Journal of Molecular Spectroscopy 298 (2014) 24–30

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Journal of Molecular Spectroscopy

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He-, Ne- and Kr-broadening coefficients of lines in the fundamentalm3 band of 12C32S2 at low temperature

http://dx.doi.org/10.1016/j.jms.2014.01.0080022-2852/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Fax: +32 81724585.E-mail address: muriel.lepere@unamur.be (M. Lepère).

1 Permanent Address: Département de Physique, Université de Kinshasa, People’sRepublic of Congo.

Pardaillan Kongolo Tshikala 1, Jean-Claude Populaire, Ghislain Blanquet, Muriel Lepère ⇑Laboratoire Lasers et Spectroscopies, Research Centre in Physics of Matter and Radiation (PMR), University of Namur, 61, Rue de Bruxelles, B-5000 Namur, Belgium

a r t i c l e i n f o

Article history:Received 26 November 2013In revised form 9 January 2014Available online 15 February 2014

Keywords:Diode-laserCollisional broadeningCarbon disulfideRare gases

a b s t r a c t

Using a tunable diode-laser spectrometer coupled with a low temperature cell, He-, Ne- and Kr-broaden-ing coefficients of 8 lines in the fundamental m3 band of CS2 have been measured at five low tempera-tures: 173 K, 198 K, 223 K, 248 K and 273 K. These lines with J values ranging from 2 to 78 are locatedin the spectral range 1514–1544 cm�1, four lines in the P branch and four in the R branch. The collisionalhalf-widths have been obtained by adjusting each experimental profile with a Voigt, Rautian and Galatrylineshape model. For each line, the collisional broadening coefficients obtained at low temperature andthose previously at room temperature have been used to deduce the n-parameter of the temperaturedependence. We have also determined the average value of the temperature dependence of collisionalbroadening coefficients for lines in the m3 band of CS2 diluted in helium, neon and krypton.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

The determination of collisional broadening coefficients of car-bon disulfide diluted in a rare gas has been realized for the firsttime by Misago et al. [1]. They have measured the Ar-broadeningcoefficients for 16 spectral lines in the fundamental m3 band ofCS2 from room temperature to low temperature and have deter-mined the temperature dependence n parameters of the collisionalbroadening coefficients. Recently, we have determined the colli-sional broadening coefficients of 28 lines of the same band of CS2

diluted in helium, neon and krypton at room temperature [2].This work is devoted to the study of collisional broadening coef-

ficients of carbon disulfide perturbed by rare gases in the sameband but at low temperatures. For that, we have used a high reso-lution tunable diode-laser spectrometer [3,4] to measure He-, Ne-and Kr-broadening coefficients for 8 lines of the m3 band of CS2. Torecord these spectra at 5 low temperatures: 173 K, 198 K, 223 K,248 K and 273 K, we have used a home-made cooling cell [5]. Theselines with J values ranging from 2 to 78 are located in the spectralrange 1514–1544 cm�1 with four lines for P branch and four for Rbranch.

The collisional half-widths have been obtained by adjusting theobserved profile with a Voigt [6,7], Rautian and Sobel’man [8] and

Galatry [9] models. From these results, we have deduced the colli-sional broadening coefficient. These values and those determinedpreviously at room temperature [2] have been used to deducethe temperature dependence n parameter of the collisional broad-ening coefficients. Finally we have determined the average value ofthe n parameters of lines in the m3 band of CS2 in the presence ofhelium, neon and krypton.

