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1. Introduction

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these coefcients using a Fourier transform spectrometer with aresolution of about 0.01 cm1 and by tting a Voigt prole. In1997, Pine [11] published newmeasurements realized by a tunabledifference-frequency spectrometer using a RautianSobelManprole and in 2000, Pine and Gabard [12] reinvestigated the resultsobtained by Pine by tting more elaborated proles taken into ac-count the speed dependence and the line mixing effects.

2. Experimental details

Liquide Corespective

50 cm cell contained the gas mixture with a small pressmethane (0.7 mbar) diluted in the perturber gas (compristween 170 and 300 mbar) whereas the 15 cm cell conpure methane with a pressure between 0.2 and 1 mbar dependingon the line understudy. These two cells were placed in parallel inthe two beams of the spectrometer. So, we recordedsimultaneously two spectra: one with the unperturbed line, theother with the perturbed and shifted line. The tting of experimen-tal lineshapes can be done separately and the position of each lineis known with a great accuracy. For each line, the measurements

Corresponding author. Fax: +32 81 72 45 85.E-mail address: muriel.lepere@unamur.be (M. Lepre).

Journal of Molecular Spectroscopy 298 (2014) 710

Contents lists availab

ul

w.eSome studies have already been devoted to the N2-shift coef-cients in the m3 band of CH4. In 1993, Benner et al. [10] determined

Methane and nitrogen were supplied by Airwith a stated purity of 99.95% and 99.999%http://dx.doi.org/10.1016/j.jms.2013.12.0040022-2852/ 2014 Elsevier Inc. All rights reserved.mpanyly. Theure ofed be-tainedof our own atmosphere and the analyze of the infrared spectra ofplanetary atmospheres. These elds need very accurate measure-ments to improve their precision [9]. Thus we have studied witha great accuracy 16 individual N2-shift coefcients of CH4 using adual beam tunable diode-laser spectrometer with a high resolution(105 cm1).

The spectra from the two beams were recorded simultaneouslyand an homemade microprocessor signal averager was used fordata acquisition. In order to increase the signal-to-noise ratio, eachspectrum was averaged over 100 scans. For the relative calibration,a confocal talon with a free spectral range of 0.007958 cm1 wasintroduced in a laser beam.Methane is an important trace gaalso a minor component in variousspheres like Jupiter [2], Mars [3] oris the third greenhouse effect gas adioxyde [5,6] and its global-warminthan those of CO2 [7]. The quantitare essentially dependant on industperature and humidity [7,8].

The measurements of molecular pcients, are useful for the understanur atmosphere [1]. It isary (or satellite) atmo-[4]. On earth, methaneater vapor and carbonntial is 21 times higherethane in atmospherevities, agriculture, tem-

ters, like the shift coef-the physical chemistry

The spectra were recorded with an improved Aero Laser dualbeam tunable diode-laser spectrometer (model L5004-IR) whichwill be described in details elsewhere [13]. Fig. 1 gives an opticalscheme of this apparatus; its resolution is about 105 cm1 andits signal-to-noise ratio better than 2000. The signal provided bya diode-laser, cooled with liquid nitrogen, is divided into two com-ponents by a beam splitter 45/55. For this study, the rst part ofthe laser beam is directed to a cell of 15 cm optical pathlengthwhereas the second part is passed through a 50 cm optical path-length absorption cell.N2-shift coefcients in the m3 band of 12C

Bastien Vispoel, Migul Dhyne, Jean-Claude PopulaiLaboratoire Lasers et Spectroscopies, Research Centre in Physics of Matter and Radiation

a r t i c l e i n f o

Article history:Received 2 November 2013In revised form 12 December 2013Available online 25 December 2013

Keywords:MethaneDiode-laser spectroscopyLine shifts

a b s t r a c t

Using a dual beam diode-lthe P-branch of the m3 banlines in the spectral rangefour different nitrogen preting to the experimental lining. The shift coefcients wsimultaneously recorded wlished results.

