Cold Collision Frequency Shift in Two-Dimensional Atomic Hydrogen

TL;DR

This study measures the cold collision frequency shift in two-dimensional atomic hydrogen on superfluid helium at ultra-low temperatures, revealing a shift much smaller than theoretical predictions, which impacts understanding of surface interactions.

Contribution

It provides the first experimental measurement of the collision frequency shift in 2D atomic hydrogen, showing a significant discrepancy with mean field theory predictions.

Findings

Measured the collision frequency shift as -1.0(1) x 10^-7 Hz cm^-2 x sigma

Found the shift to be about 100 times smaller than theoretical predictions

Separated dipolar and exchange interaction contributions using two-photon NMR

Abstract

We report a measurement of the cold collision frequency shift in atomic hydrogen gas adsorbed on the surface of superfluid 4He at T<=90 mK. Using two-photon electron and nuclear magnetic resonance in 4.6 T field we separate the resonance line shifts due to the dipolar and exchange interactions, both proportional to surface density sigma. We find the clock shift Delta v_c = -1.0(1)x10^-7 Hz cm^-2 x sigma, which is about 100 times smaller than the value predicted by the mean field theory and known scattering lengths in the 3D case.