The hot atom chemistry of muonium in alkane vapors

Abstract

Energetic positive muons thermalizing in alkane vapors are observed in either diamagnetic environments, with relative fraction fD, or as the polarized muonium atom (Mu = μ+e−), with fraction fMu. The fraction fD is found to vary from ~0.12 in CH4 to ~0.24 in n-C6H14 at pressures near 1 atm (300 K); some pressure dependence in yield is also observed, notably for the heavier alkanes. The diamagnetic fraction (yield) is interpreted as due to hot atom reactions (Mu*) incomplete analogy with past studies in hot tritium (T*) chemistry. The data have been interpreted within the Wolfgang–Estrup formalism and compared with similar analyses of T* reactions for (unmoderated) alkane systems. It is concluded that the energy loss parameter (α) is determined largely by inelastic scattering effects. If these are assumed to be the same for both Mu* and T* moderation, then the resulting reactivity integrals (I) for the noncyclic alkanes are found to exhibit the same trend with mass as in hot tritium chemistry, giving, on average, ~6:1 for the ratio I(T*)/I(Mu*).

Muonium Hot Atom Chemistry in Gases

“…As in NMR, the relaxation of the initial amplitude (λ = 1 /Γ 2 ) gives information about the interaction of the muon spin ensemble with its environment after thermalization, which is of particular relevance to the study of thermal reaction kinetics [1,[4][5][6][7], The initial amplitude of the signal gives information on the "preparation" process; i.e., during the slowing down of the muon. In this regard the initial amplitude plays the same role in μSR as it does in Ps chemistry, both then in contradistinction to hot tritium chemistry [10,11 ].…”

“…The subject of hot atom (or spur) muonium chemistry rests on the interpretation of these amplitudes [7,9,10,11], which are identified with "primary" reaction processes. The ability to directly discriminate "hot" processes from thermal ones (i.e., relaxation) is a distinct advantage that juSR (and Ps chemistry) enjoys in comparison with hot tritium chemistry [11],…”

“…15 The relative amplitudes in Table 1 are pressure independent (average) values which form the basis of the subsequent discussion. In some of the heavier alkanes, a pressure dependence in / D was observed and interpreted as likely evidence for hot atom substitution reactions [10].…”

“…In condensed phase studies, the concept of hot Mu atom reactions played an integral part in the theoretical development of muonium chemistry in such media [8], recently evolving into alternate "spur" models of muonium formation [7,9]. In the gas phase, where energy moderation effects can be more clearly defined, Mu* reactivity in the alkanes has been discussed in terms of the standard Wolfgang-Estrup (WE) formalism [10], and compared also with related results for hot tritium [ 10,11] and positronium (Ps) reactions [11].…”

“…Earlier results on the alkanes [ 10] are summarized and some new results on Mu* + H 2 , D 2 are presented.…”