Molecular dynamics simulations of pressure-broadened symmetric-top gas spectra. Application to CH3F-Ar and CH3F-He mixtures

https://doi.org/10.1016/j.jqsrt.2021.108031Get rights and content

Highlights

  • Requantized classical molecular dynamics simulations (rCMDS) were developed to compute Ar- and He-broadened CH3F spectra.

  • Line broadenings deduced from the simulated spectra are in good agreement with available measured values.

  • Speed dependence effect on the simulated spectra is detected and comparable with the unique experimental data available.

  • Line-mixing effect at high pressure in parallel bands of CH3F is correctly predicted by rCMDS.

Abstract

We present a new approach, based on requantized classical molecular dynamics simulations (rCMDS), to calculate pressure-broadened absorption spectra of symmetric-top molecules. We test it in the cases of CH3F-He and CH3F-Ar gas mixtures at room temperature for which predictions are made, without use of any adjusted parameter, based on input inter-molecular potentials from the literature. The results show that the Lorentz pressure-broadenings, deduced from fits of the computed spectra, are in good agreement with available measured values. Furthermore, the predicted effects of line-mixing at high pressure are also very consistent with observations. In addition, the study of the influence of the speed dependence of the pressure-broadening coefficient is promising but needs to be confirmed, since based on only one experimental result available. This opens renewed perspectives for predictions of symmetric-top molecules line shapes beyond the Voigt profile.

Introduction

The tremendous progress of the spectral resolution, accuracy and precision of laboratory spectroscopy experimental techniques (see Section 2 of [1]) achieved in the last decades nowadays routinely point out the limits of the Voigt profile (VP) for collisionally-isolated absorption lines (i.e. no line mixing). As a consequence, a variety of much more elaborated line shapes (see Chapt. III of [2]) are increasingly used in the analysis of measured spectra (e.g. Table 3 of [1]). Concomitantly, the success of a number of Earth observation experiments devoted to the retrieval of greenhouse gasses [3], [4], [5] requires an unprecedented accuracy in the description of atmospheric spectra which also rules out the use of the VP. This has recently motivated an evolution [6] of the HITRAN database [7], which now not only provides the “usual” pressure-broadening coefficients feeding the VP, but also some parameters involved in more refined line-shape parameterizations including the “reference” [8] Hartmann-Tran profile (HTP) proposed in [9].

The parameters involved in refined isolated-line profiles (i.e. generally limited to the speed-dependent pressure-broadening and shifting coefficients and/or the frequency of collision-induced velocity changes) can be deduced from fits of high-quality measured spectra, as more and more frequently done (see Table 3 of [1]). However, since the parameters of interest depend on the considered absorbing molecule and line as well as on the temperature and collision partner (i.e. gas mixture), filling up databases using an approach solely based on experiments is a tremendous task. An alternative and complementary solution is the use of theoretical predictions for which two types of approaches have been proposed. In the first, the parameters are directly computed using the relevant intermolecular potential, which can be done using a variety of approaches, ranging from purely quantum to purely classical ones (see Section 3 of [1]). However, while only the former type of calculation can achieve the high accuracy required, they are so far limited, due to their huge computational cost, to simple colliding pairs (e.g. [10], [11], [12], [13], [14]). In the second approach, the absorption spectrum is directly computed and its fit then provides the line-shape parameters. This can be achieved, for linear molecules, by using the requantized classical molecular dynamics simulations (rCMDS) proposed in [15] with which [16], [17], [18], [19], [20], [21] have shown that very accurate refined line-shape parameters can be obtained without use of any adjusted parameter.

As far as symmetric-top molecules are considered, for isolated lines, very few experimental results are available concerning deviations from the Voigt profile and the parameters of more refined line shapes (see Table 3 of [1] and references therein). Specifically, we can cite the series of papers by Rohart and co-workers in which non-Voigt effects were observed for the symmetric tops CH3X, for a number of pure rotational transitions [22], [23], [24], [25], [26], [27], [28]. The speed dependence of the line pressure broadening was also observed for NH3 lines in [29] and [30]. To the best of our knowledge, except for Ref. [26], no theoretical predictions of non-Voigt effect for isolated lines have been so far made. Concerning line-mixing, several studies were devoted to its experimental study in symmetric-top molecules (e.g. [31], [32], [33], [34], [35], [36], [37]). A number of theoretical models were also proposed in order to represent this phenomenon (e.g. [33], [34], [35]), based either on the Infinite Order Sudden (IOS) or the Exponential Gap (EG) approximations and on the use of some empirical parameters. In addition, theoretical predictions were performed for line mixing in N2-broadened CH3Cl spectra using a semi-classical method [38].

In the present work, we propose an extension, to the case of symmetric-top molecules, of the rCMDS approach proposed in [15] for linear molecules in which line shapes are predicted without use of any adjusted parameter. For its first test, we consider the absorption shapes of methyl fluoride (CH3F) diluted in Ar and He for which experimental results are available for the “usual” pressure-broadening coefficients [32,[39], [40], [41], [42], [43]], for the speed-dependence and velocity-changes [22,23,40,41], as well as for line-mixing effects [32,33]. The remainder of this paper is organized as follows. Section 2 presents the requantized classical molecular dynamics simulations (rCMDS) approach for symmetric-top molecules and the input data used for the calculations of CH3F-Ar and CH3F-He spectra. Section 3 then explains how these spectra are analyzed and the associated results are presented and discussed in Section 4 before some concluding remarks in Sec. 5.

