Rotationally inelastic scattering of symmetric top molecules is examined by crossed molecular beam and velocity map imaging experiments using a newly constructed instrument. The systems studied are collisions of methyl radicals and ammonia molecules with rare gases and diatomic molecules. Resonance-enhanced multi-photon ionization detection provides velocity map images for ND3 molecules in a single n't, level, and for methyl radicals in levels corresponding to a single rotational angular momentum quantum number (n'), but averaged over a subset of the projection quantum number k'. Product level-resolved angular scattering distributions are extracted from measured images, and for CD3, CH3 and ND3 + He systems are compared with full close-coupling quantum-mechanical scattering calculations that used ab initio potential energy surfaces. These calculations were performed by collaborating groups at Radboud University Nijmegen (ND3 + He), and Johns Hopkins University and the University of Maryland (CD3 / CH3 + He). The experimental measurements provide rigorous tests of the accuracy of the potential energy surfaces and computed quantum scattering dynamics, and agreement between experiment and theory is excellent. For the experimental studies of ND3, a hexapole electrostatic lens was used for the 11 initial state-selection of ND3 molecules in their electronic and vibrational ground states in a molecular beam. For selected final quantum states, the effect of collision energy on the differential cross section for the ND3 + He system was explored in the range 230 - 720 cm-I. For methyl radical scattering experiments, this initial state selection was not possible, but colling in a supersonic expansion restricted the radicals to n = 0 and 1 rotational angular momentum levels prior to the collisions of interest. The inelastic scattering of methyl radicals and ammonia molecules with He and H2 (and D2 in the case of methyl radicals) is predominantly forward scattered for small changes in the n quantum number, but the scattering shifts to the sideways and backward directions as I'!.n increases. The comparison of experimental differential cross sections with theoretical calculations also reveals the dependence of scattering on the k' - projection quantum number and on further features of the scattering dynamics such as the impact parameter. The inelastic scattering ofND3 with Ar and methyl radical with Ar and N2 is dominated by forward scattering for all final levels probed. Common features of the scattering dynamics of these two symmetric top molecules, one closed shell and the other an open-shell radical, are identified and discussed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:627964 |
| Date | January 2013 |
| Creators | Tkac, Ondrej |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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