On the effect of "glancing" collisions in the cold atom vacuum standard

TL;DR

This paper models how glancing collisions affect vacuum pressure measurements in cold atom standards, revealing non-exponential atom loss and quantifying shifts in loss rate coefficients due to these collisions.

Contribution

It introduces a probabilistic model for glancing collisions in cold atom vacuum standards, showing their impact on loss rates and pressure readings, which was not previously accounted for.

Findings

Glancing collisions cause up to 0.6% shift in loss rate predictions for lithium.

In zero trap depth limit, shifts up to 2.2% in rubidium loss rate coefficients.

Atom loss in the trap follows a non-exponential decay due to glancing collisions.

Abstract

We theoretically investigate the effect of ``glancing" collisions on the ultra-high vacuum (UHV) pressure readings of the cold atom vacuum standard (CAVS), based on either ultracold $^7$Li or $^{87}$Rb atoms. Here, glancing collisions are those collisions between ultracold atoms and room-temperature background atoms or molecules in the vacuum that do not impart enough kinetic energy to eject an ultracold atom from its trap. Our model is wholly probabilistic and shows that the number of the ultracold atoms remaining in the trap as a function of time is non-exponential. We update the recent results of a comparison between a traditional pressure standard -- a combined flowmeter and dynamic expansion system -- to the CAVS [D.S. Barker, et al., arXiv:2302.12143] to reflect the results of our model. We find that the effect of glancing collisions shifts the theoretical predictions of the total loss rate coefficients for $^7$Li colliding with noble gases or N$_2$ by up to $0.6$ %. Likewise, we find that in the limit of zero trap depth the experimentally extracted loss rate coefficients for $^{87}$Rb colliding with noble gases or N$_2$ shift by as much as 2.2 %.