Muon depolarization and the chemistry of muonium in liquids

Abstract

Polarized positive muons are stopped in liquids containing dissolved reagents. Their polarization is measured by observing positrons from the asymmetric decay as the muon spins precess in a magnetic field. As the reagent concentration is varied, the apparent initial magnitude and direction (phase) of the polarization change markedly, due to the "muonium mechanism": muons form free muonium and are depolarized in a fraction of a nanosecond via the hyperfine interaction unless the muonium reacts chemically in even shorter times to place the muon in a diamagnetic compound. The observed dependence of the magnitude and phase of the polarization upon reagent concentration confirms the validity of this model and allows extraction of chemical rate constants for fast reactions of muonium (chemically a light isotope of atomic hydrogen). A more complicated situation is also observed, in which muonium reacts to place the muon in a radical compound, where further depolarization takes place via the hyperfine interaction with the unpaired electron. The theory is expanded to include such processes.