  We report the first observation of the microwave-induced intersubband
(Rydberg) resonance in the surface-bound electrons on superfluid helium
confined in a single 4-$\mu$m deep channel. The resonance signal from a few
thousand of surface electrons comprising the Wigner Solid (WS) is detected by
observing WS melting due to the microwave absorption. The observed transition
frequency for the two lowest Rydberg states, in the range of 0.4-0.5 THz, is
determined by the image charges induced by the surface electrons in conducting
electrodes of the microchannel and the potentials due to applied voltages, and
is in a good agreement with our calculations. The observed large broadening of
the resonance on the order of 10 GHz, which is due to inhomegeneous
distribution of the pressing electric field in the microchannel, is also in a
reasonable agreement with our finite-element modeling calculations. This study
of the confined electrons is motivated by their potential use for charge and
spin qubits.
