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Approximately seventy isolated and overlapping methane absorption lines in the 5647 − 6164 cm−1 spectral region were processed with seven line profiles accounting for all main line shape forming physical mechanisms in different combinations. It was shown that at low methane pressures the conventional Nelkin–Ghatak line profile gives satisfactory results in retrieving line parameters as opposed to the Voigt profile. It was found that most of the considered lines do not interfere. The FTS instrumental function elimination technique based on Doppler-broadened line's records was applied, and its applicability was proved.

The physically grounded parameterization of a line profile including the speed-dependence was performed. It was shown that two actual parameters of the quadratic speed-dependence appear in gas mixtures instead of a single parameter in a pure gas. One of the parameters is associated with hard elastic velocity-changing collisions; the second is connected with the other sorts of collisions. For comparable concentrations of gas species, they may differ by 50% and depend nonlinearly on partial gas pressures. The dimensionless line narrowing parameter also reveals nonlinear pressure-dependence. The computational expressions for the line profile including all main physical mechanisms of its forming in conditions of gas mixtures are derived.

A theory for molecular nonlinear absorption resonance in a standing electromagnetic wave recorded with the use of frequency-modulated radiation is developed. Spectral line profiles for the second harmonic of modulation frequency derived for different actual cases have been tested by comparison with the experimental ones recorded for absorption lines of OCS and CO molecules in the mm-wave region. The derived line profiles describe all the qualitative features of the experimentally observed spectra including the apparent splitting of a nonlinear resonance at high modulation frequency and allow retrieving quantitative data on the collision line broadening coefficients.

An application of one-dimensional velocity approach to calculation of speed-dependent spectral line profiles was considered. It was shown that a mean deviation of the line profile obtained within this approach from the line profile derived with integrating over three projections of an absorbing molecule's velocity does not exceed 1.1% at mass ratios of perturbing and absorbing molecules ≤9. Analytical approximate expressions for speed-dependent line profiles, including spectral line narrowing and mixing, were obtained for one- and three-dimensional velocity approaches.

Systematic errors inherent in the Voigt line profile are analyzed. Molecular spectrum processing with the Voigt profile is shown to underestimate line intensities by 1–4%, with the errors in line positions being 0.0005 cm−1 and the decrease in pressure broadening coefficients varying from 5% to 55%.

Line profiles expedient for the description of the simultaneous action of line mixing, narrowing, and dependence of collision relaxation constants on molecular speed have been constructed. It is shown that negligence of the mixing of overlapped lines and speed-dependence in the case when masses of absorbing and perturbing molecules are comparable leads to systematic errors in the principal absorption line parameters in tens of percent.

Algebraic approximations for line profiles calculated on the basis of quantum-mechanical collision integral kernels for dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole intermolecular interaction potentials were obtained. In derivation of the profiles velocity-changing collisions of molecules with scattering on small and large angles also with the speed-dependence of collision relaxation constants have been taken into account. It was shown that the relative contribution of small-angle collisions into the frequency of elastic velocity-changing collisions is much more pronounced for long-range potentials. A sensitive criterion for analysis of a line narrowing was proposed and tested.

Analytical approximate expressions for speed-dependent Rautian-Sobel'man, Voigt, and general line profiles, including spectral line mixing, were obtained. Systematic errors of the approximate profiles are within 0.7% for the mass ratios of perturbing and absorbing molecules mb/ma≤9. The calculation time of the approximate line profiles is much shorter than the time required for calculating exact expressions for the line profiles. An approximate analytical expression for the observable homogeneous line width was derived. A least-square fitting of the singlet methane absorption line centered at 6107.17 cm−1 was performed, using the approximate line profiles obtained.

Analytical expressions for line profiles of mixed doublets and multiplets were derived in the model of hard velocity-changing collisions which describe correctly both collision line mixing and narrowing. It was shown that for multiple absorption the line mixing leads to enhancement of the line narrowing. The line asymmetry and nonlinear dependence of the doublet line shift on buffer gas pressure associated with narrowing and mixing have been considered. The regions of pressure and other parameters where the line shift can change its sign, were found by numerical calculations.