Rotationally inelastic scattering of hydrogen fluoride as probed by a hydrogen fluoride chemical laser

Rotationally inelastic collisions of HF are studied using a crossed molecular beams apparatus. The incident HF is rotationally "cold", with > 85% in the lowest three states. Differential cross sections are measured for scattering into a single HF rotational state by Ar. The rotational state is detected by using a line-tunable cw chemical laser operating at 2.6μm, driven by the F + H₂ → HF + H reaction. The laser beam rotates about the HF+Ar collision zone in tandem with a cryogenic bolometer detector, which is sensitive to the additional vibrational energy conveyed by HF molecules that are excited by the laser. We observe a rotational rainbow effect for highly excited rotational states, corresponding to strongly repulsive scattering of Ar off the repulsive core of the HF. In addition, we observe a novel "shoulder" for scattering into the rotationless state. "Exact" quantum close-coupling calculations, fully averaged over instrumental parameters, succesfully reproduce this feature, whereas classical trajectories fail completely. Semi-classical calculations qualitatively reproduce the quantum results and suggest that the shoulder is due to a delicate balancing of attractive and repulsive contributions to the scattering.