Attenuation and Lifetime of Hot Electrons Injected into Liquid Helium

We have obtained current densities up to ${10}^{\ensuremath{-}9}$ A ${\mathrm{cm}}^{\ensuremath{-}2}$ by injection of hot electrons from a cold-cathode emitter into liquid helium. The current in helium is only 1% of that available from the same devices into vacuum at liquid-helium temperatures. The current attenuation has been explained in terms of the electron-helium barrier, the back diffusion of the quasifree nonlocalized electrons near the emitter surface, and the thermalization of the quasifree electrons to a localized state while still within the influence of their image potential due to the emitter surface. Using a model based on these three factors the lifetime of the quasifree electrons has been calculated. In the present experiments the lifetime has varied from 2.5 psec, for the lowest helium densities and highest energy electrons to 0.3 psec for the reverse conditions.