Annular confinement for electrons on liquid helium

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