UBC Theses and Dissertations
Deviation from quantum diffraction universality and effects of interaction potential uncertainty on collision observables Herperger, Katherine
Quantum Diffraction Universality (QDU) is a law that allows the experimental determination of the thermalized rate coefficient ⟨σₜₒₜv⟩ between an impinging gas and a nearly stationary sensor gas in high vacuum. QDU relies on the insensitivity of the rate coefficient to the short-range interaction potential for the collision partners. This law allows one to bypass time-intensive theoretical scattering calculations for ⟨σₜₒₜv⟩ and additionally leads to the determination of the impinging gas pressure. In this thesis, I describe two projects that further our understanding of QDU. First, I conduct coupled-channel quantum scattering calculations for the collision partners Li+H₂ and Rb+H₂. These calculations for ⟨σₜₒₜv⟩, in combination with experimental measurements, show that Rb+H₂ is a system that cannot be described by QDU. The reason is that Rb+H₂ has a light reduced mass and small C6 coefficient (which characterizes the long-range interaction potential). For these reasons, one can infer that Li+H₂ also deviates from Universality. In the second project, I modify the shortrange interaction potential of Rb+H₂ and analyze how the modifications lead to a change in ⟨σₜₒₜv⟩. Furthermore, I describe how machine learning – specifically Gaussian Process Regression – can be utilized to predict ⟨σₜₒₜv⟩ for different modulations of the short-range interaction potential. This analysis will give an estimate on the error arising from interaction potential uncertainty associated with ⟨σₜₒₜv⟩ for Rb+H₂. Additionally, it serves as a second demonstration of the non-Universal behaviour of the Rb+H₂ system.
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