Single frequency laser probing of velocity component correlations and transport properties of Ba+ drifting in Ar

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Abstract
Velocity distributions for Ba+ ions drifting in argon under the influence of an external electric field are measured at directions of 0°, 45°, and 90° with respect to the electric field using single frequency laser‐induced fluorescence probing. Values for the reduced mobility, translational temperatures (second central moments), skewness (third central moment), and excess (fourth central moment) are presented as a function of field strength up to E/N values of 201 Td, which corresponds to a mean Ba+/Ar center‐of‐mass collision energy of 0.371±0.038 eV. Maxima are observed in both the reduced mobility, 2.40±0.05 cm2 V−1 s−1 at 160 Td, and in the skewness, 0.95±0.03 at 119 Td. The dimensionless skewness parameter characterizes the asymmetry of the velocity distributions and is the ratio of the cube root of the third central moment to the square root of the second central moment. A comparison of the moments of the measured 45° velocity distribution to a distribution synthesized without correlation from the 0° and 90° distributions shows that a positive correlation exists between velocity components parallel and perpendicular to the electric field. This is the first experimental verification of velocity component correlation in drifting ions.