- Email: [email protected]
N2-broadening coefficients of ethane
Journal of Molecular Spectroscopy 255 (2009) 72–74
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Note
N2-broadening coefficients of ethane Ghislain Blanquet a,*, Jean Vander Auwera b,1, Muriel Lepère a,2 a b
Laboratoire Lasers et Spectroscopies, Facultés Universitaires Notre-Dame de la Paix (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles, B-1050 Brussels, Belgium
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 6 January 2009 In revised form 27 February 2009 Available online 9 March 2009
Ó 2009 Elsevier Inc. All rights reserved.
Keywords: Ethane Diode-laser N2-broadening
Ethane plays an important role in the atmospheres of the Earth [1] and Titan [2]. To retrieve C2H6 abundances in these environments using remote sensing techniques, accurate spectroscopic line parameters are needed. Among the required information, nitrogen broadening coefficients of ethane have only occasionally been measured. In the m9 band, Blass et al. [3] measured self- and N2-broadening coefficients at 296 K and Chudamani et al. [4] reported N2-broadening parameters at 150 K. Note that these broadening coefficients are difficult to measure because of the torsional splitting of the lines, often leading to strong overlap of the resulting two components of the lines, even at relatively low pressure [5]. In the present work, we measured N2-broadening coefficients for 15 lines in the m9 band at five pressures of nitrogen. We present here the results obtained for the stronger of the two components of these lines. Indeed, the precision of measurement of the collisional broadening coefficient of the weaker component is low, because of overlap with the stronger line and sensitivity to the baseline position. The spectra have been recorded at room temperature (296 K) with the improved Laser Analytics diode-laser spectrometer of the University of Namur [6]. The samples of ethane and nitrogen were respectively purchased from Praxair (99.9% stated purity) and Air Liquide (99.995% stated purity). The measurements were carried out with a 1-m base White-type cell adjusted for four transits, yielding an optical path of 4.17 m (including the distance between the field mirror and the cell windows). For each broadened * Corresponding author. Fax: +32 81 72 45 85. E-mail address: [email protected] (G. Blanquet). 1 Senior Research Associate with FRS-FNRS (Belgium). 2 Associate Researcher with FRS-FNRS (Belgium). 0022-2852/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2009.02.020
line, spectra were recorded at five different pressures. To limit the overlap of the two components, we used relatively low pressures of nitrogen, ranging from 2.5 to 10.7 mbar; the pressure of ethane being maintained to about 0.15 mbar. The relative wavenumber calibration was obtained by introducing in the laser beam a confocal étalon with a free spectral range of 0.007958 cm1. To correct the slightly nonlinear tuning of the diode-laser radiation, the spectra were linearized in wavenumber with a constant step of about 1 104 cm1 by means of a cubic spline technique. Throughout the article, the lines are identified as DKDJ (K00 , J0 0 , r00 ), where J is the quantum number associated with the total angular momentum of the molecule, K is its projection along the molecular symmetry axis and r is the torsional symmetry number (r = 0, 1, 2 or 3) [5]. Fig. 1 shows an example of the recorded spectra near 799 cm1. Data reduction was achieved using the same method as applied previously (see for instance Ref. [8]), fitting the experimental profiles with the Voigt [9,10] and Rautian theoretical models [11]. The small instrumental distortions were taken into account as previously [12,13] through use of an effective Doppler width instead of the true Doppler width. For each line, the Voigt profile involves three adjustable parameters: the line center v0, the collisional HWHM cc and an intensity factor. For the Rautian model, an additional parameter, the collisional narrowing bc, was also adjusted. Because of the strong overlap of the two torsional components, the superimposed profiles were fitted simultaneously, as in [14]. Consequently, each fit involved the adjustment of six or eight line parameters (depending upon the line shape used for each line, three for Voigt model, four for the Rautian model). However, the only retained parameter is the line broadening coefficient for the main component, the precision of others being too poor. Specifically, the precision of the measured line intensities is
73
G. Blanquet et al. / Journal of Molecular Spectroscopy 255 (2009) 72–74
Transmission
7
1 2 3 4 5
P
P
Q(9,14) -1 798.98 cm
6
Q(9,15) -1 799.01 cm
8
P
Q(9,16) -1 799.05 cm
1: 4.209 mbar 2: 5.880 3: 7.716 4: 9.582 5: 10.592
9
-1
0.007958 cm
Fig. 1. Spectra of the PQ(9, J0 0 , r0 0 ) lines with J0 0 = 14–16 and r0 0 = 1, 3 in the m9 band of 12C2H6 recorded at 296 K: (1–5) broadened lines at five increasing pressures of N2; (6) diode-laser emission profile recorded without absorption (100% transmission); (7) low-pressure line of 12C2H6 used to determine the apparent Doppler width; (8) confocal étalon fringes, (9) 0% transmission level.
