Probing Low-Energy Resonances in Water-Hydrogen Inelastic Collisions.

Molecular scattering at collisional energies of the order of 10-100  cm^{-1} (corresponding to kinetic temperatures in the 15-150 K range) provides insight into the details of the scattering process and, in particular, of the various resonances that appear in inelastic cross sections. In this Letter, we present a detailed experimental and theoretical study of the rotationally inelastic scattering of ground-state ortho-D_{2}O by ground-state para-H_{2} in the threshold region of the D_{2}O(0_{00}→2_{02}) transition at 35.9  cm^{-1}. The measurements were performed with a molecular crossed beam apparatus with variable collision angle, thence with variable collisional energy. Calculations were carried out with the coupled-channel method combined with a dedicated high-level D_{2}O-H_{2} intermolecular potential. Our theoretical cross section 0_{00}→2_{02} is found to display several resonance peaks in perfect agreement with the experimental work, in their absolute positions and relative intensities. We show that those peaks are mostly due to shape resonances, characterized here for the first time for a polyatomic molecule colliding with a diatom.