ground state of IBr must contain at least 100 bound vibrational levels, only a small fraction of which have been observed. Obviously, another spectroscopic technique must be utilized to realize a more complete vibrational analysis of the ground state of IBr. The spectroscopic study of the IBr molecule, as well as that of any mixed intcrhalogen, presents an added difficulty. Since IBr in the vapor phase is in equilibrium with 12 and Brz (6) the spectroscopist is faced with the problem of differentiating IBr absorption lines from IZ and Brz lines. Nevertheless, early absorption studies were carried out by Badger and Yost (7) in 1931 and Brown (5) in 1932 on the “IIo++-- Ix+ infrared system and by Cordes (8) in 1932 on the infrared system and the O+ +- ‘Z+ visible system. However, except for some lower-resolution emission studies (Y-11) and a magnetic rotation spectroscopy study in 1959 by Eberhardt et al. (la), no further spectroscopic investigations had been reported until those of Selin and co-workers in 1962. The analyses of three separate band systems of IBr resulted in B,,” values for the vibrational levels of the *Z+ state which agree very well with one another. However, values for R,, exe,and ye tabulated by Selin (2) were apparently determined graphicall) and could not be reproduced by this author. Consequently, a weighted least-squares fit was made to R,” compiled from all of Selin’s absorption data (Z-4) and generated the rotational constants (in cm-l) B, = 0.05678, (Y,.= 0.000190 and y,. = -9.5X1O-7. 395 Copyright0 All rights

1976

by Academic

of reproduction

in any

Press, form

Inc. reserved

ELLIOT

396

M. WEINSTOCK

To complicate matters further, a measurement and analysis of the J = 4 + 5 and J = 5 + 6 microwave transitions for the ‘u = 0, 1, and 2 vibrational states of IBr7g by Jaseja (13) produced values (in cm-l) of R, = 0.056116 and (Y, = 0.000350, in substantial disagreement with those extracted from the electronic absorption studies. Considering the usually reliable nature of microwave measurements on diatomic molecules (14) an independent measurement is suggested. Within the last 8 yr, laser-induced fluorescence has been utilized for the spectroscopic investigation of diatomic molecules (1.5). In these studies, one or more coincidences between a molecular absorption line and an ion laser line is taken advantage of to generate fluorescence data. The recent development of tunable cw dye-lasers has greatly expanded the number of small molecules whose structure can be spectroscopically probed. The IBr molecule is particularly suitable for the fluorescence approach using a singlemode dye-laser. Such a laser facilitates location of the strongest IBr absorption lines which are not overlapped by 1~ or Brz absorption lines. EXPERIMENTAL

PROCEDURE

IBr, purchased from Alfa Inorganics, was distilled into a previously evacuated pyres cell without further purification. A Spectra Physics 580 dye-laser system was used to produce about 70 milliwatts of single-mode radiation with a linewidth of less than 20 MHz. The dye-laser was frequency stabilized by a feedback loop built around a P.A.R. JB.5 lock-in amplifier. In this way the laser frequency could generally be maintained for periods of 1 to 3 hr with only occasional adjustment of the etalon voltage control. The following procedure was used to locate IBr fluorescence. The laser was initially tuned to a wavelength approximately corresponding to an absorption line of the IBr 0+ +-- lx+ band. The monochrometer was adjusted to a wavelength corresponding to an energy 268 cm-’ (%w, for the 59 state of IBr) below the laser energy and the etalon voltage was gradually changed until IBr fluorescence could be detected. Using the etalon voltage control the laser could be tuned over a 2.5 A range. An 1~ cell was positioned in the fluorescence path immediately after the IBr cell and made it possible to distinguish between IBr fluorescence and interfering 12 fluorescence. This proved necessary since collision-induced rotational quantum jumps in 12 often spread interfering 12 fluorescence into the region of anticipated IBr fluorescence. In addition, the 1~ cell displayed a fluorescence color pattern as a function of etalon voltage. Identifying such a pattern proved useful for the relocation of the specific IBr absorption of interest at the time. All fluorescence and detected electrically

lines were scanned using a l-m Czerny-Turner

photoelectrically cooled to -20°C.