2. Experimental procedure

The spectra have been recorded at high resolution with animproved tunable diode-laser spectrometer (TDL) made by LaserAnalytics of the type LS3 [3,4]. This spectrometer has a resolutionof about 5 � 10�4 cm�1 and is interfaced with a homemade averag-er for driving the records and for data acquisition. The spectra havebeen the average of 100 scans in order to increase the signal-to-noise ratio (better than 1000). The measurements at low tempera-tures have been performed using a homemade low temperaturecell [5] with an optical path length 0.4043 m. This cell can becooled down uniformly at any temperature from room tempera-ture to 77 K by a cold gaseous nitrogen flow circulating in a coppercoil surrounding it. To obtain a good temperature stabilization(±0.5 K), a current electronically monitored by a temperature con-troller circulates in copper wires surrounding the inner cell whichis so warmed by Joule effect. Fig. 1 illustrates this low temperaturecell and the cooling system. The pressure in the cell has been mea-sured using two MKS Baratron capacitance gauges with full scalereadings respectively 1.2 and 120 mbar with an accuracy of

Fig. 1. Schematic view of the low temperature cell and cooling system: (1) absorption cell, (2) vacuum flask, (3) primary pomp, (4) cold gaseous nitrogen inlet, (5) coldgaseous nitrogen flow controller, (6) nitrogen flow, (7) copper wire, (8) temperature regulator, (9) digital thermometer, (10) absorption cell window in ZnSe, (11) vacuumflask window in BaF2, (12) gas inlets, (13) pump system, (14) pressure gauges and (15) Dewar liquid nitrogen.

P. Kongolo Tshikala et al. / Journal of Molecular Spectroscopy 298 (2014) 24–30 25

0.05%. The gas temperature is measured with a digital thermome-ter connected to a platinum resistance sensor fixed in the innercell.

The sample of carbon disulfide has been taken from a bottle ofspectrophotometer-grade solvent and has been purified by trap totrap distillation with liquid N2 cooled traps. The samples of helium,neon and krypton have been provided by L’Air Liquide with astated purity of 99.999%, 99.999% and 99.998%, respectively.

The wavenumbers of the studied lines were comprised between1514 cm�1 and 1544 cm�1. We have measured eight lines of CS2 inthe m3 band: R(2), R(4), R(6), R(46), P(22), P(36), P(62) and P(78)and a five temperatures: 173 K, 198 K, 223 K, 248 K and 273 K.The relative wavenumbers calibration has been obtained by intro-ducing in the laser beam a confocal étalon with a free spectralrange of 0.008323 cm�1.

For each gas mixture (CS2 + He, CS2 + Ne and CS2 + Kr) ninespectra have been recorded: a spectrum with the empty cell whichgives the diode-laser emission profile (100% transmission level), a

Fig. 2. Example of spectra recorded for the P(22) line in the m3 band of CS2 at 273 K:(1) diode-laser emission profile; (2) Doppler line recorded at low absorption andpressure of CS2 (p = 0.0016 mbar) to determine the apparatus function; (3) confocalétalon fringes for relative calibration; (4) 0% transmission level; (5–9) record of theCS2 line at different pressures of neon.

saturated spectrum which gives the 0% transmission level, a Dopp-ler spectrum with pure CS2 at very low pressure <0.002 mbar (thisspectrum has been used to determine the apparatus function), 5spectra of CS2 broadened to 5 pressures of the perturber and the

Fig. 3. Example of theoretical Voigt fit (j) on the experimental (–) profile a(m) forthe P(22) line of CS2 (0.0154 mbar) diluted in 20.51 mbar of Ne at 173 K. For clarity,only one calculated value out of five is plotted there. Below, we give the observedminus calculated residuals (O–C) (–), with an intensity scale expanded by 10 for thefitted Voigt, Rautian and Galatry profiles.

Fig. 4. Pressure dependence of the collisional half-width cc for the P(78) line of CS2

broadened at 5 pressures of krypton at 223 K, derived from the fits of Voigt (j),Rautian (d) and Galatry (N) profile. The self-broadening contribution has beensystematically subtracted for each point. The slope of the best-fit line for eachprofile represents c0 the collisional broadening coefficient of this line.

Table 1He-broadening coefficients c0 in the fundamental m3 band of CS2.