0.8
Table 1 N2-broadening coefficients of C2H6 in the v9 band at room temperature.
P
Q(9,15)
va (cm1)
cVoigt (103 cm1 atm1)
cRautian (103 cm1 atm1)
P
795.5934 795.7177 795.8555 798.9324 798.9786 799.0116 799.0476 799.6713 808.9836 809.0650 809.1682 809.1976 816.7144 816.7931 816.9007
79.6 ± 3.0 80.8 ± 3.0 82.4 ± 3.7 81.3 ± 2.3 82.8 ± 2.0 83.0 ± 2.4 83.4 ± 3.2 84.2 ± 2.9 81.2 ± 4.9 83.4 ± 4.6 80.7 ± 4.8 83.1 ± 3.5 80.2 ± 5.0 80.4 ± 3.6 83.2 ± 5.3
87.5 ± 3.4 88.7 ± 4.9 87.8 ± 5.0 85.6 ± 2.3 84.3 ± 2.1 87.6 ± 2.2 88.9 ± 2.4 86.7 ± 3.1 84.6 ± 6.9 86.7 ± 6.2 87.1 ± 4.2 86.0 ± 2.5 86.9 ± 3.0 86.6 ± 2.9 85.8 ± 6.3
P(2,16,2) PQ(3,14,3) P P(4,12,2) P Q(9,17,3) P Q(9,16,3) P Q(9,15,3) P Q(9,14,3) P P(2,13,2) P Q(5,17,1) P Q(5,15,1) P Q(5,12,1) P Q(5,11,1) P Q(2,17,2) P Q(2,15,2) P Q(2,12,2)
0.6
α(ν)
Raie
0.4
0.2
Note: The errors quoted are twice the standard deviation +2% of c0. a The ethane line positions are from [7].
0.0
O-C*10
0.2
Voigt 0.0 -0.2
O-C*10
0.2
Rautian 0.0 -0.2
799.008
799.012
799.016
ν (cm-1) Fig. 2. Example of theoretical Voigt fit (d) of the experimental profile a(v) (—) for the PQ(9, 15, r0 0 ) lines (r0 0 = 1, 3) with 10.592 mbar of N2. For clarity, only one calculated value out of five is plotted here. The residuals [observed (O) minus calculated (C)] are shown for the Voigt and Rautian profiles at the bottom of the figure with an intensity scale multiplied by 10.
low because of the low active gas pressure used; a study devoted to the precise determination of line intensities is ongoing. As shown in Fig. 2, the adjustment is realized for each two torsional components and is much better with the Rautian profile, in particular in this range of pressures. Table 1 lists the N2-broadening coefficients obtained with the Voigt and Rautian models for the stronger component of 15 lines of C2H6. The main contributions to the quoted experimental errors arise from the proximity of the two torsional components, perturbations due to interfering lines, the baseline location, the line shape model and the nonlinear tuning of the laser. As expected, the results derived from the Voigt profile are a few percents (2–10%) lower than those derived from the Rautian model. The average values of the N2-broadening coefficients obtained at 296 K from the Voigt and Rautian lineshapes are respectively equal to 0.0822 (17) and 0.0864 (14) cm1 atm1. They are in good agreement with previous measurements (0.090 cm1 atm1 [3] and 0.084 cm1 atm1 [4] at 296 K), but are significantly higher
74
G. Blanquet et al. / Journal of Molecular Spectroscopy 255 (2009) 72–74
100
values are in relatively good agreement with the value calculated using the diffusion theory (30.3 103 cm1 atm1). Acknowledgments
90 85
This work was accomplished in the framework of the ‘‘Laboratoire Européen Associé HiRes” and was partly supported by the F.R.S.-FNRS (FRFC 2.4522.97 and 2.4527.03).