with an extended Each absorption

SPEX monochrometer

S-20 photomultiplier line was identified

tube

thermo-

by measuring

the

wavelength of the stabilized laser line which produced the IBr fluorescence, converting it to a wavenumber in vacua, and comparing it to Selin’s absorption data (4). This procedure was sufficient to identify each absorption line used in this study. Nevertheless, since the fluorescence manifests itself as P-R doublets, the region near the laser line was scanned so that the absorption could be identified as a P type or R type.

FLUORESCENCE

OF

IBP

3Yi

The first two absorption lines were chosen because of a strong fluorescence signal and happened to belong to the IBr7g molecule. The final two absorption lines were specific+. selected to complete the analysis. A similar study of the IBrel molecule is planned. Each recorded scan of either the laser line or the fluorescent lines contained at least one, generally two, and sometimes three inert gas emission lines for calibration. The wavelength of each P-R doublet was measured at least three times with the results averaged. ANALYSIS

Table I lists the IBr7g fluorescent series used in the analysis. of a P-R doublet, A3Fv(J”) is customarily expressed as AZFY(J”)

=

R(J”

-

1) - P(J”

+ 1) = (4I3, - 6D,)(J”

The observed

+ 3) - SD,(J”

splitting

+ 4)“.

(1)

Since the fluorescence data were not judged accurate enough to determine directly a value of D,, a value D, = 4Be3/we2 ‘v 1.0 X lo-* cm-’ was used throughout the analysis. This value agrees rather well with those of Selin et al. (Z-4). For so small a I),. value, Eq. (1) becomes (the primes have been omitted for conciseness)

A weighted least squares was used to determine the rotational constants of the ground state according to Eq. (2). The weights were calculated using the variances taken from the average of the measured line positions. The data for all transitions to the zl” = 1 and v” = 2 levels were taken from the visible absorption data (3) and were weighted according to their quoted accuracy. To carry out the vibrational analysis, the energy of a fluorescent line, Fr, is expressed as fif = E’ - ,q = E’ - CG, + B,J(J

+ 1) - DvJ2(J + I>“].

(4

E’ is the energy of the exited-state level formed in the absorption process, E is the vibrational term value, is energy of the absorbing ground state. G,., the ground-state espressed by the well-known espansion G, = w,(el + 3) - w,~w,.(z’ +

+J2+ wyc(z + 3)“.

(4)

ELLIOT

398

Reference

M. WEINSTOCK

we

Be

13

0.056116

2

0.056788

1.99

*-4(a)

0.05678

1.90

-0.95

This vor!~(~)

0.05682

1.87

-1.7

0.05678

1.90 io.05

This work(')

*9.00002

0.7

Determined

from

a weighted least

(b)

Determined

directly

from fit

(c)

Determined

by

for

searching

268.71

-0.95 to.25

(a)

Eq. (4).

w,yexlo3

WeX,

3.50

t

0.83

268.640 0.015

0.8140 *0.0015

squares fit to the

By

-1.77 to.07

values

therein.

to P-R doublet.separations. rotational

constants

whichgave

a best fit to

Al1 uncertainties are believed equivalent to two standard deviations

(90% confidence limits).

Table III.

The IBr79 Fluorescence Praqression (20, 33 + v". 32-34)

G”(“bS)

” Y”

Line

0

H(2) R(34)

1 2

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 IF 19

-1 (cm )

-

P'741.43 18734.77 17575.53 17567.95 17310.34 17302.79 17046.76 17039.46 11785.33 16771.71 16524.77 16517.28 16266.40 16258.87 16009.76 16001.93 ... .. . 15500.90 15493.68 ... ... ... .. . 14751.03 14743.78 . ... . .. ... 14016.88 ::rjr;;:;: 13768.77 ... . l?P??.li; 13292.01 13063.73 13056.61

(cm-l1 134.05 134.11 liOl.14 401.15 666,54 666.55 930.12 930.11 1i91.96 1192.09 1452.73 1452.76 1711.31 1711.41 1968.17 1968.58 ... ... 2477.46 2477.33 . ... ...

