Observation of anomalous light-induced drift

In a two-component gas consisting of light-absorbing particles immersed in a buffer gas, velocity-selective excitation combined with a state-dependent kinetic collision frequency results in light-induced drift. Standard theory predicts that the drift velocity as a function of frequency detuning should have a dispersion-curve-like behavior. This was indeed found experimentally for the molecular species studied so far. We report a dramatic deviation from this behavior for ${\mathrm{C}}_2$ ${\mathrm{H}}_4$, excited into its ${\nu }_7$ vibrational mode by a ${\mathrm{CO}}_2$ laser, with Kr as a buffer gas. The extent of the deviation is found to depend strongly upon the rotational sublevels involved. A possible explanation in terms of a three-level model is attempted but found to be unlikely.