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Application of the muonium spin rotation technique to a study of the gas phase chemical kinetics of muonium reactions with the halogens and hydrogen halides Garner, David Michael


Muonium (Mu) is the atom formed by an electron bound to a positive muon "nucleus" (charge:+l, spin:1/2, lifetime: 2.2 μs). Since muons are 207 times as massive as electrons, the reduced mass of Mu is 0.996 that of the hydrogen atom, and the Bohr radii and ionization potentials of Mu and H are essentially the same. Therefore, the chemical behaviour of the Mu atom is that of a light H isotope (m[sub Mu] = 1/9 m[sub H]) with a greatly enhanced sensitivity to H isotope effects. Mu reaction rates are measured by a method called "Muonium Spin Rotation" (MSR) which resembles conventional resonance techniques such as NMR or ESR in that it monitors the characteristic Larmor precession of the Mu atom. However, unlike NMR or ESR, the MSR method does not detect the Mu Larmor precession by resonant power absorption, but rather through the peculiar spin dependent radioactive decay of the muon itself. The theoretical basis for the application of the MSR technique to the measurement of muonium reaction rates is derived. An extensive discussion is given to the practical aspects of the experimental implementation of the MSR technique. Rate constants and activation energies are reported for the gas phase reactions: Mu + F₂  MuF + F and Mu + Cl₂ + MuCl + Cl between 300 and 400K, and room temperature rate constants are reported for the reactions: Mu + Br₂  MuBr + Br and Mu + HX  [sup MuH + X][sub MuX +H], X = Cl, Br, I. While in most of these systems Mu reacts considerably faster than the heavier H isotopes, attention is focussed on hydrogen isotope effects in the Mu + F₂ and Mu + Cl₂, reactions. This discussion is based on the extensive theoretical investigations of Connor et al., which show the Mu + F₂ reaction to be dominated by quantum mechanical tunnelling at room temperature. Experimentally, quantum tunnelling manifests itself in this reaction by producing two dramatic isotope effects at 30OK: (1) the bi-molecular rate constant for the Mu reaction (1.4 x 10¹⁰ 1/mole-s) is at least six times that for the analogous H atom reaction, and (2) the apparent Arrhenius activation energy of this Mu reaction (0.9 kcal/mole) is less than half of that for H + F₂ In contrast, the Mu + reaction does not show any such strong isotope effects at 300K: (1) the bimolecular rate constant for Mu + Cl₂ (5.1 x 10¹⁰/mole-s) is no more than four times that of the analogous H reaction, and (2) the apparent activation energies for both Mu and H reactions are the same (1.4 kcal/mole) Preliminary calculations of Connor et al. on Mu + Cl₂ suggest that classical "wall reflection" partially offsets any rate enhancement due to quantum tunnelling. Quantitative isotope effects cannot be defined for the Mu + Br₂ and Mu + HX reactions and their hydrogen isotopic analogues because of the absence of sufficient experimental and theoretical data; these reactions are discussed in terms of the general theory of isotope effects.

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