Line m0 (cm�1) c0 (10�3cm�1atm�1)

Voigt

P(78) 1514.07390a 89.5 ± 3.698.5 ± 3.2

103.3 ± 4.0114.5 ± 4.6123.9 ± 4.0

P(62) 1519.1401 89.6 ± 3.994.6 ± 2.9

103.4 ± 3.0113.5 ± 5.4122.8 ± 6.4

P(36) 1526.6033 92.6 ± 3.3100.4 ± 3.8107.3 ± 5.5112.1 ± 4.2123.1 ± 5.5

P(22) 1530.2259 91.1 ± 3.397.3 ± 3.5

107.5 ± 2.9114.1 ± 4.4122.5 ± 8.2

R(2) 1536.0024 103.9 ± 6.1109.8 ± 8.9118.5 ± 4.5124.8 ± 5.5138.5 ± 7.2

R(4) 1536.4261 97.1 ± 3.299.9 ± 4.5

107.0 ± 4.2129.5 ± 3.7

R(6) 1536.8440 93.5 ± 4.7101.1 ± 5.1105.7 ± 5.6117.2 ± 4.8127.4 ± 6.0

R(46) 1544.0012 94.1 ± 4.098.5 ± 4.9

106.7 ± 4.7114.9 ± 4.2124.6 ± 5.5

a Assignments and positions of the lines are taken from Ref. [14].

26 P. Kongolo Tshikala et al. / Journal of Molecular Spectroscopy 298 (2014) 24–30

spectrum of the interference fringes of the confocal étalon givingthe relative calibration. These fringes have allowed to correct theweak nonlinear tuning of the diode-laser with a constant step ofthe spectra and to check the laser mode quality. Fig. 2 gives anexample of the spectra recorded for P(22) line of CS2 diluted inneon for (20.51, 30.38, 38.25, 45.52 and 53.68 mbar).

3. Data reduction

The Beer–Lamber’s law allows to define the absorbance a(m) atwavenumber m of a homogeneous gas sample as:

aðmÞ ¼ � lnItðmÞI0ðmÞ

� �ð1Þ

where It(m) is the proportional transmitted radiation intensity ob-tained with the cell filled the gas sample; I0(m) is the incident radi-ation intensity obtained with the cell under vacuum; a(m) definedthe absorption coefficient at wavenumbers m and l is the opticalpathlength.

To determine the collisional broadening half-width at half-max-imum (HWHM) cc (in cm�1 atm�1) of each of the five broadenedexperimental profile, we have fitted on the experimental profile

Temperature (K)

Rautian Galatry

90.4 ± 3.0 90.9 ± 3.4 27399.1 ± 3.6 99.1 ± 3.6 248

105.1 ± 3.9 105.1 ± 3.8 223115.6 ± 4.1 115.6 ± 4.0 198124.5 ± 3.3 124.6 ± 3.2 173

90.2 ± 3.7 90.3 ± 3.8 27395.0 ± 3.1 95.0 ± 3.1 248

103.7 ± 4.1 103.8 ± 3.0 223113.9 ± 5.4 113.9 ± 5.4 198123.3 ± 6.6 123.4 ± 6.5 173

92.9 ± 2.7 93.0 ± 2.5 273100.8 ± 3.5 100.9 ± 3.5 248107.5 ± 5.5 107.5 ± 5.5 223112.5 ± 4.2 112.5 ± 4.2 198121.2 ± 5.7 123.0 ± 6.0 173

91.8 ± 2.9 91.8 ± 2.8 27399.3 ± 3.9 99.4 ± 3.9 248

108.3 ± 3.6 108.3 ± 3.3 223114.5 ± 4.5 114.5 ± 4.5 198122.8 ± 8.6 120.9 ± 10.3 173

113.0 ± 7.6 113.3 ± 7.2 273119.8 ± 8.3 119.3 ± 6.8 248126.8 ± 5.1 126.8 ± 5.0 223135.2 ± 5.8 135.3 ± 5.8 198145.3 ± 7.0 145.4 ± 6.9 173

