The influence of ions and radiation on tunneling of electrons from the surface of liquid helium

In work previously reported by Saville et al. [1] and Saville [2], we found that electrons tunneling from their shallow potential well above the surface of liquid helium behave as an almost ideal 1D hydrogen atom. Computations [3], without approximation or adjustable parameters, using the time dependent Schrödinger equation are in excellent agreement with measured tunneling rates. The same computation method was used to compute the influence of AC fields on the tunneling rates [3] as preparation for planned experiments. In this paper I discuss phenomena which appear to set upper and lower limits on the observable tunneling rates. Lower limits were determined by photo assisted escape due to thermal radiation and by cosmic ray generation of ions in the bulk liquid helium. Upper limits were set by the creation of excitations in the superfluid film on the top of the sample chamber when electrons impacted with energy greater than 21.6 eV. Through its influence on the tunneling rate, we were also able to measure very small changes in the electron potential at the surface due to a sub-monolayer coverage of³He.