N2- and O2-broadenings and lineshapes of the 551.53-GHz line of 14NO

For the purpose of atmospheric applications, the collisional relaxation of the J = 5.5 <-- 4.5 rotational line of (NO)-N-14 (551.53 GHz) has been investigated in the 235 to 345-K temperature range. Experiments were performed with N-2 and O-2 as buffer gases, which allow the derivation of the air-induced broadening parameters. Using a video-type spectrometer, small but significant departures from the usual Voigt profile have been demonstrated. These departures are characteristic of narrowed lineshapes: they have been fitted using the speed-dependent Galatry profile that takes account of both the dependence of relaxation rates on molecular speeds and the molecular diffusion via velocity changing collisions. Retrieved broadening parameters, as well as their temperature dependencies, are in agreement with previous determinations dealing with rovibrational lines: they are fairly reproduced by semi-classical calculations that consider exact collisional trajectories. The detailed lineshape properties have been studied thanks to some further experiments using atomic buffer gases, namely He, Ar, and Xe. If we discard the case of He-induced relaxation, theoretical arguments suggest that observed line narrowings are mainly related to the speed dependence of relaxation and that molecular diffusion plays a negligible role. (C) 2001 Academic Press.