Rotational state-to-state differential cross sections for the HCl–Ar collision system using velocity-mapped ion imaging

Abstract

Rotational state-resolved differential cross sections (DCSs) for the j-changing collisions of HCl by Ar are presented. A new crossed molecular beam velocity-mapped imaging apparatus is used to measure the full (θ=0–180°) DCS for j=0→j′=1, 2, ..., 6 rotational energy transfer at a center of mass energy of ∽538 cm⁻¹. The j=0 state accounts for over 97% of the initial HCl rotational state population, and the scattering products are state-selectively ionized via (2+1) resonance enhanced multi-photon ionization through the E state, allowing for the direct extraction of state-to-state DCSs in the center of mass frame. The angular distributions for the experimental DCSs become increasingly backscattered as Δj increases, but do so non-monotonically, as j′=3 is more forward scattered than j′=2. Images for the even Δj 0→2 and 0→4 are similar, and those for the odd Δj 0→1 and 0→3 also have similarities. The calculated cross sections, based upon the HCl–Ar H6(4,3,0) potential of Hutson [J. Phys. Chem., 1992, 96, 4237], agree qualitatively with the experimental cross sections. However, there are significant differences between the theoretical and experimental results, where many of the principal features in the calculated DCSs lie 10–30° more backscattered than the same features in the experimental DCSs. These results may suggest that an adjustment to the repulsive region of the H6(4,3,0) potential is required.