80
References
-3
γ0 (10 cm-1 atm-1)
95
75 70
12
14
16
18
20
J+K*0.2 Fig. 3. N2-broadening coefficients c0 measured in the m9 band of 12C2H6 at 296 K. The experimental values with error bars are derived from the Rautian profile. For clarity, the values of c0 are plotted versus J + K 0.2.
than the air-broadening parameter measured by Pine and Stone, equal to 0.0671 cm1 atm1 at 296 K [15]. Fig. 3 shows the N2-broadening coefficients, plotted versus J + K 0.2. Taking the experimental errors into account, no rotational dependence is observed over the rather limited range of rotational quantum numbers investigated. The adjustment of the Rautian profile provides an additional parameter, the collisional narrowing coefficient b0. The average of the measured values is equal to (25 ± 9) 103 cm1 atm1 at 296 K. In spite of the difficulty to measure this parameter because of its high sensitivity to the baseline position, the experimental
[1] C.P. Rinsland, N.B. Jones, B.J. Connor, J.A. Logan, N.S. Pougatchev, A. Goldman, F.J. Murcray, T.M. Stephen, A.S. Pine, R. Zander, E. Mahieu, P. Demoulin, J. Geophys. Res. 103 (1998) 28197–28217. [2] A. Coustenis, R.K. Achterberg, B.J. Conrath, D.E. Jennings, A. Marten, D. Gautier, C.A. Nixon, F.M. Flasar, N.A. Teanby, B. Bézard, R.E. Samuelson, R.C. Carlson, E. Lellouch, G.L. Bjoraker, P.N. Romani, F.W. Taylor, P.G.J. Irwin, T. Fouchet, A. Hubert, G.S. Orton, V.G. Kunde, S. Vinatier, J. Mondellini, M.M. Abbas, R. Courtin, Icarus 189 (2007) 35–62. [3] W.E. Blass, G.W. Hasley, D.A. Jennings, J. Quant. Radiat Transfer. 38 (1987) 183–184. [4] S. Chudamani, P. Varanasi, L.P. Giver, F.P. Valero, J. Quant. Radiat. Transfer 34 (1985) 359–364. [5] N. Moazzen-Ahmadi, J. Schroderus, A.R.W. McKellar, J. Chem. Phys. 111 (1999) 9609–9617. [6] M. Lepère, G. Blanquet, J. Walrand, J.P. Bouanich, J. Mol. Spectrosc. 180 (1996) 218–226. [7] J. Vander Auwera, N. Moazzen-Ahmadi, J.-M. Flaud, Astrophys. J. 662 (2007) 750–758. [8] F. Misago, M. Lepère, J.P. Bouanich, G. Blanquet, J. Mol. Spectrosc. 248 (2008) 14–19. [9] B.H. Armstrong, J. Quant. Spectrosc. Radiat. Transfer 7 (1967) 61–88. [10] P.L. Varghese, R.K. Hanson, Appl. Opt. 23 (1984) 2376–2385. [11] S.G. Rautian, I.I. Sobel’man, Sov. Phys. Uspekhi Engl. Transl. 9 (1967) 701–716. [12] A. Mouchet, G. Blanquet, P. Herbin, J. Walrand, C.P. Courtoy, J.-P. Bouanich, Can. J. Phys. 63 (1985) 527–531. [13] G. Blanquet, J. Walrand, J.-P. Bouanich, J. Mol. Spectrosc. 159 (1993) 137–143. [14] G. Blanquet, P. Coupé, J. Walrand, J.-P. Bouanich, J. Quant. Radiat. Transfer 51 (1994) 671–678. [15] A.S. Pine, S.C. Stone, J. Mol. Spectrosc. 175 (1996) 21–30.
