  We discuss annular confinement for electrons on liquid helium surface induced
by a tubular electrode submerged beneath the surface. For shallow liquid layer
above the tube the resulting potential has a minimum off the axis of the tube.
The symmetry transitions in the ground-state driven by external magnetic field
can be resolved by the microwave spectroscopy provided that the confinement
radii in the first and second Rydberg subbands of the vertical quantization are
different. This condition can be fulfilled for submersion depth of the tube
comparable to its radius. Then, discontinuities in the main absorption line
appear with the period that corresponds to the subsequent magnetic flux quanta
passing across the area within the ground-state confinement radius.
