The dependence of the mobility on driving electric field for electrons
localized in two-dimensional electron layer over liquid helium surfac
e is studied experimentally at temperatures below 1 K in the region of
electron-ripplon scattering. The measurements are carried out at elec
tron concentrations lower than those leading to a screening the holdin
g electric field by the field of electron layer (unsaturated case). Th
e electron mobility in ground surface level is shown to be monotonical
ly increasing function of driving field. The results of the investigat
ion agree with theoretical dependences obtained under the supposition
of coinciding the short-wave region of ripplon spectrum with long-wave
asymptotics.