  It is shown that a nonequilibrium filling of an upper surface subband induced
by the microwave resonance can be the origin of the absolute negative
conductivity and zero-resistance states for the two-dimensional electron system
on liquid helium under magnetic field applied normally. Contrary to the similar
effect reported for semiconductor systems, an oscillating sign-changing
correction to the dc-magnetoconductivity appears due to quasi-elastic
iter-subband scattering which does not involve photons. The analysis given
explains remarkable magnetooscillations and zero-resistance states recently
observed for electrons on liquid helium.
