Date of Award
Doctor of Philosophy (PhD)
This thesis presents developments and applications of the mixed quantum/classical theory (MQCT) for inelastic scattering. In this approach, translational motion of collision partners is treated classically, while the internal degrees of freedom – rotational and/or vibrational motion – are treated quantum mechanically. Within this framework calculations of rotationally inelastic cross sections are carried out in a broad range of collision energies and results are compared against the exact full quantum data for several real systems. For CO +He, N2 + Na and H2 + He the agreement is excellent through six orders of magnitude range of cross sections values and for energies 1 < E < 10000 cm-1. Elastic and differential cross section for N2 + Na are described very accurately. For ro-vibrational transitions in CO + He and H2 +He MQCT reproduces full quantum results even for highly excited rotational states. For H2O + He it is found that at lower energies the typical errors for cross sections are on the order of 10%, which is acceptable. It is showed that computational cost of the fully-coupled MQCT scales as n2- 3, where n is the number of channels which is far more favorable in comparison with full quantum scaling n5-6. This enables calculations on larger molecules and at higher collision energies, than was possible using the standard approach. The largest system ever considered for rotational scattering, HCOOCH3 + He, is also treated by MQCT. At energies where quantum results are available (≤ 30 cm-1) the agreement is found very good. Then MQCT calculations for this system are extended up to E = 1000 cm-1. Finally, theoretical framework for treatment of molecule + molecule scattering is developed and applied to H2+H2 and N2+H2 systems where excellent agreement with exact quantum results is found. We also apply MQCT method to H2O + H2O rotationally inelastic scattering and obtain the first and only data for this process in a broad range of collisional energies. Success of MQCT makes this theory a practical tool for obtaining the state-to-state transition rates for astrochemical modeling and other applications.