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Note
N2-broadening coefficients of ethane Ghislain Blanquet a,*, Jean Vander Auwera b,1, Muriel Lepère a,2 a b
Laboratoire Lasers et Spectroscopies, Facultés Universitaires Notre-Dame de la Paix (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles, B-1050 Brussels, Belgium
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 6 January 2009 In revised form 27 February 2009 Available online 9 March 2009
Ó 2009 Elsevier Inc. All rights reserved.
Keywords: Ethane Diode-laser N2-broadening
Ethane plays an important role in the atmospheres of the Earth [1] and Titan [2]. To retrieve C2H6 abundances in these environments using remote sensing techniques, accurate spectroscopic line parameters are needed. Among the required information, nitrogen broadening coefficients of ethane have only occasionally been measured. In the m9 band, Blass et al. [3] measured self- and N2-broadening coefficients at 296 K and Chudamani et al. [4] reported N2-broadening parameters at 150 K. Note that these broadening coefficients are difficult to measure because of the torsional splitting of the lines, often leading to strong overlap of the resulting two components of the lines, even at relatively low pressure [5]. In the present work, we measured N2-broadening coefficients for 15 lines in the m9 band at five pressures of nitrogen. We present here the results obtained for the stronger of the two components of these lines. Indeed, the precision of measurement of the collisional broadening coefficient of the weaker component is low, because of overlap with the stronger line and sensitivity to the baseline position. The spectra have been recorded at room temperature (296 K) with the improved Laser Analytics diode-laser spectrometer of the University of Namur [6]. The samples of ethane and nitrogen were respectively purchased from Praxair (99.9% stated purity) and Air Liquide (99.995% stated purity). The measurements were carried out with a 1-m base White-type cell adjusted for four transits, yielding an optical path of 4.17 m (including the distance between the field mirror and the cell windows). For each broadened * Corresponding author. Fax: +32 81 72 45 85. E-mail address: [email protected] (G. Blanquet). 1 Senior Research Associate with FRS-FNRS (Belgium). 2 Associate Researcher with FRS-FNRS (Belgium). 0022-2852/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2009.02.020
line, spectra were recorded at five different pressures. To limit the overlap of the two components, we used relatively low pressures of nitrogen, ranging from 2.5 to 10.7 mbar; the pressure of ethane being maintained to about 0.15 mbar. The relative wavenumber calibration was obtained by introducing in the laser beam a confocal étalon with a free spectral range of 0.007958 cm1. To correct the slightly nonlinear tuning of the diode-laser radiation, the spectra were linearized in wavenumber with a constant step of about 1 104 cm1 by means of a cubic spline technique. Throughout the article, the lines are identified as DKDJ (K00 , J0 0 , r00 ), where J is the quantum number associated with the total angular momentum of the molecule, K is its projection along the molecular symmetry axis and r is the torsional symmetry number (r = 0, 1, 2 or 3) [5]. Fig. 1 shows an example of the recorded spectra near 799 cm1. Data reduction was achieved using the same method as applied previously (see for instance Ref. [8]), fitting the experimental profiles with the Voigt [9,10] and Rautian theoretical models [11]. The small instrumental distortions were taken into account as previously [12,13] through use of an effective Doppler width instead of the true Doppler width. For each line, the Voigt profile involves three adjustable parameters: the line center v0, the collisional HWHM cc and an intensity factor. For the Rautian model, an additional parameter, the collisional narrowing bc, was also adjusted. Because of the strong overlap of the two torsional components, the superimposed profiles were fitted simultaneously, as in [14]. Consequently, each fit involved the adjustment of six or eight line parameters (depending upon the line shape used for each line, three for Voigt model, four for the Rautian model). However, the only retained parameter is the line broadening coefficient for the main component, the precision of others being too poor. Specifically, the precision of the measured line intensities is
73
G. Blanquet et al. / Journal of Molecular Spectroscopy 255 (2009) 72–74
Transmission
7
1 2 3 4 5
P
P
Q(9,14) -1 798.98 cm
6
Q(9,15) -1 799.01 cm
8
P
Q(9,16) -1 799.05 cm
1: 4.209 mbar 2: 5.880 3: 7.716 4: 9.582 5: 10.592
9
-1
0.007958 cm
Fig. 1. Spectra of the PQ(9, J0 0 , r0 0 ) lines with J0 0 = 14–16 and r0 0 = 1, 3 in the m9 band of 12C2H6 recorded at 296 K: (1–5) broadened lines at five increasing pressures of N2; (6) diode-laser emission profile recorded without absorption (100% transmission); (7) low-pressure line of 12C2H6 used to determine the apparent Doppler width; (8) confocal étalon fringes, (9) 0% transmission level.
