Laser probing of velocity-subgroup dependent rotational alignment of N-2(+) drifted in He

Results are presented for Doppler-resolved laser-induced fluorescence measurements of collision-induced rotational alignment of N-2(+)(v ''=0) drifted in He in a drift-tube apparatus. A single-frequency ring dye laser is used to probe the R(1)(N ''=15) rotational line of the B (2) Sigma(u)(+)-X (2) Sigma(g)(+) 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 A(2) With higher velocity subgroup, up to a maximum value of A(0)((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 N-2(+)-He pair with field strength. (C) 1997 American Institute of Physics.