Inelastic Collisions Between Excited Alkali Atoms and Molecules IX. An Isotope Effect in the Cross Sections for 6²P1/2↔6²P3/2 Mixing in Cesium, Induced in Collisions with Deuterated Methanes

The temperature dependence of cross sections for 6 2 P 1/2 ↔ 6 2 P 3/2 mixing in cesium, induced in collisions with CH 4 , CH 3 D, CH 2 D 2 , CHD 3 , and CD 4 molecules, has been investigated in a series of sensitized fluorescence experiments over a temperature range 290–650 K. The various cross sections which are of the order of 10 −15 cm 2 , and which exceed similar cross sections for cesium–noble gas collisions by 4 – 6 orders of magnitude, exhibit differences in their variation with temperature. This isotope effect in the collision cross sections is interpreted on the basis of a novel semiclassical theory of electronic to rotational energy transfer. The cross section for mixing induced by collisions with CF 4 , which was determined in a subsidiary experiment, and which is 2–3 times larger than the methane cross sections, does not show comparable behavior with temperature, probably because the energy transfer takes place to closely lying molecular vibrational states.