Explanation of 100-fold reduction of spectral shifts for hydrogen on helium films

TL;DR

This paper explains a 100-fold reduction in spectral shifts for hydrogen on helium films by showing helium film-mediated interactions, resolving previous discrepancies between theory and experiment, and discussing implications for stability and phase transitions.

Contribution

It introduces helium film-mediated hydrogen interactions as the key factor explaining spectral shift reductions and anomalous behaviors, providing a new understanding of cold collision shifts on helium films.

Findings

Helium film-mediated interactions cause a two-order magnitude reduction in spectral shifts.

These interactions explain the dependence of collision shifts on helium-3 coverage.

The gas is predicted to become mechanically unstable before the Kosterlitz-Thouless transition.

Abstract

We show that helium film-mediated hydrogen-hydrogen interactions account for a two orders of magnitude discrepancy between previous theory and recent experiments on cold collision shifts in spin-polarized hydrogen adsorbed on a helium film. These attractive interactions also explain the anomalous dependence of the cold collision frequency shifts on the $^3$He covering of the film. Our findings suggest that the gas will become mechanically unstable before reaching the Kosterlitz-Thouless transition unless the experiment is performed in a drastically different regime, for example with a much different helium film geometry.