. .

3228.00 3227.98 ... . ... 3962.85 3962.96 4204.13 4204.04 ... ... 46Rl.20 4681.35 4916.9( 4917.02

G"(calc) -1 (cm ) 134.12 401.13 666.51 930.23 1192.31 1452.71 1711.43 19GP.47 2223.80 2477.42 2729.33

nv (chs) G, (talc) -0.07 -0.01 0.01 0.02 0.03 0.04 -0.11 -0.12 -0.35 -0.22 0.02 0.05 -0.12 -0.02 -0.30 0.11 ... . 0.04 -0.09 . ... ...

2979.50 3227.92 3474.60 3719.51 39G2.65 4204.00 4443.56 4031.31 4917.?4

rJ.')$ 0.06 . . . . 0.20 0.31 0.13 0.04 ... ... -0.03 0.04 -0.28 -0.72

399

FLUORESCENCE OF IBP

Y"

Line

0

N33) P(35)

I. 2

3 4 5 6 7 a 9 10 11 18 13 14 15 16 17 18 19

For each fluorescence

" -1 (cm I

G"(ObS) -1 (cm )

17840.75 17832.83 17573.62 17565.85 17308.47 17300.70 17045.18 17037.31 16783.17 16775.40 16523.33 16515.68 16264.28 16256.59 16007.73 15999.96 ...

133.81 133.91 401.15 401.13 666.52 666.52 930.02 930.15 1192.26 1192.31 1452.32 1452.28 1711.60

. ..

15499.15 15491.64 15247.50 15240.01 ...

.. .

14749.43 14741.91 14503.01 14495.44

.. . 14015.20 14007.73 13774.09 13766.04

...

13297.54 13290.17 13062.16 13054.78

:C::$ 1968.51 ... ... 2477.41 2477.35 2729.30 2729.24 1.. .*. 3227.85 3227.88 3474.51 3474.65 ... ... 3962.81 3962.89 4204.17 4204.06 ... ... 4681.24 4681.30 4g16.88 4916.99

Gv(calc) (Clll

-11

134.12 401.13 666.51 930.23 1192.31 1452.71 1711.43 1968.47 2223.80 2477.42 2729.33 2979.50 3227.93 3474.60 3719.51 3962.65 4204.00 4443.56 4681.31 4917.24

GV(obs)GJdC) -0.31 -0.21 0.02 0.00 0.01 0.01 -0.21 -0.08 -0.05 0.00 -0.39 -0.43 0.17 0.19 -0.10 0.04 ... ... -0.01 0.07 -0.03 -0.09 ... ... -0.06 0.05 -0.09 0.05 ... ... 0.16 0.24 0.17 0.06 ... .. . -0.07 -0.01 -0.36 -0.25

series, an initial value of E’ is derived using the laser line energy

(El) : E’ = El + E = EI + G,I + BJI(JE

+ 1) - D.,J?(Jl + 1)“.

(5)

Using Eqs. (3) and (S), and the results of the rotational analysis, G, values can be obtained. The vibrational constants are then determined from a weighted least-squares fit to Eq. (4). Equation (5) produces only a preliminary value for E’ because the G,z can only be written in terms of the vibrational constants provided by Selin (2). Consequently, the least-squares program was allowed to converge to a final G,I value with a convergence.limit of 0.0001 cm-‘. When the rotational constants obtained from the fit to Eq. (2) were used to convert the Ef to G, using Eq. (3), a systematic trend appeared in the data. That is, for any vibrational level, the Gr can either increase or decrease with increasing J unless the rotational constants which express the vibration-rotation interaction are correct. By allowing CY.and ye to vary, the trend was minimized and a best fit to Eq. (4) was obtained. The variances of B, and (Yeare determined from 40 Monte Carlo simulations to the rotational data. The variance of -ye is estimated from observation of the fit of the data to Eq. (4). These variances are needed so that all the data in the vibrational analysis can be properly weighted.

