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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.

The line strenghts of H 2 O bands ν 1 + ν 2 + ν 3 and ν 2 + 2 ν 3 have been measured using a high-resolution Nd-glass single-mode laser spectrophotometer. The parameters of 33 lines were found by fitting a Voigt contour to experimental line profile. The vibration-rotation interaction affecting the line strength was taken into account by using the F factor in the three-parametric form, and its constants for ν 1 + ν 2 + ν 3 and ν 2 + 2 ν 3 bands were found. There is a good agreement between line intensities calculated using these constants and the experimental values.

The high resolution spectrophotometer developed on the basis of a single-mode running wave Nd-glass with line frequency continuously tuned during a generation pulse is described. The overlapping lines of water vapor near 9257.1, 9325.1 and 9343.6 cm -1 were resolved and their strengths and positions were measured. The air broadening parameters of 9304.4 and 9305.4 cm -1 were determined.