Laser probing of ion velocity distributions in drift fields: Parallel and perpendicular temperatures and mobility for Ba+ in He

Abstract

Measurements of ion velocity distributions are presented for Ba+ drifted in helium under well characterized conditions using single-frequency laser-induced fluorescence probing. We present the reduced mobilities and the Doppler profiles parallel and perpendicular to the electric field vector as a function of the ratio of the field strength (E) to the buffer gas density (N) up to 33.5 Td. The reduced mobility decreases monotonically with increasing E/N from the zero-field value of 16.7±0.4 cm2 V−1 s−1 at 313 K. The parallel and perpendicular ion temperatures are in very good agreement with both a repulsive Maxwell model and a parametrized version of the three-temperature theory of Lin et al. The parallel temperature is always higher than the perpendicular one. Effects of optical pumping on the Doppler profiles are also presented.