Laser probing of rotational-state-dependent velocity distributions of N2+ (ν″=0,J) drifted in He

Rotational state-resolved velocity component distribution functions of N2+ (ν″=0) drifted in helium are measured in a flow-drift apparatus by the technique of single-frequency laser-induced fluorescence (LIF). A single-mode ring dye laser is used to probe Doppler profiles of various rotational lines in the R-branch of the (ν′,ν″)=(0,0) band of the B 2Σu+–X 2Σg+ system at 390 nm, with the laser propagation direction set either parallel or perpendicular to the drift field. A small but definite (3%) increase in ion mobility with increasing rotational state from J=13.5 to J=22.5 is observed at a fixed field strength of 12 Td. Mobilities of J=15.5 measured over the range of 1.5–16 Td yield a K0(0) of 22.0 cm2 V−1 s−1 and are in good agreement with earlier arrival-time measurements. Parallel translational temperatures are found to be significantly higher than perpendicular temperatures; a difference of at least 140 K between these temperatures is measured for J=15.5 at 16 Td. No discernible difference between the parallel translational temperatures for different rotational states is observed. There is evidence for a small degree of positive skewness (third central moment) in the parallel velocity component distributions, the first observation of such an effect in a molecular ion-atomic buffer system. Previous results that indicated poor agreement between CO+–He pulsed-field arrival-time and LIF mobilities are discussed; the LIF results were most likely hampered by space-charge effects.