Self and polar foreign gas line broadening and frequency shifting of CH3F: Effect of the speed dependence observed by millimeter-wave coherent transients

The shape of the (J, K: 2, 1 <-- 1, 1) millimeter line of CH3F in collision with polar buffer molecules has been investigated in the temperature range 140-300 K. The experiments exploit a Stark switching coherent transient technique, namely the optical free precession phenomenon, the Fourier transform of which is the usual steady state absorption lineshape. Using various buffer gases (CH3Br, CH3F, and NH3), the observed time domain signals provide the first experimental evidence in the millimeter range that line broadenings as well as frequency shiftings depend on the relative speed of collision partners; that is, lineshapes can become narrowed and asymmetric according to the molecular mass ratio and the type of collisional interaction involved. The experimental signals are analyzed with a time domain speed-dependent Voigt profile: for the polar buffer molecules considered, it is shown that a simultaneous interpretation of the broadening and narrowing parameters as well as of their temperature dependence can be satisfactorily obtained only with a realistic collision theory; in contrast with atomic buffer gases, velocity changing collisions play a negligible role. (C) 1997 Academic Press.