Journal of Molec

journal homepage: ww4 at room temperature

Ghislain Blanquet, Muriel Lepre R), University of Namur, 61, Rue de Bruxelles, B-5000 Namur, Belgium

spectrometer, we have studied with accuracy the N2-shift coefcients inf methane. The experiments were performed at room temperature for 1662948 cm1 with J values between 7 and 11. Each line was recorded ates ranging from 17 to 302 mbar. The collisional shifts were obtained by t-ape a RautianSobelMan prole that takes into account the Dicke narrow-determined using a procedure where a non perturbed line of pure CH4 wasN2-shifted transitions. Finally, we give a comparison with previous pub-

2014 Elsevier Inc. All rights reserved.

le at ScienceDirect

ar Spectroscopy

lsevier .com/ locate / jms

were realized at four perturber pressures. The pressures were

ularity and the level of the saturated line. After being recorded the

Fig. 1. Aero Laser dual beam tunable diode-laser spectrometer: (1) diode-laser Dewar, (detector.

8 B. Vispoel et al. / Journal of Molecularspectra were linearized, to correct the weak nonlinear tuning of thediode-laser, with a constant step of about 1 104 cm1.

For each line under study, a set of 12 spectra was recorded: (1)diode-laser emission prole (100% transmission level); (2) lowabsorption line at low pressure of pure CH4 to avoid collisional ef-fects (Doppler line) in view to determine the apparatus function;(3) confocal talon fringes; (4) saturated line (0% transmission le-vel); (58) the N2-broadened lines at four different total pressuresand (912) the unperturbed absorption line of pure methane. Anexample of the spectra obtained for the N2-shift of the P(7)F1 lineof the m3 band at 2947.6679 cm1 is shown in Fig. 2. The assign-ments and wavenumbers of the measured lines of methane are ta-ken from HITRAN [14]. The labelling of the transitions consideredin our work is also explained in Ref. [14].

3. Data reduction and experimental results

The measured absorbance a(m) at wavenumber m (cm1), for thelight intensity transmitted through a gas column, is obtainedthrough the BeerLambert law asmeasured by two MKS Baratron gauges with a full sale of 1.2 and1200 mbar and all spectra were recorded at room temperature(297 K).

Before recording spectra, the purity of the mode was checked bythe smoothness of the talon fringe pattern, the laser emission reg-Fig. 2. Example of the spectra recorded for shift determination at 296 K for theP(7)F1 line in the m3 band of CH4 perturbed by N2. (1) diode-laser emission prole(100% transmission level); (2) low absorption line at low pressure of pure CH4(Doppler line), (3) confocal talon fringes, (4) saturated line (0% transmission level),(58) perturbed line at different pressures of N2 (154.3, 202.2, 252.1 and302.0 mbar respectively) and (912) unshifted line of CH4.am lnI0m=Itm 1where I0(m) and It(m) are transmitted intensities measured with thecell under vacuum and lled with the gas sample, respectively. Thesmall instrumental distortion capp is taken into account as previ-ously [15] through an effective Doppler width cD obtained fromthe pure CH4 lines at low pressure and systematically used in placeof the true Doppler width cDth. As the apparatus function can beassimilated to the Gaussian function [16] and theoretical Doppler,the apparatus half-width is given by:

capp c2D c2Dth

q2

where

cDth 3:5812TM

r3

is the theoretical Doppler half-width with T (K) the temperature, M(a.m.u.) the molecular mass of CH4, and m0 (cm1) the line centerwavenumber. The apparatus half-width capp deduced from Eq. (2)for recorded lines has a typical value of 5 104 cm1 in this work.

To determine the N2-shift coefcient of a line, we assumed thatthe pressure broadened lines are symmetrical and they are consid-ered sufciently isolated to be individually tted by a theoreticalprole. We used the RautianSobelMan model [17] which incor-porates the Dicke effect [18]. When the lines were not well iso-lated, like for P(7)E-1 and P(7)F2-2 lines, which are onlyseparated of 0.1 cm1, a multiline t method described in Ref.[19] was considered.

1 1

2) monochromator, (3) and (4) absorption cells, (5) confocal talon and (6) double

Spectroscopy 298 (2014) 710The collisional shift d0 (in cm atm ), is determined for eachline at each partial pressure of N2 and is the difference betweenthe position of the unshifted line of pure methane and the N2-shifted line of methane. The values of dc (in cm1) determined atthe four different total pressures were plotted versus the pressureof the perturber (N2).

