Rotational excitation of HDO and D₂O by H₂: Experimental and theoretical differential cross-sections

Abstract : We present state-to-state differential cross sections (DCSs) for rotationally inelastic scattering of HDO by normal- and para-H₂ at collision energies of 580 cm^-1 and 440 cm^-1. (2+1) resonance enhanced multiphoton ionization is used to detect rotationally cold HDO molecules before collision and as scattering products, which occupy higher rotational states due to collision with H₂. Relative integral cross sections of HDO are obtained by integrating its DCSs measured at the same experimental conditions. Experimental and theoretical DCSs of HDO scattered by normal- and para-H₂ are in good agreement in 30°-180° range of scattering angles. This partial agreement shows the accuracy of the recently tested potential of H₂O-H₂, but now by using a completely different set of rotational transitions that are (unlike in H₂O), not forbidden by nuclear spin restrictions. Similar results are presented for D₂O scattered by normal-H₂ at collision energy of 584 cm^-1. The agreement between experiment and theory is, however, less good for forward scattering of HDO/D₂O. A critical analysis of this discrepancy is presented.