Lineshapes of the 172 and 602Â GHz rotational transitions of HC15N

Experimental lineshapes of the 172 and 602 GHz millimeter lines of HC15N in collision with H2, N2, O2, CH3CN and some rare gases are studied for the purpose of atmospheric applications and detailed collision analysis. Using of a sensitive frequency modulation technique allows highlighting clear deviations from the usual Voigt profile, these departures being generally considered as related to molecular diffusion effects or to the dependence of collisional relaxation rates on molecular speeds. Except the light buffer gases H2 and He, the linefits using the Galatry profile lead to nonlinear pressure dependencies of relaxation rates, that rules out the frequent hypothesis of an exclusive role of molecular diffusion (Dicke effect). This observation is shown to be in accordance with the fact that optical radii related to relaxation are usually greater than kinetic Lennard-Jones radii tied to diffusion. In contrast, the actual lineshapes are well reproduced by the Speed-Dependent Voigt profile taking into account the speed dependence of relaxation rates which display linear pressure dependencies. For the particular cases of the N2- and Ar-HCN pairs, the experimental results are rather well explained by semi-classical computations based on the Robert-Bonamy formalism improved by the model of exact trajectories. These computations show that relaxation rates are proportional to some power of the colliders' relative speed. A detailed comparison of relaxation parameters deduced from low-pressure experiments with Galatry and Speed-Dependent Voigt profiles allows to infer that the optical diffusion rates are much smaller that kinetic ones, so that the experimental lineshapes depend nearly exclusively on the speed dependence of relaxation rates and are weakly affected by molecular diffusion effects. Extension of these conclusions to other molecules of atmospheric interest is discussed. Finally, an appendix presents unpublished results dealing with the collisional relaxation of some rotational lines of HC15N (at 258 GHz for different temperatures and at 344 GHz) and HC14N (at 355 GHz).