Photo-Conductivity Response at Cyclotron-Resonance Harmonics in a Non-Degenerate 2D Electron Gas on Liquid Helium

We report the first observation of an oscillatory photo-conductivity response at the cyclotron-resonance harmonics in a non-degenerate 2D electron system formed on the free surface of liquid helium. The dc conductivity oscillations are detected for electrons occupying the ground surface subband. Their period is governed by the ratio of the microwave frequency to the cyclotron frequency. Theoretical analysis of the photo-response in a strongly interacting electron system indicates that the observation can be explained by an oscillatory correction to the electron distribution function that appears for a large inelastic relaxation time because of photon-assisted scattering.