0.8
Table 1 N2-broadening coefficients of C2H6 in the v9 band at room temperature.
P
Q(9,15)
va (cm1)
cVoigt (103 cm1 atm1)
cRautian (103 cm1 atm1)
P
795.5934 795.7177 795.8555 798.9324 798.9786 799.0116 799.0476 799.6713 808.9836 809.0650 809.1682 809.1976 816.7144 816.7931 816.9007
79.6 ± 3.0 80.8 ± 3.0 82.4 ± 3.7 81.3 ± 2.3 82.8 ± 2.0 83.0 ± 2.4 83.4 ± 3.2 84.2 ± 2.9 81.2 ± 4.9 83.4 ± 4.6 80.7 ± 4.8 83.1 ± 3.5 80.2 ± 5.0 80.4 ± 3.6 83.2 ± 5.3
87.5 ± 3.4 88.7 ± 4.9 87.8 ± 5.0 85.6 ± 2.3 84.3 ± 2.1 87.6 ± 2.2 88.9 ± 2.4 86.7 ± 3.1 84.6 ± 6.9 86.7 ± 6.2 87.1 ± 4.2 86.0 ± 2.5 86.9 ± 3.0 86.6 ± 2.9 85.8 ± 6.3
P(2,16,2) PQ(3,14,3) P P(4,12,2) P Q(9,17,3) P Q(9,16,3) P Q(9,15,3) P Q(9,14,3) P P(2,13,2) P Q(5,17,1) P Q(5,15,1) P Q(5,12,1) P Q(5,11,1) P Q(2,17,2) P Q(2,15,2) P Q(2,12,2)
0.6
α(ν)
Raie
0.4
0.2
Note: The errors quoted are twice the standard deviation +2% of c0. a The ethane line positions are from [7].
0.0
O-C*10
0.2
Voigt 0.0 -0.2
O-C*10
0.2
Rautian 0.0 -0.2
799.008
799.012
799.016
ν (cm-1) Fig. 2. Example of theoretical Voigt fit (d) of the experimental profile a(v) (—) for the PQ(9, 15, r0 0 ) lines (r0 0 = 1, 3) with 10.592 mbar of N2. For clarity, only one calculated value out of five is plotted here. The residuals [observed (O) minus calculated (C)] are shown for the Voigt and Rautian profiles at the bottom of the figure with an intensity scale multiplied by 10.
low because of the low active gas pressure used; a study devoted to the precise determination of line intensities is ongoing. As shown in Fig. 2, the adjustment is realized for each two torsional components and is much better with the Rautian profile, in particular in this range of pressures. Table 1 lists the N2-broadening coefficients obtained with the Voigt and Rautian models for the stronger component of 15 lines of C2H6. The main contributions to the quoted experimental errors arise from the proximity of the two torsional components, perturbations due to interfering lines, the baseline location, the line shape model and the nonlinear tuning of the laser. As expected, the results derived from the Voigt profile are a few percents (2–10%) lower than those derived from the Rautian model. The average values of the N2-broadening coefficients obtained at 296 K from the Voigt and Rautian lineshapes are respectively equal to 0.0822 (17) and 0.0864 (14) cm1 atm1. They are in good agreement with previous measurements (0.090 cm1 atm1 [3] and 0.084 cm1 atm1 [4] at 296 K), but are significantly higher
74
G. Blanquet et al. / Journal of Molecular Spectroscopy 255 (2009) 72–74
100
values are in relatively good agreement with the value calculated using the diffusion theory (30.3 103 cm1 atm1). Acknowledgments
90 85
This work was accomplished in the framework of the ‘‘Laboratoire Européen Associé HiRes” and was partly supported by the F.R.S.-FNRS (FRFC 2.4522.97 and 2.4527.03).
