Molecular fly-by collisions take little time, something like 10 s. 
Optical transition of collisional complexes of molecules generate spectral 
"lines" that are very broad - roughly five orders of magnitude broader 
than the most familiar "ordinary" spectral lines (Heisenberg's uncertainty 
relation). The resulting spectral "lines" usually strongly 
overlap so that collision-induced spectral bands typically appear as continua 
(as opposed to the bands of often discernible lines of ordinary molecules).
Collision-induced spectra appear at the frequencies of the rotovibrational and 
electronic transition bands of the unperturbed molecules, and also at sums and 
differences of such transition frequencies: simultaneous transitions in two (or 
more) interacting molecules are well known to generate optical transitions of
molecular complexes.