  An ultra-strong photovoltaic effect has recently been reported for electrons
trapped on a liquid Helium surface under a microwave excitation tuned at
intersubband resonance [D. Konstantinov et. al. : J. Phys. Soc. Jpn. 81, 093601
(2012) ]. In this article, we analyze theoretically the redistribution of the
electron density induced by an overheating of the surface electrons under
irradiation, and obtain quantitative predictions for the photocurrent
dependence on the effective electron temperature and confinement voltages. We
show that the photo-current can change sign as a function of the parameters of
the electrostatic confinement potential on the surface, while the photocurrent
measurements reported so far have been performed only at a fixed confinement
potential. The experimental observation of this sign reversal could provide a
reliable estimation of the electron effective temperature in this new out of
equilibrium state. Finally, we have also considered the effect of the
temperature on the outcome of capacitive transport measurement techniques.
These investigations led us to develop, numerical and analytical methods for
solving the Poisson-Boltzmann equation in the limit of very low temperatures
which could be useful for other systems.