400

ELLIOT

M. WEINSTOCK

Table v. TheI&79 Fluorescence

C”(ObS)

v Y” 0

1 2 3 4 5 6 7

IArE

N57) W59)

(cm-l) 17820.69 17807.35 17554.34 17541.11 17289.61 17276.43 17026.57 17017.47 16765.16 16752.16 16505.41 16492.45 16247.16 16231.30 15991.07 15978.03

8 9 10 11 12 13 14 15 16 17 18

Progression

\clU-l) 134.17 134.26 401.15 401.15 666.51 666.53 930.19 030.17 1192.25 1192.10 1452.65 1452.60 1711.37 :;tZ: 1968.45 ... . .. . .

. .. . .. . .. .. . . 14734.97 14722.38 14488.96 14476.42 ...

... . ... 3227.90 3227.81 3474.63 3474.54

..

13285.86 13273.48

58 + Y". 57-59) G”(dC) (cm

-1

GJObS)J

134.12 401.13 666.51 930.23 1192.31 1452.71 1711.43 1968.47 2223.80 2477.42 2729.33 2979.50 3227.93 3474.60 3719.51

.

14002.27 13989.80 13761.84 13749.33 ...

(21.

3962.77 3962.72 4203.95 4203.98 ... ... 4681.43 4681.44

3962.65 4204.00 4443.56 46Rl.31

G”hlC) 0.05 0.14 0.02 0.w 0.00 0.02 -0.01 -0.06 -cl.06 -0.13 -3.OF -3.11 -0.06 0.02 -3.14 0.02 ... . .. . ... . . ... ... -0.03 -0.12 0.03 -0.06 ... 0.12 0.07 -3.05 -0.02 ... . 0.12 0.13

It turned out that the set of rotational constants which gave a best fit to Eq. (4) agreed extremely well with those determined from Selin’s data. It should be noted that an attempt to fit to Eq. (4) by varying (Yeand keeping ye = 0 was made but the systematic trend in the data could not be removed in this fashion. Table II lists the rotational and vibrational constants from previous studies as well as from this work. The uncertainties of the vibrational constants determined from this study are estimated from the fit to the data and to the variation of vibrational constants with variation of the rotational constants. The uncertainties of the rotational constants are estimated from the fit to Selin’s B, values and from the variation of the standard deviation of the fit to Eq. (4) with values of cr and y. Tables III-VI contain the observed fluorescent lines, the G, value computed using the “best” rotational constants, the fitted G, values, and the difference between the measured G, and the calculated G,. DISCUSSION

Using the technique of laser-induced fluorescence the rotational constants of Selin have with some modification been verified. In addition, improved vibrational constants have been determined. More accurate wavelength measurements of the fluorescent series, using photographic plates and a Fabry-Perot etalon for calibration purposes, could certainly be used to improve the accuracy of the data and, in turn, of the constants. However, it would

FLUORESCENCE

G”bbS) -1 (cm )

Y

Y”

Line

0

;t::,’

1 2

3 4 5 6 7

a 9 10 11 12 13 14

(Cm-‘1 17801.90 17785.16 17535.78 17519.19 17271.43 17254.90 17008.58 16992.02 16747.65 16731.15 16488.31 16472.05 16230.58 16214.37 16974.69 16958.43 ... .. .. 15468.02 15451.95 ... . . . .. .. . 14720.60 14704.69 14475.20 14459.46

.. .

13989.42 13973.78 13749.24 13733.50 ...

15 16 17

. ..

