Inelastic Collisions Between Excited Alkali Atoms and Molecules IX. An Isotope Effect in the Cross Sections for 6²P_1/2↔6²P_3/2 Mixing in Cesium, Induced in Collisions with Deuterated Methanes

Abstract

The temperature dependence of cross sections for 62P1/2 ↔ 62P3/2 mixing in cesium, induced in collisions with CH4, CH3D, CH2D2, CHD3, and CD4 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 cm2, 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 CF4, 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.

Temperature dependence of cross sections for 72P1/2 ↔ 72P3/2 excitation transfer and for quenching in cesium, induced in collisions with H2, N2, CH4, and CD4

“…The large sizes of the cross sections Q~~ suggest that the energy defect between the fine structure states is interconverted not only with the kinetic energy of relative motion as it is in the case of the collisions with noble gas atoms, but also with molecular rotational energy. A similar effect had been found previously in the cases of collisional transfer between the resonance fine-structure states in Rb (16) and Cs (2). The molecular vibrational states probably do not participate in the mixing process, because none of the molecules has vibrational levels at energies close to the 72P fine-structure splitting.…”

“…for collisions of cesium atoms with N2, Hz, CH,, and CD,. The various cross sections exhibit a variation with temperature, which is about as extensive as in the case of the cesium resonance doublet (2,13). Their values exceed by some two orders of magnitude the corresponding cross sections for collisions with noble gas atoms except for helium (3,14), and are comparable with cross sections reported by Rocchiccioli (15) for transfers from C S~~P~,~ to 8?P31?, 72D312, and 72D512 induced in collisions with N2 and H2, bearing in mind differences in the energy defect AE.…”

“…Equat~ons [2] and [3] are strictly valid only if imprisonment of radiation is negligible. Radiation trapping has been avo~ded by working at a cesium vapor pressure lower than 4.5 x lo-' Torr, at which it was found to be absent.…”

“…The cross sections for excitation transfer between the 6'P resonance fine-structure substates in cesium, induced in collisions with noble gas atoms, exhibit a pronounced temperature dependence ( I ) which becomes considerably less pronounced when molecules are used instead as collision partners (2). The cross sections for the very much less adiabatic 7'PI,' o 7'P3,2 transfers were also found to vary somewhat with temperature (3).…”

“…The theoretical treatment of electronic-to-rotational energy transfer proposed by Baylis and co-workers (2,4) can be applied to collisions with polyatomic molecules such as the methanes. The interaction between the excited alkali atom and the methane molecule is given by a time dependent function V(t) of the electron coorlong-range dependence as the van d e r Waals interaction, varying as R-6.…”