Velocity-dependent collision rates from light-induced drift experiments:C2H4–noble-gas mixtures

Light-induced drift arises in a binary mixture when velocity-selective optical excitation is combined with a state-dependent kinetic collision frequency (or cross section). This phenomenon is studied experimentally for rovibrationally excited ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ immersed in the atomic buffer gases Ne, Ar, Kr, or Xe at temperatures between 275 and 500 K. In several cases, the laser-frequency dependence of the drift is observed to be anomalous and to change dramatically with temperature. This means that the relative change in collision frequency upon excitation depends strongly on velocity. To evaluate the data, an analytic model description is presented and applied to the experimental data. It is found that for each particular buffer gas, all data are well described with a relative difference in collision rate depending on a single parameter only, i.e., the average relative velocity. This dependence is found to be different for the various noble gases. This suggests that the attractive part of the interaction potential is crucial for the occurrence of anomalous light-induced drift.