Mobility of excess electrons andO2−formation in dense fluid helium

Abstract

Mobility measurements of excess electrons in dense fluid helium have been made at temperatures of 20.3, 52.8, 77.3, and 160 K, as a function of the helium density. An intrinsic mobility was observed, at all temperatures, which indicates increased electron self-trapping with increasing fluid density (constant $T$), and with decreasing temperature (constant $\rho$). These trends are continuations of those previously observed at lower temperatures. For 52.8 K and above, an additional low mobility signal was also observed. The intensity of this other signal is temperature dependent and nonmonotonic with helium density, and is interpreted as ${\mathrm{O}}_2^{-}$ formation. The probability of electron attachment to the minute concentration of ${\mathrm{O}}_2$ impurities is enhanced for helium densities at which the excess electron energies are in resonance with vibrational states of ${\mathrm{O}}_2^{-}$. The temperature effects indicate that electrons which are self-trapped are the ones predominantly involved in the ${\mathrm{O}}_2^{-}$ formation, as would be expected from their large geometrical cross section.