Laser-induced fluorescence measurements of rotationally resolved velocity distributions for CO+ drifted in He

Abstract

Measurements of ion-velocity distributions of CO+ in a He buffer gas are presented as a function of an applied electric field. The distributions are obtained by single frequency, laser-induced fluorescence from various initial rotational states with the laser beam propagating parallel and perpendicular to the drift velocity vector. All distributions are well represented by a Maxwellian for the observed E/N range of 0–13 Td. The reduced mobilities, calculated from the shift of the mean velocity as a function of electric field, increase from 18.7±1.0 cm2 V−1 s−1 at very low fields to 26.4±0.7 cm2 V−1 s−1 at 13 Td. From the width of the Doppler profiles, translational ‘‘temperatures’’ are calculated, which are compared to simple attractive and repulsive Maxwell models as a function of the field. The measured values disagree with the predictions, which are well established for atomic ion systems. The differences are discussed in terms of rotationally inelastic energy transfer in the collisions, which is predicted by kinetic theory models. This argument is strengthened by the fact that even though the rotational states rapidly equilibrate, measurements on different lines yield higher temperatures for higher rotational levels. Finally, the small influence of selective quenching of the electronically excited CO+ on the Doppler profiles is demonstrated by measuring effective lifetimes as a function of the applied drift field.