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

Abstract

Rotational state-resolved velocity component distribution functions of N 2 ϩ (Љϭ0) drifted in helium are measured in a flow-drift apparatus by the technique of single-frequency laser-induced fluorescence ͑LIF͒. A single-mode ring dye laser is used to probe Doppler profiles of various rotational lines in the R-branch of the (Ј,Љ)ϭ(0,0) band of the B 2 ⌺ u ϩ -X 2 ⌺ g ϩ 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 Jϭ13.5 to Jϭ22.5 is observed at a fixed field strength of 12 Td. Mobilities of Jϭ15.5 measured over the range of 1.5-16 Td yield a K 0 (0) of 22.0 cm 2 V Ϫ1 s Ϫ1 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 Jϭ15.5 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 moment͒ 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 ϩ -He pulsed-field arrival-time and LIF mobilities are discussed; the LIF results were most likely hampered by space-charge effects.

Modeling of the N2+ ion in cold helium plasma: dynamics of N2+/He collisions and cross-sections

“…The dependence of calculated cross-sections on the initial rotational-vibrational excitation of the N + 2 ion has been analyzed as well as the effect of the polarization of the N + 2 angular momentum, a well-known phenomenon in systems with strongly anisotropic interactions [45]. In addition, state-of-the-art experimental data [40,41,42] on the mobility of N + 2 in helium have been inverted via an inverse-method (IM) approach [37] to get "experimental" estimates of the NRS cross-sections for an assessment of the reliability of calculated data and validation of employed computational approaches.…”

“…[41], and dashdotted line -IM cross-sections based on Ref. [42]. In the inset, related experimental mobilities are provided for comparison (circles Ref.…”

“…A survey of the most extensive experimental data on the N + 2 /He mobility [40,41,42] is provided in the inset of Fig. 1, related type 1 MTCSs derived from the IM are shown in the main panel of the figure, and for comparison, the N + 2 /He mobility values calculated from the IM cross-sections are also shown in the inset.…”

3 Optical studies of ion–molecule reactions

“…These studies are of considerable importance in understanding the coupling between rotational and translational motion of ions, the effects of potential surfaces on internal state distributions and relating drift field measurements to their thermal counterparts. For both N 2 1 , 249 and CO 1 , 251 a rotational state dependence to the ion mobility was reported. The incorporation of polarisers in the LIF detection has also allowed the determination of the rotational alignment of the ions as a function of both rotational state and velocity sub-group for N 2 1 drifting in He.…”

Ion Dynamics Related to Hypersonics

“…21 " 23 Using Doppler resolved laser-induced fluorescence, we have examined the velocity distributions and molecular alignment of ions in drift fields. 17 " 20 In this last funding period, we have completed (1) an experimental and theoretical study of the mobilities of several cluster ions drifting in helium and in nitrogen, 26 (2) measurement of the rotational-state-dependent velocity distributions of N 2 + drifted in helium, 27 and (3) detailed characterization of the alignment of N 2 + in helium. 28 …”