Section snippets

Initialization

We first define the positions of the CH3F atoms in the body-fixed (BF) x, y, z molecular frame, with the center of mass at the origin, the C-F bond along the z axis and one H atom in the (x, z) plane. Then, using randomly chosen Euler angles θm(t = 0), ϑm(t = 0) and ψm(t = 0) for each CH3F molecule m, we compute the quaternions qm,i(t = 0) (i = 0,4) using Eq. (3.35) of [44]. The latter directly yield, through Eq. (3.36) of [44], the rotation matrix Am(t = 0) from the space-fixed (SF) laboratory

Spectra examples

Figs. 1 and 2 present computed [with ω0 = 0 and A(ω + ω0) = 1 in Eq. (2)] spectra of CH3F infinitely diluted in Ar at P = 0.5 atm for a parallel and a perpendicular band, respectively.

In the first case, the dipole (μ0=z) is carried by the C-F bond and its evolution with time is thus independent of the rotation of the molecule around this axis. This implies that, for a given J value, all lines associated with the various K values (from -J to +J) are centered at the same position. The resulting

Results and discussion

The line-shape parameters obtained from fits of the rCMDS-simulated spectra are presented and compared with available experimental values. The pressure broadening coefficients and their speed dependent coefficients are discussed in Section 4.1, while Section 4.2 is devoted to line-mixing effects at high pressures.

Conclusion

Requantized classical molecular dynamics simulations (rCMDS) for symmetric-top molecules were developed and used to compute Ar- and He-broadened spectra of CH3F in parallel and perpendicular bands for a large range of pressure. The pressure-broadening coefficients were then deduced from fits of the simulated spectra at 0.5 atm. Comparisons between the obtained parameters and available measured values of the Lorentz pressure broadening coefficient show a good agreement for both perturbers. Using

CRediT authorship contribution statement

Ngoc Hoa Ngo: Formal analysis, Writing – review & editing. Minh Thu Le: Software. Ha Tran: Conceptualization, Methodology, Writing – review & editing. Jean-Michel Hartmann: Conceptualization, Methodology, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors warmly thank Franck Thibault for providing the experimental and Lorentz-calculated values displayed in Fig. 9 of [33] and reproduced in the right panel of Fig. 8.

References (52)

  • F. Rohart et al.

    Self- and polar foreign gas line broadening and frequency shifting of CH3F: effect of the speed dependence observed by millimeter wave coherent transients

    J Mol Spectrosc

    (1997)
  • J.M. Colmont et al.

    K-Dependence and temperature dependence of N2-, H2-, and He-broadening coefficients for the J = 12–11 transition of acetonitrile CH3C14N located near 220.7GHz

    J Mol Spectrosc

    (2006)
  • M. Guinet et al.

    Experimental studies by complementary terahertz techniques and semi-classical calculations of N2- broadening coefficients of CH335Cl

    J Quant Spectrosc Radiat Transf

    (2012)
  • J. Buldyreva et al.

    Speed dependence of CH335Cl-O2 line-broadening parameters probed on rotational transitions: measurements and semi-classical calculations

    J Quant Spectrosc Radiat Transf

    (2013)
  • A.S. Dudaryonok et al.

    Experimental studies, line-shape analysis and semi-empirical calculations of broadening coefficients for CH335Cl–CO2 submillimeter transitions

    J Quant Spectrosc Radiat Transf

    (2014)
  • V. Gupta et al.

    Line-shapes and broadenings of rotational transitions of CH335Cl in collision with He, Ar and Kr

    J Quant Spectrosc Radiat Transf

    (2015)
  • A.S. Pine et al.

    Self- and foreign-gas-broadened line shapes in the ν1 band of NH3

    J Mol Spectrosc

    (2004)
  • F. Frichot et al.

    Pressure and Temperature Dependences of Absorption in the ν5 RQ0 Branch of CH3Cl in N2: measurements and Modeling

    J Mol Spectrosc

    (1996)
  • I.M. Grigoriev et al.

    Line-mixing effects in the ν3 parallel absorption band of CH3F perturbed by rare gases

    J Quant Spectrosc Radiat Transf

    (1997)
  • H. Tran et al.

    Line mixing in the ν6 Q branches of self- and nitrogen-broadened methyl bromide

    J Quant Spectrosc Radiat Transf

    (2008)
  • H. Aroui et al.

    Self-broadening, self-shift and self-mixing in the ν2, 2ν2 and ν4 bands of NH3

    J Quant Spectrosc Radiat Transf

    (2009)
  • C. Bray et al.

    Line mixing in the QQ sub branches of the ν1 band of methyl chloride

    J Quant Spectrosc Radiat Transf

    (2012)
  • M.M. Beaky et al.

    Hydrogen and helium pressure broadening of CH3F between 1K and 600K

    J Mol Struct

    (1995)
  • I.M. Grigoriev et al.

    Diode laser measurements of He-broadening coefficients in the ν6 band of CH3F

    J Mol Spectrosc

    (1997)
  • M. Lepère et al.

    Collisional broadening coefficients in the ν6 band of 12CH3F perturbed by Ar

    J Mol Spectrosc

    (1998)
  • D. Dhib et al.

    Pressure broadening coefficients in the ν6 band of CH3F in collision with He

    J Mol Spectrosc

    (2018)
  • Cited by (1)

    View full text