Non-negligible collisions of alkali atoms with background gas in buffer-gas-free cells coated with paraffin

TL;DR

This study quantifies the collision rate between alkali atoms and background gas in paraffin-coated cells, revealing that background gas effects are significant and influenced by ripening, impacting cell performance.

Contribution

It provides the first measurement of velocity-changing collision rates in buffer-gas-free coated cells and shows how ripening affects these collisions and cell relaxation.

Findings

VCC rate is approximately 1 million per second in coated cells

Background gas has a non-negligible effect on alkali atom motion

Ripening increases VCC rate but improves anti-relaxation performance

Abstract

We measured the rate of velocity-changing collisions (VCCs) between alkali atoms and background gas in buffer-gas-free anti-relaxation-coated cells. The average VCC rate in paraffin-coated rubidium vapor cells prepared in this work was $1 \times 10^{6}$ s$^{-1}$, which corresponds to $\sim 1$ mm in the mean free path of rubidium atoms. This short mean free path indicates that the background gas is not negligible in the sense that alkali atoms do not travel freely between the cell walls. In addition, we found that a heating process known as "ripening" increases the VCC rate, and also confirmed that ripening improves the anti-relaxation performance of the coatings.