Abstract
We measured the escape rates of surface-state electrons from an electron layer confined at the liquid-helium--vacuum interface in the temperature range of 30--450 mK, for densities (0.02--2.2)\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. We compared the measured escape rates with calculated tunneling rates in a model where the interactions between the escaping electron and the other electrons are described by an effective single-particle potential. Below 200 mK the escape rates were temperature independent. The single-particle rates were enhanced exponentially as the density was increased up to a critical density ${\mathit{n}}_{\mathit{c}}$. In this regime the calculated and measured rates were in good agreement. At ${\mathit{n}}_{\mathit{c}}$ we observe a one order of magnitude steep rise and the time development of density becomes extremely nonlinear. We show that this nonlinearity can be explained by the electron impact excitation of helium atoms on the walls of the cell. As the barrier is raised so that the escape rates decrease below 5.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$, a new mechanism seems to dominate the escape and the rates become very weakly density and external field dependent. Thermally activated escape was observed above 250 mK and the activation energies were in good agreement with the calculated values.
Published Version
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