80
References
-3
γ0 (10 cm-1 atm-1)
95
75 70
12
14
16
18
20
J+K*0.2 Fig. 3. N2-broadening coefficients c0 measured in the m9 band of 12C2H6 at 296 K. The experimental values with error bars are derived from the Rautian profile. For clarity, the values of c0 are plotted versus J + K 0.2.
than the air-broadening parameter measured by Pine and Stone, equal to 0.0671 cm1 atm1 at 296 K [15]. Fig. 3 shows the N2-broadening coefficients, plotted versus J + K 0.2. Taking the experimental errors into account, no rotational dependence is observed over the rather limited range of rotational quantum numbers investigated. The adjustment of the Rautian profile provides an additional parameter, the collisional narrowing coefficient b0. The average of the measured values is equal to (25 ± 9) 103 cm1 atm1 at 296 K. In spite of the difficulty to measure this parameter because of its high sensitivity to the baseline position, the experimental
[1] C.P. Rinsland, N.B. Jones, B.J. Connor, J.A. Logan, N.S. Pougatchev, A. Goldman, F.J. Murcray, T.M. Stephen, A.S. Pine, R. Zander, E. Mahieu, P. Demoulin, J. Geophys. Res. 103 (1998) 28197–28217. [2] A. Coustenis, R.K. Achterberg, B.J. Conrath, D.E. Jennings, A. Marten, D. Gautier, C.A. Nixon, F.M. Flasar, N.A. Teanby, B. Bézard, R.E. Samuelson, R.C. Carlson, E. Lellouch, G.L. Bjoraker, P.N. Romani, F.W. Taylor, P.G.J. Irwin, T. Fouchet, A. Hubert, G.S. Orton, V.G. Kunde, S. Vinatier, J. Mondellini, M.M. Abbas, R. Courtin, Icarus 189 (2007) 35–62. [3] W.E. Blass, G.W. Hasley, D.A. Jennings, J. Quant. Radiat Transfer. 38 (1987) 183–184. [4] S. Chudamani, P. Varanasi, L.P. Giver, F.P. Valero, J. Quant. Radiat. Transfer 34 (1985) 359–364. [5] N. Moazzen-Ahmadi, J. Schroderus, A.R.W. McKellar, J. Chem. Phys. 111 (1999) 9609–9617. [6] M. Lepère, G. Blanquet, J. Walrand, J.P. Bouanich, J. Mol. Spectrosc. 180 (1996) 218–226. [7] J. Vander Auwera, N. Moazzen-Ahmadi, J.-M. Flaud, Astrophys. J. 662 (2007) 750–758. [8] F. Misago, M. Lepère, J.P. Bouanich, G. Blanquet, J. Mol. Spectrosc. 248 (2008) 14–19. [9] B.H. Armstrong, J. Quant. Spectrosc. Radiat. Transfer 7 (1967) 61–88. [10] P.L. Varghese, R.K. Hanson, Appl. Opt. 23 (1984) 2376–2385. [11] S.G. Rautian, I.I. Sobel’man, Sov. Phys. Uspekhi Engl. Transl. 9 (1967) 701–716. [12] A. Mouchet, G. Blanquet, P. Herbin, J. Walrand, C.P. Courtoy, J.-P. Bouanich, Can. J. Phys. 63 (1985) 527–531. [13] G. Blanquet, J. Walrand, J.-P. Bouanich, J. Mol. Spectrosc. 159 (1993) 137–143. [14] G. Blanquet, P. Coupé, J. Walrand, J.-P. Bouanich, J. Quant. Radiat. Transfer 51 (1994) 671–678. [15] A.S. Pine, S.C. Stone, J. Mol. Spectrosc. 175 (1996) 21–30.