Quantum Effects in Collisional Energy Transfer Simulated Using Mixed Quantum/Classical Theory

Mixed quantum/classical theory (MQCT) was used to study collisional energy transfer between the rotational states of molecules with the focus on reproducing quantum effects related to this process. Namely, rotational energy transfer in the N2 + O system was studied to replicate quantum interference effects observed as oscillations of scattering cross section as a function of collision energy. Both MQCT code and the full-quantum code MOLSCAT were used for calculations, and results were in excellent agreement with the experiment and the full-quantum infinite-order sudden method from literature. The CO + CO system was used as a case study for diatom + diatom collisions. First, two CO molecules were treated as distinguishable in order to compare results with available full-quantum coupled-states data. Excellent agreement between the two methods was achieved. It was found that for strong transitions with large cross sections, the results of MQCT are reliable, especially at higher collision energy. For weaker transitions and lower collision energies, the cross sections predicted by MQCT may be up to a factor of 2–3 different from those obtained by full-quantum calculations. Then, the treatment of two colliding molecules as indistinguishable was developed and applied to study H2 + H2, CO + CO, and H2O + H2O systems. MQCT results showed that if a posterior correction by a factor of 2 is applied, the distinguishable approach agrees well with the indistinguishable method, as well as with available full-quantum data. The results of the two treatments agree within 5% for most but may reach 10–20% for some transitions. At low collision energies dominated by scattering resonances, these differences can be larger, but they tend to decrease as collision energy is increased. It is also shown that if the system is artificially forced to follow the same collision path in the indistinguishable and distinguishable treatments, then all differences between the results of the two treatments disappear. This interesting finding gives new insight into the collision process and indicates that the indistinguishability of identical collision partners comes into play through the collision path itself, rather than through matrix elements of inelastic transitions.