Vector correlations in gas-phase inelastic collision dynamics

Thesis

This thesis presents a joint experimental and theoretical study of vector correlations in the electronically, vibrationally, and rotationally inelastic collisions of simple molecules with rare-gas atoms.

In the first instance, empirical and calculated data are presented for rotationally inelastic scattering in the NO(X)+Ar and ND₃(X(tilde))+Ar systems at collision energies in the range 405 − 2210 cm⁻¹. These experiments – the first to be conducted on a newly commissioned crossed-molecular beam machine – measured the k − k′ correlation, i.e. that between the vectors describing the relative velocities before and after collision, respectively. The empirical data were subjected to rigorous comparison with both quantum-mechanical and quasi-classical trajectory (QCT) calculations. For both the NO(X)+Ar and ND₃(X(tilde))+Ar systems, there is generally good agreement between experiment and theory at all four collision energies investigated.

Two chapters of this thesis focus on the development of trajectory surface-hopping (TSH) QCT models of the OH(A, v = 0)+Kr and OH(A, v = 0)+Xe systems. Experimental data relating to scalar quantities (rotational energy transfer (RET) and electronic quenching) and to the j − j′ correlation (which quantifies the depolarisation of the angular momentum of the OH(A) radical) are compared to variable-collision-energy TSH QCT calculations in which the length of the OH bond is fixed. The algorithms involve all three PESs of the OH(A/X)+Kr system, and the full range of electrostatic and roto-electronic mechanisms that couple them, for the first time. The most complete model succeeded in accounting for 93% of experimentally observed quenching. For the OH(A/X)+Xe system, coupling matrix elements were estimated from those of OH(A/X)+Kr, and the most complete model recovered 63% of experimentally observed quenching.

This thesis also presents a novel theoretical study of rotationally inelastic dynamics in the OH(A, v = 1)+Kr system. Provisional results from adiabatic calculations in which the OH bond length is allowed to vary over the course of a trajectory are presented alongside experimental data that were reported previously. To date, these calculations continue to underestimate the extent of empirical RET data. Reasons for the observed discrepancy, and suggestions to resolve it, are outlined in detail.

Authors: McCrudden, G

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: molecular beam; non-adiabatic; quenching; inelastic scattering; quasi-classical trajectory
• Subjects: Inelastic scattering; Non-adiabatic dynamics; Quasi-classical trajectory theory