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 inve
stigated 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-ind
uced broadening parameters. Using a video-type spectrometer, small but sign
ificant departures from the usual Voigt profile have been demonstrated. The
se departures are characteristic of narrowed lineshapes: they have been fit
ted using the speed-dependent Galatry profile that takes account of both th
e dependence of relaxation rates on molecular speeds and the molecular diff
usion via velocity changing collisions. Retrieved broadening parameters, as
well as their temperature dependencies, are in agreement with previous det
erminations 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 obse
rved line narrowings are mainly related to the speed dependence of relaxati
on and that molecular diffusion plays a negligible role. (C) 2001 Academic
Press.