97.8 ± 2.4 97.9 ± 2.4 273100.3 ± 4.4 100.3 ± 4.5 248108.0 ± 3.7 108.1 ± 3.7 223129.6 ± 3.7 129.6 ± 3.7 173

93.7 ± 5.2 93.8 ± 5.2 273100.0 ± 5.2 101.5 ± 4.9 248104.9 ± 6.4 106.2 ± 5.9 223117.7 ± 4.6 116.4 ± 3.6 198126.1 ± 5.6 126.2 ± 5.6 173

98.4 ± 3.4 98.4 ± 3.4 27399.7 ± 4.9 99.7 ± 4.9 248

109.7 ± 3.7 109.8 ± 3.7 223116.9 ± 4.4 117.0 ± 4.4 198125.2 ± 5.4 125.2 ± 5.4 173

P. Kongolo Tshikala et al. / Journal of Molecular Spectroscopy 298 (2014) 24–30 27

the Voigt [6,7] lineshape, but also the models developed by Rautianand Sobel’man [8] and by Galatry [9] which take into account thecollisional narrowing due to the molecular confinement (Dicke ef-fect [10]). To fit an experimental profile, we must take into accountthe instrumental distortion which has been determined through aneffective Doppler half-width cD deduced from a fit of a Gaussianfunction on the observed profile of the spectrum of pure CS2 at verylow pressure [11,12].

The theoretical Voigt profile [6,7] is a model which gives theabsorption coefficient as a convolution between a Gauss function(Doppler effect) with a Lorentz function.

The Voigt profile is usually described by:

aV ðx; y;AÞ ¼ Ayp

Z þ1

�1

expð�t2Þy2 þ ðx� tÞ2

dt ð2Þ

with three parameters given by:

A ¼ Sffiffiffiffiffiffiffiffiln 2p

cD

ffiffiffiffipp ; y ¼

ffiffiffiffiffiffiffiffiln 2p cc

cD; x ¼

ffiffiffiffiffiffiffiffiln 2p m� m0

cDð3Þ

By fitting the Voigt profile, we determine these parameters (A, xand y) which allow to deduce respectively the line intensity S (3a),the collisional half-width cc (3b), the line center position m0 (3c);cD is the Doppler half-width.

Table 2Ne-broadening coefficients c0 in the fundamental m3 band of CS2.

Line m0 (cm�1) c0 (10�3cm�1atm�1)

Voigt

P(78) 1514.0739 64.0 ± 2.168.3 ± 2.171.9 ± 2.979.2 ± 2.1

P(62) 1519.1401 67.5 ± 2.271.5 ± 2.576.5 ± 2.782.2 ± 2.690.3 ± 2.7

P(36) 1526.6033 73.2 ± 2.678.2 ± 2.484.3 ± 2.390.6 ± 2.398.9 ± 3.2

P(22) 1530.2259 77.3 ± 2.079.4 ± 2.686.1 ± 3.196.1 ± 3.5

102.4 ± 3.8

R(2) 1536.0024 89.1 ± 3.192.4 ± 4.0

100.9 ± 4.7109.0 ± 4.2122.7 ± 14.3

R(4) 1536.4261 80.7 ± 2.788.9 ± 3.193.6 ± 2.5

104.2 ± 3.3113.5 ± 2.8

R(6) 1536.8440 81.7 ± 2.286.3 ± 2.992.9 ± 2.3

100.6 ± 2.7111.6 ± 3.7

R(46) 1544.0012 71.2 ± 2.877.0 ± 2.381.8 ± 2.187.4 ± 2.197.7 ± 3.8

The Rautian profile [8] is given by:

aRðx; y; z;AÞ ¼ AReWðx; yþ zÞ

1�ffiffiffiffipp

zWðx; yþ zÞ

� �ð4Þ

with

Wðx; yÞ ¼ ip

Z þ1

�1

expð�t2Þðxþ iy� tÞdt ð5Þ

where x, y and A are the parameters defined in Eq. (3) and where z isthe narrowing parameter? This additional parameter is describingthe Dicke effect and is given by:

z ¼ffiffiffiffiffiffiffiffiln 2p bc

cDð6Þ

with bc the coefficient of collisional narrowing and cD is the Dopplerhalf-width.

