THE EFFECT OF INITIAL VELOCITY ON ROTATIONAL ENERGY-TRANSFER IN (CH3F)-C-13

The dependence on initial velocity of collisionally-induced rotational transitions in CH3F has been studied by infrared-infrared four-level double resonance experiments in which the offset Of the pump laser was varied in order to vary the velocity component in the direction of th e pump beam of the excited molecules. For this purpose, the frequency of the carbon dioxide pump laser was stabilized to a saturation dip (L amb dip) in the Stark spectrum in a CH3F sample in a cell outside the laser cavity, which provided a tunability of approximately 30 MHZ for each laser line. It was found that the rms change in a velocity compon ent during collisionally-induced transitions is roughly proportional t o the rms relative speed of the colliding molecules. Velocity dependen ce of the rates of rotational energy transfer has also been observed. Classical elastic scattering theory has been used to calculate the one -dimensional collision kernel at different initial values of the compo nent of the velocity of the active molecule in the direction of the be am. The results of this calculation do not agree with the qualitative features of the observation.