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Relaxation and formation processes of the muon and muonium in the gas phase Mikula, Randall John


The positive muon is an unstable (Tµ =2.2µsec) particle but one with properties very similar to the proton from a chemical and atomic physics point of view. It has, however, a mass only 1/9 that of the proton, a fact which makes it ideal for studying any mass dependence in chemical and physical phenomena. The µSR and MSR techniques have been utilized to study various relaxation phenomena that occur in the gas phase when the muon or muonium (the muon and electron bound state analogous to the hydrogen atom) interacts with its environment. The fraction of muons that thermalize as muonium has also been measured. It was found that 83% of the muons stopping in n₂ formed muonium, the remainder staying in the charged state. Other gases investigated were H₂ (61 %) , CH₄ (86%), NH3(90%), He(0%), Ne(4%), Ar(74%), Kr(100%) and Xe100%). The amplitude of the signal was found to be strongly pressure dependent and this has been explained in terms of the thermalization time of the muons in these gas targets. Various gas mixtures were also studied where it was found that relaxations of the µSR signal occured as a function of added reagent gas concentration. This phenomena has been attributed to reactions of muon molecular ions with the reagent gas, forming muonium thermally. This represents the first reliable measurements of muon relaxation in the gas phase. Evidence is presented indicating that such reactions occur from the first vibrational state of these muon molecular ions. The systems studied and their room temperature rate constants The spin exchange interactions of muonium with NO and 0₂ were also studied as a function of temperature in the range 295K to 478K. The measured rate constants exhibit a temperature dependence consistent with T ½ and hence a constant spin exchange cross section. The temperature averaged spin exchange cross sections found are: for Mu+O₂, 6~SE=9.0±1x10⁻¹⁶cm² and for Mu+NO, 6se = 1 0 . 5±1 x ⁻¹⁶cm². These values are essentially the same as the cross sections found for hydrogen atom spin exchange with the same molecules, in qualitative agreement with current theoretical predictions.

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