Exploring Rotational Energy Transfer of Intermolecular Collisions in Complex Systems: A MQCT Approach

The accurate modeling of molecular spectra in astrophysical environments requires detailed understanding of collisional energy transfer processes, which remains a significant challenge due to the computational complexity of quantum mechanical calculations for larger molecules, heavier projectiles, and higher collision energies. This dissertation addresses this challenge through the development and application of Mixed Quantum/Classical Theory (MQCT), a hybrid approach that combines quantum mechanical treatment of internal molecular motion with classical description of translational degrees of freedom. The methodology was first validated through detailed study of rotational energy transfer in the ND3 + D2 system, demonstrating excellent agreement with full quantum results while offering significant computational advantages. An alternative method for computing state-to-state transition matrices was formulated and implemented in a new version of MQCT code, enhancing computational efficiency for complex scattering calculations. MQCT was then extended to the astronomically important H2O + H2 system, performing the most comprehensive calculations to date including 200 rotational states of water and states of H2 up to j=10, for collision energies up to 12,000 cm-1. This work significantly expanded existing collisional databases and provided the first detailed analysis of collisions with highly excited H2 molecules. The results revealed that rate coefficients for rotational transitions in H2O increase with H2's rotational excitation, typically exceeding ground-state values by a factor of two - crucial information for modeling water in high-temperature astrophysical environments. Systematic analysis methods were developed to characterize collisional energy transfer, demonstrating that the values of cross sections correlate not only with energy gap ΔE, but also with the changes of quantum numbers Δj and Δτ. The database of state-to-state transition rate coefficients generated in this work for H2O + H2 is a significant contribution to the molecular datasets used by astrophysical community. This work promotes MQCT as a powerful and computationally efficient tool for studying complex molecular collision systems that are intractable with full quantum methods, advancing capabilities for modeling molecular collisions in diverse astrophysical environments.