Laser probing of rotational-state-dependent velocity distributions of N-2(+) (nu('')=0,J) drifted in He

Rotational state-resolved velocity component distribution functions of N-2( +) (nu' = 0) drifted in helium are measured in a flow-drift apparatus by th e 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 (nu',nu ") = (0,0) band of the B (2)Sigma(u)( +)-X (2)Sigma(g)(+) system at 390 nm, with the laser propagation direction set either parallel or perpendicular to the drift field. A small but defini te (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 K-0(0) of 22.0 cm (2) V-1 s(-1) and are in good agreement with earlier arrival-time measureme nts. Parallel translational temperatures are found to be significantly high er than perpendicular temperatures; a difference of at least 140 K between these temperatures is measured for J = 15.5 at 16 Td. No discernible differ ence between the parallel translational temperatures for different rotation al states is observed. There is evidence for a small degree of positive ske wness (third central moment) in the parallel velocity component distributio ns, the first observation of such an effect in a molecular ion-atomic buffe r system. Previous results that indicated poor agreement between CO+-He pul sed-field arrival-time and LIF mobilities are discussed; the LIF results we re most likely hampered by space-charge effects. (C) 2000 American Institut e of Physics. [S0021-9606(00)01021-7].