LASER-INDUCED FLUORESCENCE MEASUREMENTS OF DRIFT-VELOCITY DISTRIBUTIONS FOR BA+ IN AR - MOMENT ANALYSIS AND A DIRECT MEASURE OF SKEWNESS

Velocity distributions for Ba+ ions drifting in argon under the influence of an electric field are measured using single-frequency, laser-induced fluorescence techniques. A moment analysis of these data yields values for the ion mobilities, temperatures, and, for the first time, skewness parameters of the ion velocity distributions. Doppler profiles of the drifting ions are obtained with the laser beam propagating both parallel and perpendicular to the electric field direction as a function of E/N up to 148 Td. The velocity-component distributions perpendicular to the electric field direction are nearly Maxwellian at all E/N values studied, but the parallel distributions become noticeably skewed toward high velocities at the largest electric field strengths. The reduced mobilities for Ba+ in Ar, obtained from the first moments of the parallel velocity-component distributions, increase from 1.80±0.16 cm2 V-1 s-1 at zero field to 2.38±0.11 cm2 V-1 s-1 at 148 Td. The second central moments, which describe the widths of the distributions, increase monotonically with increasing E/N values for both the parallel and perpendicular distributions. Temperatures derived from the second central moments agree well with those calculated from a parametrized version of the three-temperature theory [M. Waldman and E. A. Mason, Chem. Phys. 58, 121 (1981)]. The third central moments of the parallel velocity-component distributions characterize the asymmetry of the distributions. Skewness parameters derived from the third central moments increase with electric field strength over the range of E/N studied. © 1990 American Institute of Physics.