Novel techniques for cold/ultracold molecular collisions
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] This thesis presents the successful implementation of a slice velocity map imaging apparatus capable of performing molecular beam scattering experiments at collision energies from well above the room temperature to below 1 K. The novel techniques introduced here offers a platform to study collisions at a regime where the scattering is mainly governed by quantum mechanics. Experimental developments of low energy collisions will challenge the theoretical description of molecular interactions, and aid in validating the accuracy of the calculated potential energy surfaces. Scattering studies involving two different configurations of the apparatus, intrabeam and near-copropagating beam configurations are reported in this thesis. The intrabeam scattering approach exploits beam slippage, the velocity difference of different species in the same beam, to establish the relative velocity. The beam slippage in the current setup was created by a pulsed electric discharge which induced a velocity upchirp in the excited species. Implementing a unique dual-slit chopper that can separately fix the velocities of the excited and non-excited species at the interaction region, we achieve precise control over the relative velocity and narrow its spread. With this configuration l-changing collisions occurring between Xe Rydberg atoms and Xe ground state atoms at subKelvin temperatures have been observed. The near-copropagating beam apparatus coupled with the stimulated emission pumping method and sliced velocity map imaging detection enabled us to measure the state-to-state differential cross sections for inelastic collisions between vibrationally excited NO molecules and Ar at broadly tunable energies. Experimentally measured rotationally inelastic state-to-state differential cross sections have enjoyed excellent agreement with quantum scattering calculations performed on state-of-the-art coupled-cluster potential energy surfaces. In contrast, state-to-state differential cross sections obtained for collision-induced multiplet changing transitions at collision energies near 1 K have showed better agreement with quantum scattering calculations involving multireference potential energy surfaces compared to coupled-cluster surfaces. This study provided a platform to validate the accuracy of the potential energy surfaces for collisions which are mainly governed by the attractive part of the interaction potential.