Effective mass of surface electrons in helium under non-equilibrium conditions

The mass shift of surface electrons in helium has been investigated theoretically caused by electron-ripplon interaction as a function of electromagnetic field power absorbed by a system. It is shown that the mass shift is negative at weak powers of external field but it increases as the power increases and changes its sign. The results of calculation and comparison with the experimental data testify to the favourable role of two-ripplon scattering processes in energy relaxation of surface electrons above helium.