We have also used Galatry profile [9] which is given by:

aGðx; y; z;AÞ ¼Affiffiffiffipp Re

Z þ1

�1expð�ixt � yt � zt � 1þ e�zt

2z2

� �ð7Þ

with the same definition of the parameters.Fig. 3 shows the comparison of the theoretical Voigt profile with

the experimental one for the P(22) line of CS2 diluted in neon at173 K and gives also the residuals observed minus calculated

Temperature (K)

Rautian Galatry

64.8 ± 1.7 64.9 ± 1.8 27369.6 ± 1.8 69.7 ± 1.9 24873.8 ± 2.6 73.9 ± 2.7 22381.2 ± 3.2 81.3 ± 3.5 198

68.6 ± 3.2 68.6 ± 3.3 27373.4 ± 2.5 73.5 ± 2.6 24877.5 ± 2.0 77.5 ± 2.1 22383.0 ± 2.7 83.0 ± 2.7 19890.8 ± 2.7 90.8 ± 2.7 173

74.1 ± 2.4 74.2 ± 2.4 27379.8 ± 2.4 79.9 ± 2.5 24884.7 ± 2.7 84.8 ± 2.6 22390.3 ± 2.4 90.6 ± 2.3 19899.2 ± 3.4 99.2 ± 3.4 173

78.4 ± 2.7 78.3 ± 2.9 27383.0 ± 4.2 83.2 ± 4.4 24887.8 ± 2.3 87.1 ± 4.3 22397.4 ± 2.9 97.5 ± 2.9 198

102.8 ± 4.5 102.8 ± 4.6 173

96.9 ± 4.2 97.3 ± 4.2 27397.4 ± 3.3 97.9 ± 3.0 248

107.3 ± 5.5 107.5 ± 5.4 223115.7 ± 4.6 115.8 ± 4.5 198129.0 ± 4.5 129.0 ± 4.5 173

80.7 ± 3.4 81.0 ± 3.4 27389.7 ± 3.0 89.8 ± 3.1 24894.0 ± 2.6 94.0 ± 2.6 223

104.5 ± 3.0 104.4 ± 3.0 198114.0 ± 3.1 114.0 ± 3.2 173

82.1 ± 2.1 82.2 ± 2.2 27386.7 ± 3.2 86.6 ± 2.8 24893.4 ± 2.5 93.4 ± 2.6 223

101.1 ± 2.8 101.1 ± 2.9 198112.0 ± 3.9 112.0 ± 3.8 173

73.1 ± 2.3 73.2 ± 2.2 27378.3 ± 2.7 78.3 ± 2.7 24882.8 ± 2.7 82.8 ± 2.7 22388.3 ± 2.6 88.3 ± 2.6 198

100.8 ± 3.3 100.8 ± 3.3 173

Table 3Kr-broadening coefficients c0 in the fundamental m3 band of CS2.

Line m0 (cm�1) c0 (10�3cm�1atm�1) Temperature(K)

Voigt Rautian Galatry

P(78) 1514.0739 66.9 ± 1.6 68.4 ± 2.1 68.5 ± 2.2 27374.9 ± 2.7 76.7 ± 3.2 76.7 ± 3.2 24879.2 ± 2.0 81.4 ± 2.5 81.5 ± 2.6 22385.3 ± 3.2 88.5 ± 3.4 88.6 ± 3.5 198

P(62) 1519.1401 70.4 ± 1.8 71.9 ± 2.6 72.0 ± 2.7 27373.8 ± 2.1 75.6 ± 2.9 75.7 ± 2.9 24879.2 ± 2.2 80.2 ± 2.7 80.3 ± 2.8 22388.8 ± 3.4 88.9 ± 3.3 88.9 ± 3.3 19896.0 ± 3.0 97.9 ± 3.3 97.9 ± 3.4 173

