Thermal Emission Spectrum of C<Sup>2</Sup>Π-X<Sup>2</Sup>Σ Transition of BaCl Molecule

Indian J . P hys. 67B (1_), 47-5 2 ₍1993₎

Thermal emission spectrum of C^Tl — transition of BaCl molecule

M M Joshi, R Gopal and K N Uttam

Saha's Spectroscopy Laboratory, Deparunent of Physics, Allahabad University, AUahabad*211002, India

Received 4 August 1992, accepted 1 September 1992

Abstract : Thermal emission spectrum of BaQ molecule has been recorded by using Saha’s high lemperatuie vacuum graphite furnace in the visible spectral region XX 5410— 5022 A. The spectrum has been photographed at a reciprocal Imear dispersion of 3.5 A/mm about a temperature 2200°C. About 61 new band heads have been recorded and analysed into =■ 0, ± 1 and ~2 sequences of C^fl—X^E system. The vibrational constants are determined.

K eywords : Thennal emission spectrum, BaCl molecule, vibrational analysis.

PACS Nos, : 33.20. Kf. 33.10 Gx

1. In tro d u c tio n

The spectra of alkaline earth monohalidcs have been the subject of interest for a number of spectroscopists. Recently in the case of BaCl molecule a long searched metastable A state has been reported by Martin and Royen [¹]. Therefore, more information about BaCl is desirable.

The band spectrum of BaCl molecule has been investigated by several earlier workers [2, 3J. The spectrum was known to consist of several systems of bands corresponding to transitions from the ground state to various excited states designated as A. B, C. D, E, F and G in order of increasing energy. Ryzlewicz et al [4] derived vibrational constants for the ground slate from the microwave spectroscopy. Recently Gustavsson et al [5] and Gustafsson et al [⁶, 71 recorded high resolution spectra of B— X and G— X systems and reported the molecular constants.

Parker [2] was the first to report vibrational analyses of the C^FI—X^Z transition of BaCl molecule. He recorded large number of band heads in the green spectral region AA5047—5322 A and classified the observed four heads Q\, Q², and R² in Av = ⁰,

± 1 sequences of C—X system. Pages et al [⁸], on the basis of rotational study of LIF, suggested that assignments to the heads made by Parker 12] were incorrect. They also reported an emission spectrum of BaCl molecule consisting of small number of bands from the hollow cathode lamp and suggested that band heads could be determined more accurately

© 1993 lACS

48 M M Joshi, R Gopal and K N Lfltam

and p)reciscly from the emission spectrograms than the absorption spectrograms obtained with a King’s furnace.

It IS evident from the above liicraiiirc that information regarding the visible spectrum of BaCI C ~ X system is inadequate. On the other hand the study of therml emission specu-um of magnesium monochloridc by the authors [9J have revealed the existence of a number of new bands assigned to the system. Therefore, it has been thought fruitful to study the spectra of BaCl molecule using thermal exciUition technique.

2. Experim ental details

A small quantity of pure barium chloride (B.D.H) was kept inside the experimental tube of Saha’s high temperature vacuum graphite furnance [¹()| The furance chiimber, after evacuation, was filled with pure argon gas at a pressure about 50 cm of rnercury. The spectra have been photographed on C.Z, two meter Ebert Plane Grating Spectrograph (P.G.S-²) with a grating blazcxJ at A5600 A and total lines ruled 45600 at a rccipirocal linear dispersion of about 3.5 A/mm. A temperature of the order of 2 2 0 0 * ^ 0 was founif sufficient to record the spectra on ORWO 400 ASA black and white film with the exposure time of 4 minutes in second order. Iron d.c. arc served as source of comparison standard. The measurements were performed on Carl Zeiss Abbe comparator with a least count of 0.(X)01 cm.

