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

•Analysis of the speed dependence of relaxation rates on CH3Cl lines.•Examination of the optical diffusion contribution.•Measurement of collisional broadenings with a frequency-modulation technique.

Rotational lines of methyl chloride CH335Cl in collision with He, Ar, and Kr have been recorded at room temperature in the 186-901 GHz range (6.2-30.1 cm−1) for J=6→7, 10→11, 17→18, 22→23, 31→32, 33→34 and K=0-6. Owing to the high sensitivity of the frequency-modulated spectrometer used, clear deviations of the recorded line-shapes from the usual Voigt profile have been observed and their additional analyses with more refined models (Speed-Dependent Voigt, Galatry and Speed-Dependent Galatry profiles) have been performed. It has been evidenced that the observed line-shapes are mainly governed by the speed-dependence of relaxation rates (“wind effect”) and that the optical diffusion related to velocity changing collisions (Dicke effect) has a nearly negligible role.