Isolated electrons resting near a helium surface have a spectrum close to that of a quantum-defect atom. A precisely solvable model with Rydberg spectrum is sugguested and discussed.
@article{arxiv.quant-ph/9912016,
title = {Quantum-defect Electron States Near a Helium Surface},
author = {B. A. Lysov and O. A. Lysova and O. F. Dorofeyev},
journal= {arXiv preprint arXiv:quant-ph/9912016},
year = {2007}
}