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Title: Two and three vector correlations in the rotationally inelastic scattering of state-selected NO(X)
Author: Gordon, Sean Dennis Steven
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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In this thesis, an experimental and theoretical study of two and three vector correlations in the inelastic scattering of NO(X) with various rare gas atoms is presented. Vector correlations for a selection of rare gas systems were determined experimentally, and the observations were interpreted using a variety of classical and quantum mechanical models. The experiment is able to provide state-to-state resolution of the dynamics by means of an electrostatic hexapole and 1+1' resonantly enhanced multi-photon ionisation (REMPI). The simplest vector correlation of interest is the differential cross section (DCS), given by the k-k' correlation. The DCSs were determined experimentally for the NO(X)--Kr and NO(X)--Xe collision systems, both characterised by the relatively deep (≈140cm-1) attractive well and large extent of the attractive potential. The agreement between the experimental angular distributions and quantum mechanical DCS is very good for both systems. Classical calculations fail to correctly reproduce the form and magnitude of the DCS for either system, reflecting the inherently quantum mechanical nature of the collision. The classical calculations do however provide mechanistic insight into regions where the attractive part of the potential plays an important role in determining the dynamics. In order to investigate narrow angular features in the forward scattered direction, several experimental improvements to molecular beams and the detection ion-optic stack were made. Investigation into these structures revealed a strong contribution from molecular diffraction into the classical shadow of the NO(X), and the simple Fraunhofer model revealed a preference for scattering from an individual m→m' sub-state. Such measurements are in a region of the DCS where scattering is forbidden classically, and reveal the purely quantum nature of the collision interaction in the forward scattered direction. The low order k-k' correlation was then extended by using linearly or circularly polarised laser excitation. The interaction of the light with the molecular dipole allows the measurement of the k-k'-j' correlation. When linearly polarised light was used for the excitation laser, two of the rank two p{2}q(θ) renormalised polarisation dependent differential cross sections (PDDCSs), which describe rotational alignment, were obtained. With circularly polarised light, the rank one p{1}1-(θ) renormalised PDDCSs describing rotational orientation were determined. The collision induced alignment in NO(X)--Xe scattering was found to be well reproduced by classical and impulsive theories, highlighting the fact that the alignment is dominated by the propensity for the projection of j onto the kinematic apse to be conserved. The attractive part of the potential does augment the alignment renormalised PDDCSs, and this is most evident in states with strong features of the attractive part of the potential such as ℓ-type rainbows. The orientation is more strongly influenced by the attractive part of the potential and is also influenced by parity. In addition to the parity effect, there exist two limiting classical mechanisms which govern the orientation, one caused by attraction and the other repulsion. Finally, the bond axis of the NO(X) can be oriented by means of hexapole state selection combined with adiabatic orientation using a set of guiding rods. The integral steric effect, an r-k correlation, was measured for the NO(X)--Kr and NO(X)--Ar spin-orbit changing systems. There are large oscillations in the sign of the steric asymmetry which occur for scattering with the various rare gases. There are also large differences between the rare gases as the potentials become more attractive, and more isotropic. The steric asymmetry is well reproduced by quantum mechanics, however, a classical mechanism becomes dominant at high Δj.
Supervisor: Brouard, Mark Sponsor: Cambio Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: physical chemistry ; chemical physics ; reaction dynamics ; stereodynamics ; spectroscopy ; molecular orientation ; molecular alignment ; rotational energy transfer ; diffraction