3. R e su lts ^

Thermal emission spectrum of BaCl molecule, shown in Figure 1, has yielded a large number of bands in the region AX5410 —5022 A. The spectrum is well develoixid and free from atomic lines. About 224 bands have been recorded and analysed into the two subsystems : ( V ^²—^ - bands ob.scrved, 61 band heads have been reported first time while rest include the bands reported by Parker [2J and later Pages et al [⁸|. The following arc the vibrational analysis proposed by authors :

.¹./ Cl — X subsystem :

A total of 123 band heads have been assigned to the Av = 0, ±1 and -2 sequences ol Cl r j^\/7 —X Xsubsystem. The sequence Av = ~2 has been photographed and analysed here for the first time. The subsystem is well developed and as high as v = 42 vibrational quanta has been identified. The bands in Av = 0, ^{± 1} itre close to each other which might be due to the lact that the vibrational frequencies inupperand lower electronic slates arc almost the same.

3.2. C'²— X subsystem :

The remaining 101 band heads have been attributed to the Av = (), ±l sequences of the

—X^X subsystem. In this case, the band separation is larger than those in Ci— X which IS contrary to the Parker’s [2] suggestion.

Thermal emission spectrum o f ( f transition etc 49

0 c1

1

... ... .. 1I— Q .

c) 0 ¹0 D.

0 __________ _____________________________________________________ Q

0 j---

Figure 1. Thermal Itmission Spcciunn of C^;r— system of BaCl molccula ai a high lemperaiure with the reciprocal lincan dispersion of 3 5 A/mm (reprcsenls absorption speamm)

The following vibralional conslanls are determined by the authors from the detailed vibrational analyses;

For subsystem

U.„ =

19064.1 2 i 19074.3 /?, For C ^ n— subsystem

Unn =

19469.9 ^{/ ? 2} 19454.4 Qi

ft)', = 281.25 ft)", = 279.89

ft)', = 284.3 ft)', = 279.89

ft)', X, = 0.765 cm' - 1

ft)", x"t = 0.802 cm '.

ft)', x', = 0.765 cm ' ft)', x't = 0.802 cm '.

Pages ei al [⁸] have assigned the (2i, ^ i. Q² ^nd R² heads as /fi²t ^ u . ^{^ 2 1 ^ 2 2} respectively but have not given any justification.

50 M M Joshi, R Gopal and K N Vttam

The additional band head data together with their intensities and classification have been collected in Table 1 for the C—X system.

Table 1. Bind 4iead data of C—X system of BaC molecule.

(cm'*) heads 18509.9 18514.8 18519.9 18525.0 18529.9 18535.0 18540.2 18545.0 18550.5 18555.7 18560.5 18565.6 18571.2 18576.3 18581.5 18587.0

Int.

1 2 3 3 5 6 7 7 8 8 8 8 8

6

7 6

(V’. V”)

(0^.2⁾ (1.3) (2.4) (3.5) (4.6) (5.7) (6, 8) (7.9)

(8.10)

(9,11) (10.12) (11.13) (12.14) (13.15) (14.16) (15.17)

(v\ V")

Ry heads

(⁰.²)^

(1. 3)*

(2. 4)*

(3. 5)*

(4. 6)*

(5.7)^

(⁶. ⁸)*

(7. 9)*

(⁸.^{1 0})*

(9.11)*

(^{1 0}. ^{1 2})*

(11.13)*

(12. 14)*

(cm‘*)

heads 18597.8 18603.3 18611.6 18616.8 18622.0 18630.7 18636.6 18643.9 18648.8 19123.6 19126.1 19127.6 19132.0 19134 9 19139.7 19143.0

Int.

6 5 3 2 1 1 1

1

1 2 2 1 1

I I 1

(V.v”)

(17.19) (18.20) (19.21) (

20

22

(21.23) (22.24) (23.25) (24.26) (25.27) (25.25) (26, 26) (27.27) (28.28) (29,29) (30. 30) (31,32)

(V.v”)

R\ heads

(21.23) * (22.24) t (23.25) ^ (24.26) * (22. 22)*

(23,23)*

(25, 25)*

* Bands arc not well resolved.