P(36) 1526.6033 78.0 ± 2.7 78.1 ± 2.8 76.6 ± 2.2 27382.5 ± 2.5 85.3 ± 2.4 85.4 ± 2.5 24886.8 ± 2.4 90.2 ± 3.4 90.2 ± 3.4 22394.9 ± 3.1 97.5 ± 3.1 97.5 ± 3.1 198

105.5 ± 3.3 105.5 ± 3.3 103.8 ± 2.4 173

P(22) 1530.2259 83.3 ± 2.9 86.0 ± 4.1 86.1 ± 4.2 27389.2 ± 2.3 91.7 ± 3.0 91.8 ± 3.1 24894.8 ± 2.8 95.7 ± 3.8 95.7 ± 3.8 22399.9 ± 6.3 103.1 ± 4.4 103.2 ± 4.5 198

113.1 ± 4.4 114.3 ± 4.4 114.3 ± 4.4 173

R(2) 1536.0024 102.3 ± 4.1 116.6 ± 5.6 116.3 ± 6.5 273108.1 ± 4.5 121.2 ± 5.6 121.4 ± 5.3 248118.0 ± 3.5 129.2 ± 3.5 129.4 ± 3.5 223127.1 ± 4.8 135.1 ± 4.0 136.0 ± 4.0 198140.1 ± 7.4 149.4 ± 6.2 149.6 ± 6.1 173

R(4) 1536.4261 93.9 ± 3.1 95.6 ± 3.7 95.7 ± 3.8 273103.7 ± 2.6 106.2 ± 3.4 106.3 ± 3.6 248112.2 ± 2.8 113.5 ± 3.9 113.5 ± 4.0 223121.0 ± 4.9 122.5 ± 4.1 122.5 ± 4.1 198127.4 ± 5.4 129.8 ± 6.1 127.9 ± 6.0 173

R(6) 1536.8440 95.0 ± 3.3 97.3 ± 4.2 97.7 ± 4.4 27398.1 ± 2.8 102.7 ± 3.0 102.8 ± 3.0 248

107.4 ± 3.5 109.0 ± 4.6 109.1 ± 4.7 223114.9 ± 3.3 116.5 ± 3.8 116.5 ± 3.9 198127.9 ± 6.0 130.4 ± 3.4 130.1 ± 3.7 173

R(46) 1544.0012 73.7 ± 2.0 77.2 ± 2.7 77.2 ± 2.7 27378.9 ± 3.0 82.9 ± 2.7 82.9 ± 2.7 24884.9 ± 2.5 90.1 ± 3.3 90.2 ± 3.3 22390.1 ± 3.4 92.1 ± 3.7 92.1 ± 3.7 198

102.9 ± 4.2 108.8 ± 4.3 108.8 ± 4.3 173

Fig. 5. Rotational dependence of collisional broadening coefficients c0 of lines in thefundamental m3 band of 12C32S2 diluted in krypton from room to low temperature.(m = �J in the P-branch and m = J + 1 in the R-branch). These experimental resultsderived from the Rautian profile.

28 P. Kongolo Tshikala et al. / Journal of Molecular Spectroscopy 298 (2014) 24–30

(O–C) multiplied by ten for the different models. From these resid-uals, it appears that the results from the Rautian or Galatry modelsare preferable to those derived from the Voigt profile. The smallasymmetry which appears in Fig. 3 is probably due to the neigh-bouring of hot band or isotopic band lines.

4. Results

The collisional half-width of a CS2 line of broadened by a per-turber gas is given by:

cobs ¼ cself pCS2þ c0p0 ð8Þ

where cobs is the observed collisional halfwidth, cself is the self-broadening coefficient of CS2 [13], pCS2

the pressure of CS2, c0 thebroadened coefficient and p0 the pressure of the perturber (in thiscase, He, Ne or Kr).

