Classical reactive scattering in a quantum spirit: improving the shape of rotational state distributions for indirect reactions in the quantum regime

Abstract

For indirect triatomic chemical reactions under single-collision conditions, we propose a new implementation of the quasi-classical trajectory (QCT) approach to rotational state distributions, of particular interest in the quantum regime where only a few rotational states are available to the products. This method is directly inspired from the amendments to be introduced in classical phase space theory (PST) in order to make it in exact agreement with quantum PST. The approach is applied to the \(\hbox {D}^{+} + \hbox {H}_2\) and \(\hbox {H}^{+} + \hbox {D}_2\) reactions, and the population of the rotational ground state is found to be in much closer agreement with the exact quantum one than the one obtained by means of standard QCT calculations. The impact on the whole distribution is all the stronger as the number of available states is small. Lastly, the shape of the distribution appears to be controlled to a large extent by three factors, respectively, called parity, edge and rotational shift factors. The method is a complementary alternative to the statistical–dynamical methods, often used to study indirect processes. However, contrary to what is done in these methods, trajectories are not stopped at the entrance of the strong coupling region within our approach, thus expected to lead to reasonable predictions also for indirect processes that are not fully statistical, like those involving an osculating complex.