Excess electron transport and delayed muonium formation in condensed rare gases

Abstract

Experimental studies of excess electron transport in solid and liquid phases of Ne and Ar are presented and compared with those for He. The technique of muon spin relaxation in frequently reversed electric fields was used to study the phenomenon of delayed muonium formation, whereby excess electrons liberated in the ${\ensuremath{\mu}}^{+}$ ionization track converge upon the positive muons and form ${\ensuremath{\mu}}^{+}{e}^{\ensuremath{-}}$ atoms. This process is shown to be crucially dependent upon the electron's interaction with its environment (i.e., whether it occupies the conduction band or becomes localized) and upon its mobility in these states. The characteristic lengths involved are ${10}^{\ensuremath{-}6}$ to ${10}^{\ensuremath{-}4} \mathrm{cm};$ the characteristic times range from nanoseconds to tens of microseconds. Such a microscopic length scale sometimes enables the electron to spend its entire free lifetime in a state which may not be detected by conventional macroscopic techniques. The end-of-track processes are compared in (i) liquid and solid helium (where the electron is known to be localized in a bubble in the liquid phase and is thought to behave in a similar manner in the solid); $(\mathrm{ii})$ liquid and solid neon (where both localized and bandlike electrons are found in the liquid phase while most are delocalized in the solid); and $(\mathrm{iii})$ liquid and solid argon (where most electrons are bandlike in both phases). This scaling from light to heavy rare gases enables us to demonstrate new features of excess electron localization on the microscopic scale and provides insight into the structure of the end of the muon track in condensed rare gases.