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

• Received 27 May 2021
• Revised 26 October 2021
• Accepted 22 November 2021

DOI:https://doi.org/10.1103/PhysRevB.104.235402