Tunneling from Electronic Bubble States in Liquid Helium through the Liquid-Vapor Interface

The escape of electrons from their bubble state in liquid helium through the free liquid surface into the vapor phase is investigated. The measured escape rates are calculated in terms of a tunneling model which predicts a unique temperature and electric field dependence in excellent agreement with the experimental data. A fit of this model to the data yields the binding energy and the radius of the electronic bubble. The binding energy is found to be 0.70 eV in pure ${\mathrm{He}}^4$ and 0.56 eV in pure ${\mathrm{He}}^3$. The bubble radii are found to scale according to the bubble model; however, their absolute values appear to be too large. The results obtained from two different ${\mathrm{He}}^3$-${\mathrm{He}}^4$ mixtures are anomalous and not yet understood.