Fig. 4 presents an example of the collisional halfwidth cc of theP(78) line versus the Kr pressure at a temperature of 223 K. Asshown in Fig. 3, the standard deviation on the fits varies followingthe lineshape model, but is always very weak. This statistical erroris about 5 times larger for the Voigt profile (�0.3%) than for theother models (�0.06%). This error is taken into account in theexperimental part of the uncertainty (2% of c0). The very weak

standard deviation on the fits does not influence largely the totalerror given in Tables 1–3.

The self-broadening contribution has been systematically sub-tracted and the collisional broadening coefficient c0 is the slopeof the straight-line derived from a linear regression through thefive measurements of cc. Due to the weak CS2 pressure, the self-broadening contribution is very weak (�0.1 � 10�3 cm�1). It isthe reason why we consider the same model (Rautian profile) forthis self-perturbation correction. On this figure, it is clear thatthe straight-lines for data from Rautian and Galatry models arepractically identical and consequently, the collisional broadeningcoefficients are quite the same and the result for the Voigt profileis always a little lower. The whole results are presented in Tables1–3 which for each perturber He, Ne, Kr respectively, give theassignment, the wavenumbers of the line, the broadening coeffi-cients deduced from the Voigt, Rautian and Galatry profiles andthe corresponding experimental temperature. The given absoluteerrors have been estimated to twice the standard deviation on c0

derived from the linear least-squares fit plus 2% of c0 to take intoaccount the experimental uncertainties. The main sources of errorshave been the base line location, the weak non-linear tuning of thediode-laser, the perturbations due to interfering lines and the line-shape model. In fact the standard deviation on the linear regressionis quite the same for each line profile and the total error is quiteconstant for all J values and models.

As expected the coefficients given by the Rautian and Galatrymodels are very similar (quite equal) and always a little larger thanthose obtained with the Voigt profile. Fig. 5 shows the rotationaldependence of the Kr broadening coefficients versus |m| (m = �Jfor the P-branch, J + 1 for the R-branch) for the five consideredtemperatures as well as the values obtained previously at roomtemperature [2]. For clarity, only the values derived from the Rau-tian profile are presented. As expected, we observe the same vari-ation of c0 increasing when the temperature decreases. For theother pertubers (He, Ne), these graphs are similar.

The dependence of the collisional broadening coefficient infunction of temperature is represented by the simple power law:

c0ðTÞc0ðT0Þ

¼ TT0

� ��n

ð9Þ

where T0 is the reference temperature (298 K).This n parameter is easily deduced by the negative slope of the

best-fit lines obtained by a linear regression on the different values

Fig. 6. Temperature dependence of broadening coefficients for the P(62) line in them3 band of CS2 diluted in Ne.

Table 4Temperature dependence parameters n of He, Ne, Kr-broadening coefficients of linesin the m3 band of CS2.

Perturber Line n

Voigt Rautian Galatry

He R(2) 0.611 ± 0.160 0.546 ± 0.161 0.548 ± 0.152R(4) 0.586 ± 0.066 0.592 ± 0.068 0.592 ± 0.068R(6) 0.647 ± 0.084 0.645 ± 0.087 0.637 ± 0.085P(22) 0.623 ± 0.086 0.622 ± 0.086 0.628 ± 0.089P(36) 0.588 ± 0.076 0.580 ± 0.082 0.594 ± 0.084R(46) 0.617 ± 0.079 0.631 ± 0.068 0.626 ± 0.072P(62) 0.637 ± 0.089 0.643 ± 0.093 0.643 ± 0.090P(78) 0.629 ± 0.068 0.646 ± 0.061 0.642 ± 0.061