Table 1. (Conid)

Im. (V , V") Ini (v. V )

(cm'*)

Ry heads

(cm *)

/?2 heads

18585.7 1 (13.15) 19343.0 1 (22. 23)

18590.8 1 (14. 16) 19349.5 1 (23. 24)

18596.4 1 (15.17) 19809.0 1 (17. 16)

18600.9 2 (16. 18) 19813.0 1 (18. 17)

18608.8 5 (17, 19) 19816.8 1 (19.18)

18614.4 5 (18.20) 19820 8 1 (20,19)

18619.9 3 (19.21) 19824 7 1 (21,20)

18625.5 1 (20.22) 19830.6 1 (22,21)

191195 2 (21.21) 19834.2 1 (23. 24)

19129.1 1 (24,24) Unclassiiied bands

19136.7 1 (26.26) 19614.3 1

19138.6 1 (27.27) 19620.5 3

19141.5 1 (28,28) 19626.6 6

^ 2 heads 19632.4 6

19811.0 1 (21,20) 19632.4 6

19814.9 1 (22,21) 19639 0 6

19818.1 1 (23, 22) 19644.8 5

19823.2 1 (24.23) 19650 6 2

19829.1 1 (25.24) 196.56.0 1

19832.6 1 (26.25) 19661.8 1

4 . D iscussion )

Thermal emission spectrum o f transition etc 51

Barium monochloride emits strong radiation in green spectral region. A glance on the spectrogram, given in Figure ¹, reveals the presence of absorption due to the high temperature while the spectrogram given in the Figure ² has been recorded at a low temperature and represents the emission spectrogram. This conforms that the band system do arise from the ground state. Further since the observed bands are four headed,the system arises from the transition which is in agreement with the earlier workers [²].

--- (

10 0 0 '

1fi--- . . _ _ _ _ _ a 0 --- 1

Figure 2. Thermal Hmission Speciumi ol C^n— system of BaCl molccula at a reciprocal linear dispersion of 3.5 A / mm

Thermal emission spectrum of BaCl,molecule has yielded a number of new bands . All these bands have been satisfactorily explained by the proposed analyses. The new vibrational constants are in close agreement with those which were proposed by Parker [2], Harrington [²] and Pages et al L⁸J. From the nature oflhc evaluated constants, it is clear that a narrow condon parabola is expected which is in agreement with the observed spectra.

52 M M Joshi, R Gopal and K N Ultam

This jusiifics our analyses. The ground suite constants have been taken the same as suggested by Ryslcwicz el al [4J and used by Pages et al |81 andGustavssoner al (5J.

Since dispersion is not enough, the heads of the ob.scrvcd subsystem are not clearly resolved for a number oJ bands. Eight bands whose existence were doubtful as reported by Parker |2) do not appear in our spectrograms. However, a group of 9 bands lying at A5095

A

Acknowledgments

The authors are grateful to Prof S K Kor, Head, Department of Physics, University, of Allahabad, Allahabad for his keen iiucrcsl in this work. R.G. and K.N.U. arc also thanl^ful to University Grants Commission and Council ol Scientific and Industrial Research, New

Delhi, respectively, for the financial support. \

kcfc'rcnccs

HI H Marlin ami P Roycn 1983 Chem Phyy Lell 97 127

(2J A li Parker, R h JlamngicMi 1979 Molecular sfjfi^ctra arui Molecular Structure p 64 (ConsUiils ol Diaionuc Molecules cd hy K P Wuber and (i llc r /k r g (New York . 1) Van Nosiraiid)

13| R 1' Hanow and A D Counl 1953 r r o c Roy Soc A219 120

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|7| G (iuslalsson, II Martin, P Royen and M /ackrission 1990 7 M(A Spectrosc 139 163

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