Imaging inelastic scattering of CO with argon: polarization dependent differential cross sections

Abstract

Rotationally inelastic scattering of carbon monoxide (CO) with Argon at a collision energy of 700 cm⁻¹ has been investigated by measuring polarization dependent differential scattering cross sections (PDDCSs) for rotationally excited CO molecules using a crossed molecular beam apparatus coupled with velocity-map ion imaging. A simple and robust (1 + 1′ + 1′′) VUV (Vacuum Ultra-Violet) REMPI (Resonance Enhanced Multi Photon Ionization) scheme is used and images are obtained by setting the VUV light polarization direction parallel or perpendicular to the scattering plane. Clear differences between the images for the two polarizations are observed, indicating strong collision induced alignment of the rotational angular momentum of scattered CO. A direct image analysis procedure as described in our previously published paper (A. G. Suits et al., J. Phys. Chem. A., 2015, 119, 5925), is employed to extract the fully quantum state resolved alignment-free differential cross sections (DCSs) and the state-to-state angle-dependent alignment moments for each final rotational state. The experimental results are compared with advanced theory, in particular with the predictions of CC QM (Close-Coupling Quantum Mechanical) and QCT (Quasi-Classical Trajectory) calculations. The agreement between experiment and theory is generally found to be quite good throughout the entire scattering angle range for all the final states probed, showing the reliability of the experiment and use of the direct extraction method, as well as the accuracy of the potential surface over the studied collision energy range. A classical kinematic apse (hard shell) model was found to be useful in interpreting the measured collision induced alignment moments.