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

摘要

Rotational state-resolved velocity component distribution functions of N-2(+) (nu' = 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 (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 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 K-0(0) of 22.0 cm(2) 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. (C) 2000 American Institute of Physics. [S0021-9606(00)01021-7]. [References: 55]