Cross-calibration of atomic pressure sensors and deviation from quantum diffractive collision universality for light particles

Abstract

Abstract The room-temperature, velocity-averaged, total cross section for atom–atom and atom–molecule collisions can be approximated using a universal function depending only on the magnitude of the leading order dispersion coefficient, C 6. This feature of the total cross section together with the universal function for the energy distribution transferred by glancing angle collisions ( p QDU6 (Booth et al 2019 New J. Phys. 21 102001)) can be used to empirically determine the total collision cross section and realize a self-calibrating, vacuum pressure standard. This was previously validated for Rb+N2 and Rb+Rb collisions. However, the post-collision energy distribution is expected to deviate from p QDU6 in the limit of small C 6 and small reduced mass. Here we observe this deviation experimentally by performing a direct cross-species loss rate comparison for Rb+H2 and Li+H2 collisions. We measure a velocity averaged total collision cross section ratio of R = ⟨ σ tot v ⟩ Li+H 2 : ⟨ σ tot v ⟩ Rb+H 2 = 0.83 ( 5 ) . Based on an ab initio computation of ⟨ σ tot v ⟩ Li+H 2 = 3.104 × 10 − 15 m3 s−1, we deduce ⟨ σ tot v ⟩ Rb+H 2 = 3.6 ( 2 ) × 10 − 15 m3 s−1, in agreement with a Rb+H2 ab initio value of ⟨ σ t o t v ⟩ R b + H 2 = 3.574 × 10 − 15 m 3 s − 1 . By contrast, fitting the Rb+H2 loss rate as a function of trap depth to the universal function we find ⟨ σ tot v ⟩ Rb+H 2 = 5.52 ( 9 ) × 10 − 15 m3 s−1. This work demonstrates the utility of sensor-atom cross-calibration experiments to check the validity of theoretical computations to extend and enhance the performance of cold atom based pressure sensors.