Scattering dynamics in HF+He, Ne, and Ar: State-to-state cross sections, Dopplerimetry, and alignment measurement via direct infrared laser absorption in crossed supersonic jets

Absolute state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of HF with He, Ne, and Ar at mean center-of-mass collision energies of 480, 390, and 350 cm(-1), respectively. HF seeded in Ar diluent gas is cooled into the J=0 ground rotational state in a pulsed supersonic expansion, followed by single collision rotational excitation with rare gas atoms from a second pulsed supersonic jet. The column-integrated densities of HF in both the initial and final scattering slates are probed in the jet intersection region via direct absorption of light from a narrow bandwidth (0.0001 cm(-1)), continuously tunable, color center laser. Total inelastic cross sections for collisional loss out of J=0 and collisional excitation into J>0 states are determined in absolute units from the dependence of infrared absorption signals on collider gas concentration, Full close coupling scattering calculations are performed on several ab initio and empirical potential energy surfaces for each of the three HF+rare gas systems. Agreement for He+HF and Ar+HF integral cross sections is remarkably goad, but significant discrepancies are noted for the less accurately determined Ne+HF surface, Photoelastic polarization modulation of the IR laser is used to probe for rotational alignment in the scattered HF flux; the measurements set an upper polarizance limit for collisionally populated J=1 HF molecules [probed on P(1)] of /P/<2%. High resolution IR laser Doplerimetry reveals velocity structure in the collisionally excited J=1 Doppler profiles, which is in excellent qualitative agreement with theoretical predictions of rainbow features in the J=1<--0 state-to-state differential cross section. (C) 1997 American Institute of Physics.