Date of Award
Spring 2021
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Babikov, Dmitri
Second Advisor
Reid, Scott A.
Third Advisor
Huang, Jier
Abstract
A mixed quantum/classical methodology and an efficient computer code, named MQCT, were developed to model molecular energy transfer processes relevant to astrochemical environments and planetary atmospheres and applied to several real systems. In particular, the rotational energy transfer in N2 + Na collisions was studied with the focus on quantum phase, differential cross-sections, and scattering resonances, and excellent agreement with full quantum results was found. For H2O + H2, detailed calculations were carried out with the focus on allowed vs. forbidden transitions between the ortho/para states of both collision partners. Again, excellent agreement with full quantum calculations was achieved. Calculations of rotational energy transfer in a collision of two asymmetric-top rotors, a unique capability of this code, were tested using H2O + H2O system where the full-quantum calculations are unfeasible. To make MQCT calculations practical, an approximate, very efficient version of the method was developed, in which the classical-like equations of motion for the translational degrees of freedom (scattering) are decoupled from the quantum-like equations for time-evolution of the internal molecular states (rotational, vibrational). The code MQCT was made publicly available to serve as an efficient computational tool for other members of the community. It can perform scattering calculations on larger molecules and at higher collision energy than it is currently possible with full quantum methods and codes. To study the rotational quenching of isotopically substituted sulfur molecules, such as 32S32S, 32S34S, and 34S34S, a new accurate potential energy surface was developed for S2+Ar system. Rotational state-to-state transition cross sections were computed using MQCT, and the master equation modeling of energy transfer kinetics was carried out. It is found that isotopically substituted asymmetric molecules such as 32S34S promote energy transfer due to symmetry breaking and transitions with odd ∆j that become allowed. This process may be responsible for mass-independent isotopic fractionation of sulfur isotopes, typical to the Archean surface deposits.