THEORETICAL INVESTIGATION OF COLLISIONS OF CH2 WITH He: ENERGY TRANSFER WITHIN AND BETWEEN THE a ̃ AND X ̃ ELECTRONIC STATES OF CH2
| dc.contributor.advisor | Alexander, Millard H | en_US |
| dc.contributor.author | Ma, Lifang | en_US |
| dc.contributor.department | Chemistry | en_US |
| dc.contributor.publisher | Digital Repository at the University of Maryland | en_US |
| dc.contributor.publisher | University of Maryland (College Park, Md.) | en_US |
| dc.date.accessioned | 2014-10-11T05:49:02Z | |
| dc.date.available | 2014-10-11T05:49:02Z | |
| dc.date.issued | 2014 | en_US |
| dc.description.abstract | This dissertation focuses on energy transfer (rotational, vibrational, and electronic) of CH<sub>2</sub> in its ground and first excited electronic states (X<super>3</super>B<sub>1</sub> and a<super>1</super>A<sub>1</sub>), by collisions with the helium atom. Initially we investigate energy transfer within the two electronic states separately. We carry out <italic>ab initio</italic> calculations to determine the potential energy surfaces for the interaction of He with CH<sub>2</sub> in these two states. The PES for He-CH<sub>2</sub>(a) is more anisotropic than for He&mdashCH<sub>2</sub>(X). In the former case we perform quantum scattering calculations and report state-to-state and overall removal cross sections, from which we compute room temperature rate constants. For He&mdashCH<sub>2</sub>(X\) we determined the dependence of the PES on the CH<sub>2</sub> bending degree of freedom. By averaging over the bending vibrational wave functions, we were able to investigate collisional relaxation of both rotation and the molecular bending. The PES of the X state is less anisotropic than that of the a state, resulting in a less efficient relaxation process. Vibrational relaxation is very inefficient, with cross sections less than 1% of those for rotational relaxation. By taking into account the weak spin-orbit coupling between the a and X states, we explore collision-induced electronically inelastic processes. We invoke, the mixed-state model, in which transitions are due entirely to the mixing of nearly-degenerate pairs of rotational levels. We compare the computed removal rate constants with experimental results by Hall and Sears at Brookhaven. Finally, we simulate the time evolution of the singlet-triplet relaxation of CH<sub>2</sub> by solving the relaxation master equation. The simulation shows that relaxation occurs in three stages: immediate re-distribution between the two mixed states, fast rotational relaxation within the a state and a much slower relaxation within the X state. Eventually, most of the population relaxes to the X state. | en_US |
| dc.identifier | https://doi.org/10.13016/M2BG6V | |
| dc.identifier.uri | http://hdl.handle.net/1903/15769 | |
| dc.language.iso | en | en_US |
| dc.subject.pqcontrolled | Chemistry | en_US |
| dc.title | THEORETICAL INVESTIGATION OF COLLISIONS OF CH2 WITH He: ENERGY TRANSFER WITHIN AND BETWEEN THE a ̃ AND X ̃ ELECTRONIC STATES OF CH2 | en_US |
| dc.type | Dissertation | en_US |
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