Differential cross sections for rotationally state-resolved inelastic scattering of HF by argon

We present differential cross section (DCS) measurements for scattering of HF by Ar. These crossed-beam experiments employ rotational state sensitivity, allowing determination of the DCS as a function of the scattered HF rotational state. The initial HF rotational distribution is generated by nozzle expansion, without further state selection. Its composition is mostly J = 0 and J = 1, with small admixtures for J > 1. The DCS for each final state J′ is measured using a stabilized cw HF chemical laser, in conjunction with a rotatable liquid He-cooled bolometer. Measurable signals are obtained for scattering into 0≤J′≤5, where J′ = 6 is the thermodynamic limit for our collision energy of 120 meV. The measured DCS's show a strong forward peak, largely from elastic scattering. In addition, the DCS's evolve from a broad shoulder in the θ≈25°-40° region for J′ = 0 - through a flattening of the wide-angle scattering for J′ = 2 and J′ = 3 - to an increase in the scattering beyond ∼40° for J′ = 4. The DCS for scattering into J′ = 5 also shows increased intensity at wide scattering angles, but its onset is delayed until ∼70°. These features are shown to be independent of the laboratory → center-of-mass kinematic transformation. The wide-angle scattering into J′ = 4 and J′ = 5 corresponds to transferring up to 40% and 60%, respectively, of the available kinetic energy into HF rotation. Since the center-of-mass scattering angles are up to ∼110°, we interpret the observed features for J′ = 4-5 in terms of rotational rainbow scattering from the hard core of the HF + Ar potential energy surface. The origin of the shoulder for J′ = 0 scattering is less clear, but it may arise from the strongly anisotropic nature of the HF + Ar van der Waals attraction.