Scalar and vector correlations in molecular collision dynamics
McGurk, Stephen James
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This thesis concerns the fundamental scalar and vector attributes of molecular collisions. A translationally relaxed sample of fully state-selected and rotationally anisotropic CN(A² ∏,v = 4, jFɛ) was prepared within a thermal bath (~298 K) of partner gas (either Ar, N2, O2 or CO2) by ns-pulsed laser excitation. The collisional evolution of the prepared polarised rotational angular momentum was monitored using highresolution frequency modulation spectroscopy (FMS). The total removal and depolarisation of oriented or aligned rotational angular momentum was measured in CN(A² ∏,v = 4, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5, F1e). The state-to-state rotational energy transfer (RET) and orientation transfer from CN(A² ∏,v = 4, j = 6.5 F1e or j = 10.5 F2f) to ∆j ≤ |5| was investigated. The results for the CN(A² ∏,+Ar system generally agree very well with complementary exact quantum scattering (QS) calculations on the best available ab initio potential energy surfaces (PESs). For all systems, a three-level multiple-collision kinetic model satisfactorily reproduces the observed removal of population and polarisation. Elastic depolarisation is found to be a relatively minor pathway relative to population removal and inelastic depolarisation, as confirmed by complete master equation (ME) simulations for CN(A² ∏, v=4, j = 6.5 F1e)+Ar. The total removal efficiencies lie in the order CO2 > N2 > O2 > Ar, loosely correlated with long-range attractive forces. O2 and CO2 exhibit rapid removal channels in addition to RET, likely to be electronic quenching to CN(X2∑+). There are substantial parity-dependent alternations with Dj in state-to-state RET and polarisation transfer, sufficient for a striking change in sign of orientation for specific transitions. This is attributed to the near-homonuclear nature of CN(A² ∏) and consequent even character of the PESs. QS calculations indicate that the dynamics of parity-conserving and changing transitions differ fundamentally. A preference for spinorbit conservation, strongest for Ar, comes from the near-Hund’s case-(a) character of CN(A² ∏) at low-j. Despite the additional dimensions available, the qualitatively similar behaviour of the molecular partners with Ar suggests that these systems have comparable interaction potentials with CN(A² ∏). Therefore, small centrosymmetric molecules, such as N2, O2 and CO2, may approximately be treated as spherical targets. This is supported by recent spherically-averaged CN(A² ∏)-N2 PESs and associated QS calculations from the literature.