K-dependence of broadening coefficients for CH₃F-N₂ and for other systems involving a symmetric top molecule

The variations of N-broadening coefficients of CHF with the rotational quantum number K have been studied in the v and v bands, using a diode-laser spectrometer. Four different line families with same J values ranging from 3 to 17, and same ΔJ and ΔK values have been recorded in the spectral range 1478-1573 cm. The collisional halfwidths are obtained by fitting Voigt and Rautian profiles to the measured shapes of the lines. A semi-classical calculation of the N broadening has provided results that are in overall agreement with the experimental data. The general trend for the broadening coefficients belonging to the same J transition is to decrease with increasing K; this decrease is more significant at low J than at high J values. A similar K behavior has been observed and predicted for other binary systems: CHF self-perturbed or perturbed by O, He, Ar; CHD self- perturbed or perturbed by N, O. A different K behavior is observed for the broadening coefficients of a symmetric top molecule having an inversion motion such as NH. (C) 2000 Elsevier Science B.V.