The d0 collisional shift coefcient is determined by the follow-ing relation:

d0 dN2c PN2 4where dN2c represents the N2-shift of CH4, PN2 is the partial pressureof N2. The slope of the best t line passing through the experimentalpoints gives the N2-shift coefcient. Fig. 3 shows an example ofthese ts for the P(11)A2-1 line. The small contribution of self-shiftwas neglected because of its weak value due to the low pressure ofactive gas. However, the difference of pressure in the two absorp-tion cells (factor 2.5) can explain why the best t line does not passby the origin. These N2-shift coefcients d0 and their experimentalerrors are listened in Table 1, while Fig. 4 presents, for A, E and F-species, these coefcients versus the wavenumbers. The absolute

Fig. 3. Pressure dependence of the N2-shift coefcient dc of P(11)A2 absorption linebelong to the m vibrational band of CH at 2906.7348 cm1. The collisional shift

B. Vispoel et al. / Journal of Molecular3 4

coefcient is the slope of the best t line.errors are estimated to be twice the standard deviation given bythe linear least-squares procedure plus 2% of c0 representingexperimental error. This experimental error comes mainly from

with the increasing value of the wavenumbers. That has been also

Fig. 4. N2-shift coefcients of P(7), P(10) and P(11) absorption lines belong to the m3vibrational band of CH4. They are represented in function of their symmetries andtheir wavenumbers.

Table 1N2-shift coefcients d0 in the m3 band of 12CH4 at room temperature.

Lines m0 (cm1) d0 (103 cm1 atm1)

P(11) F2(42) F1(2) 2906.2824 5.84 0.27aP(11) E(28) E(1) 2906.5886 6.11 0.35P(11) F1(40) F2(2) 2906.6477 6.43 0.67P(11) A1(15) A2(1) 2906.7348 6.79 0.20P(10) F1(37) F2(3) 2916.2014 3.98 0.41P(10) E(24) E(2) 2916.3017 6.04 0.28P(10) F2(36) F1(2) 2916.3961 7.61 0.38P(10) A2(14) A1(1) 2916.7538 5.85 0.31P(10) F2(37) F1(1) 2916.9661 5.11 0.45P(10) F1(38) F2(2) 2917.0662 7.78 0.48P(7) F2(26) F1(2) 2947.6680 2.81 0.73P(7) E(17) E(1) 2947.8109 7.76 0.33P(7) F1(24) F2(2) 2947.9121 8.60 0.27P(7) A1(10) A2(1) 2947.1079 6.35 0.81P(7) F1(25) F2(1) 2947.4214 4.75 0.49P(7) F2(27) F1(1) 2947.4741 8.43 0.36

a The errors quoted are twice the standard deviation plus 2% of d0.observed in the work of Antony et al. [20].Fig. 5 compares our results obtained for six P(10) symmetries of

the m3 vibrational band with earlier published determinations fromPine [11], Benner et al. [10] and, Pine and Gabard [12] for the A-,E- and F-species. Generally, our values are in good agreement withthese previous data.

4. Conclusions

Using a tunable dual beam diode laser spectrometer, theN2-shift coefcients for 17 lines belong to the P-branch of them3 vibrational band of CH4 were measured at room temperature.The trend of our results for the m3 band with rotational state isthe baseline location, the perturbations due to neighboring lines,the slight nonlinear tuning of the laser and the lineshape modelused.

As may be seen in Fig. 4, it is not possible to highlight a generalbehavior for both symmetries of methane lines. But, we can ob-serve a possible scattering of the values of the shift coefcients

Fig. 5. Comparison of the N2-shift coefcients of the P(10) absorption lines belongto the m3 vibrational band of CH4 obtained with the RautianSobelMan prole withthose earlier published by Pine [11] (h), Benner et al. [10] (D), Pine and Gabard [12](s).

Spectroscopy 298 (2014) 710 9essentially the same than for those observed by Antony et al.[20]. The comparison with earlier published data [1012] is in goodagreement.

Acknowledgments

This work is accomplished in the framework of the Interna-tional Research Group HiResMIR. B. Vispoel is supported by theFonds pour la formation la Recherche dans lIndustrie et lAgri-culture (FRIA).

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10 B. Vispoel et al. / Journal of Molecular Spectroscopy 298 (2014) 710

N2-shift coefficients in the 3 band of 12CH4 at1 Introduction2 Experimental details3 Data reduction and experimental results4 ConclusionsAcknowledgmentsReferences