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.
Results are presented for Doppler-resolved laser-induced fluorescence measurements of collision-induced rotational alignment of N2+(v″=0) drifted in He in a drift-tube apparatus. A single-frequency ring dye laser is used to probe the R1(N′′=15) rotational line of the B 2Σu+–Χ 2Σg+ system both parallel and perpendicular to the drift field at three different field strengths and at several different Doppler-selected velocities. A strong correlation is found between the degree of rotational alignment and the velocity subgroup probed along the field direction. For field strengths of 8 and 16 Td and laser probe parallel to the drift field, there is a monotonic increase in the quadrupole alignment parameter A0(2) with higher velocity subgroup, up to a maximum value of A0(2)=−0.150 (6) for 16 Td at the high-velocity tail. There is evidence that the correlation between alignment and velocity increases with increasing field strength. The mechanisms of the alignment are discussed and these results are attributed primarily to the change in anisotropy of the relative velocity vector distribution of the N2+–He pair with field strength.
For the generation of spin-polarized photocurrents in topological insulators, a coupling between photon angular momentum and electron spin is often assumed. Such a coupling seems to be supported by dichroism reported in E(k y)-intensity maps in photoemission. We show in three dimensional two-photon photoemission and one-step photoemission calculations that the circular dichroism is in fact threefold in E(k x , k y) maps although it may appear antisymmetric in E(k y). The threefold symmetry is inconsistent with the previously assumed coupling between photon momentum and electron's chiral spin via the orbital momentum. Instead it reflects the surface point group. The only antisymmetric patterns appear in the energy range in which surface and bulk states hybridize. In general, a threefold-symmetric dichroic signal does not support unidirectional photocurrents. Nevertheless, the residual asymmetry of up to 3.5% in our photoemission spectra is compatible with previously observed helicity-dependent photocurrents.
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