Ne R(2) 0.728 ± 0.113 0.784 ± 0.080 0.781 ± 0.078R(4) 0.623 ± 0.058 0.626 ± 0.068 0.624 ± 0.067R(6) 0.678 ± 0.062 0.677 ± 0.067 0.677 ± 0.068P(22) 0.605 ± 0.072 0.614 ± 0.080 0.617 ± 0.082P(36) 0.636 ± 0.066 0.610 ± 0.070 0.610 ± 0.069R(46) 0.639 ± 0.071 0.666 ± 0.064 0.663 ± 0.064P(62) 0.640 ± 0.067 0.614 ± 0.070 0.611 ± 0.072P(78) 0.655 ± 0.091 0.667 ± 0.104 0.661 ± 0.111

Kr R(2) 0.696 ± 0.093 0.718 ± 0.077 0.735 ± 0.076R(4) 0.616 ± 0.075 0.616 ± 0.090 0.597 ± 0.090R(6) 0.681 ± 0.077 0.639 ± 0.074 0.629 ± 0.078P(22) 0.594 ± 0.078 0.606 ± 0.077 0.562 ± 0.079P(36) 0.649 ± 0.076 0.617 ± 0.082 0.616 ± 0.068R(46) 0.670 ± 0.074 0.682 ± 0.079 0.682 ± 0.079P(62) 0.706 ± 0.070 0.682 ± 0.073 0.679 ± 0.075P(78) 0.744 ± 0.059 0.750 ± 0.104 0.749 ± 0.108

Fig. 7. Rotational dependence of the n parameter for lines in the m3 band of CS2

diluted in He, Ne and Kr.

P. Kongolo Tshikala et al. / Journal of Molecular Spectroscopy 298 (2014) 24–30 29

of c0 versus the absolute temperature T in log–log scales. For thesedeterminations, we have six values of c0: five at low temperaturesfrom this study and one obtained previously at room temperature[2]. Fig. 6 gives an example of this determination for the P(62) linefor the Ne perturber. The results for the n parameter are presentedin Table 4 for each perturber and each line but only for those ob-tained with the Rautian model. Indeed the adjustment is betterfor Galatry and Rautian models than for the Voigt profile and thevalues deduced from these models are nearly identical. To showthe rotational dependence of the n parameter, we present inFig. 7 the n parameter versus |m| for the three perturbers (He,Ne, Kr). As seen, n is practically constant, it is the reason why wegive for it an average value: 0.621 ± 0.028 for He; 0.654 ± 0.026for Ne and 0.661 ± 0.028 for Kr. In the case of CS2 + Ar, Misagoet al. [1] have obtained exactly the same results: it appeared norotational J value dependence. In addition the average value(0.61) given in Ref. [1] is in the same order than our average valuesof the n parameter.

5. Conclusion

Using a diode-laser spectrometer, we have recorded the spectraof 8 spectral lines in the fundamental m3 band of CS2 diluted in raregases (helium, neon and krypton). The measurements have beenperformed at 5 low temperatures: 173 K, 198 K, 223 K, 248 K and273 K. To obtain a low temperature we have used a homemadecooling cell giving a stability of ±0.5 K.

The He-, Ne- and Kr-broadening have been determined by usingthe Voigt, Rautian and Galatry lineshapes. Our results are in agree-ment with the values obtained by Misago et al. for the CS2 + Ar [1]and our previous values at room temperature [2] for CS2 + He,CS2 + Ne, CS2 + Kr. To our knowledge, Ar-, He-, Ne- and Kr-broaden-ing coefficients determination are the only studies devoted to CS2

perturbed by rare gases. At low temperature as well as the roomtemperature, it appears no evident mass perturber dependencefor the collisional broadening coefficient for CS2-rare gases mix-tures. These measurements at different temperatures have allowedto determine the n-parameter of temperature dependence of colli-sional broadening coefficients of lines in the m3 band of CS2 mixedwith these rare gases. The appearance of rotational dependence ofn parameters leads us to determine the average value of tempera-ture dependence of He-, Ne- and Kr-broadening coefficients of linesin the fundamental m3 band of CS2. �n ¼ 0:621� 0:028 for helium,�n ¼ 0:654� 0:026 for neon and �n ¼ 0:661� 0:028 for krypton.

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