13274.41 13258.81

18

401

OF IBPs

G,(dC) (cm

-1

)

133.95

134.12

t:;.:: 401:15 666.51 666.51

401.13

;;::4’4 1192.33 1192.42 1452.72 1452.63 1711.50 1711.43 1960.46 1968.49 ... .. . 2477.30 2477.26 . .. .. . ... .,. 3228.07 3228.06 3474.61 3474.49 .. . .. . 3962.71 3962,61 4204.06 4204.14 .. . ... 4681.28 4681.35

666.51 930.23 1192.31 1452.71 1711.43 1968.47 2223.80 2477.42 2729.33 2979.50 3227. h 3474.60 3719.51 3962.65 4204.00 4443.56 4681.31

GJobs)G"(calc) _-0.17 0.02 0.01 0.02 0.00 0.00 0.14 0.23 0.02 0.11 0.01 -0.08 0.07 0.00 -0.01 0.02 . .. . .. -0.12 -0.16 .. . . .. . .. .. . 0.14 0.13 0.01 -0.11 .. . ... 0.06 -0.04 0.06 0.14 . .. ... -0.03 0.04

appear that fluorescence from an electronic state with more favorable Franck-Condon factors will be required before more of the ground-state potential curve can be explored. Toward this end utilization of an apparent coincidence of the 1849.6 A atomic line of mercury (16) with a transition within the C-X band system of IBr is under investigation.

ACKNOWLEDGMENTS The author’s thanks go to L. M. Schwartz for his aid in the data analysis, to J. D. Caddick for his help with the initial experimental setup and laser operation, and to J. C. Baird for the use of his lock-in amplifier. This work was supported by the Research Corporation under a Cottrell Research Grant. RECEIVED:

February

9, 1976 REFERENCES

1. M. S. CHILD AND R. B. BERNSTEIN, J. Chem. Phys. 59, 5916 (1973). 2. L.-E. &&IN, Ark. Fys. 21, 479 (1962). 3. L.-E. SELIN AND B. S~~DERBORG,Ark. Fys. 21,515 (1962). 4. L.-E. SELIN, Ark. Fys. 21, 529 (1962). 5. W. G. BROWN, Phys. Rev. 42,355 (1932). 6. M. BODEN~TEIN AND A. SCHMIDT, Z. Phys. Chem. 123,28 (1926). 7. R. M. BADCER AND D. M. YOST, Phys. Rev. 37, 1548 (1931). 8. H. Z. CORDES,Z. Phys. 74, 34 (1932). 9. R. K. ASUNDI AM) P. VENKATESWARLU, Indian J. Phys. 21, 78 (1947). 10. P. B. V. HARANATH AND P. T. RAO, Indian J. Phys. 31, 368 (1957).

402

ELLIOT

M. WEINSTOCK

II. P. VENKATESWARLU AND R. D. VERMA, Proc. Indian Acad. Sci. A 47, 150, 161 (1958). 12. W. H. EBERHARDT,W.-C. CHENG,ANDH. RENNER, J. Mol. Spectrosc. 3, 664 (1959). 13. T. S. JASEJA,J. Mol. Spectrosc. 5,445 (1960). 14. See B. ROSEN, “Selected Constants, Spectroscopic Data Relative to Diatomic Molecules,” Pergamon Press, New York, 1970, pp. 221-222, for Jaseja’s rotational constants.

1.5. See, for example, W. J. TANGO, J. K. LINK, AND R. N. ZARE, J. Chew Phys. 49, 4268 (1968) ; R. VELASCO,CH. OTTINGER,ANDR. N. ZARE, J. Chem. Phys. 51,5522(1969); J. I. STEINFELD,J. D. CAMPBELL,AND N. A. WEISS, J. Mol Spectrosc. 29, 204 (1969); G. ENNEN AND CH. OTTINGER, Cite-m.Phys. Lett. 36, 16 (1975). 16. F. W. LOOMISAND A. J. ALLEN, Phys. Rev. 33, 639 (1929).