Rotational effects in collisionally induced fine-structure transitions

Cross sections for transitions between the P_3/2 and P_1/2 levels of cesium induced by collisions with isotopes of methane are found to be several orders of magnitude larger than with krypton, with which one might expect similar interaction potentials. This enhancement is explained by a semiclassical theory in terms of Fourier components which arise from the rotation of CH₄ and which lie close to the transition frequency. The theory predicts the experimentally found isotope effect.