Mixed Quantum/Classical Theory for Rotational Excitation of HDO in Collisions with H ₂ : Symmetry Breaking Effects and Time-Dependent Dynamics

The application of the mixed quantum/classical theory of inelastic scattering (MQCT) to the rotational excitation of the isotopically substituted water molecule, HDO, in collisions with hydrogen molecules is presented. Modification of the potential energy surface of thewater + hydrogen system, due to isotopic substitution, is described, and the effect of symmetry breaking is explored. It is found that the additional terms of potential expansion are significant and, therefore, are expected to play an important role during the collision process. The modeling of HDO + H₂ collision is carried out using both the MQCT method and the full-quantum CC method of MOLSCAT, using the latter as a reliable benchmark. Both para- and ortho-states of projectile H₂ are considered. The focus is on those transitions that become allowed in HDO due to symmetry breaking, in contrast to the symmetric H₂O where the para-ortho transitions are forbidden. Very good agreement between the two methods is observed, not only for the total but also for partial cross sections of rotationally inelastic transitions in HDO. Moreover, the time-dependent MQCT method offers an additional insight into the process of molecule-molecule collision, rotational state excitation, and energy transfer between the collision partners. It reveals an important role of the excited rotational states of the projectile H₂ molecule and shows that, at certain values of collision impact parameters (or orbital angular momentum), the stepladder of sequential state-to-state transitions may lead to a surprising distribution of final state populations in HDO.