  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.
