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A µSR study of spin relaxation of small molecules in the gas phase

University of British Columbia

Current understanding of the underlying mechanisms of muonium and Mu-radical spin relaxation in the gas phase is presented. Models and formulae describing the effects of the three contributing processes, spin exchange, chemical reaction and collisional relaxation, on the muon spin polarization are developed and employed to extract reaction rate constants, cross sections and other kinetic parameters in reactions of Mu with atoms and small molecules; notably Mu + Cs, Mu + NO, Mu + N₂O and Mu + CO. The experimental data obtained are consistent with theory and the models so introduced. The radical relaxation rates obtained for larger molecules ( MuC₂H₄ and MUC₄H₈) are well described by the phenomenological model presented, which serves as a useful bridge linking the observed relaxation rates and the physical, chemical and magnetic properties of the reactants and offers invaluable insight into the underlying mechanisms causing muon spin relaxation. The ratios of thermal spin-flip cross sections (σ[sup H]/σ[sup Mu]) in electron spin exchange interactions (Mu + Cs and Mu + NO) are found to be about 3, consistent with previous experimental measurements for Mu + NO and Mu + O₂ systems. Reaction rates for Mu + NO, Mu + N₂O and Mu + CO were measured over a wide range of pressures (1 to 60 atm, up to 272 atm for CO) and in one case (N₂O) over a range of temperatures (300-600 K). Dramatic kinetic isotope effects are observed in all of these systems: a small "inverse" effect ([sup k]Mu/[sup k]H=0.23) in the Mu + NO reaction and large "normal" effects (([sup k]Mu/[sup k]H > 100) for the Mu + N₂O and Mu + CO reactions at room temperature. These kinetic isotope effects can be qualitatively understood within the trends indicated by reaction rate theory and the experimental results of the analogous H(D) reactions, but a quantitative comparison with theory must await specific calculations of the Mu reaction rates. This thesis data represents the first observation of large tunneling effects at room temperature in H-isotope addition reactions in the gas phase.

Thesis/Dissertation

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2009-02-18

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http://hdl.handle.net/2429/4764

Chemistry

University of British Columbia

UBCV

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