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Elastic Rate Coefficients of Li+H2 collisions in the calibration of a Cold Atom Vacuum Sensor



Constantinos Makrides, Daniel S. Barker, Julia K. Scherschligt, Stephen P. Eckel, Eite Tiesinga


On going efforts at the National Institute of Standards and Technology in creating a Cold Atom Vacuum Sensor device have prompted theoretical investigations of atom-molecule collisions processes that characterize its operation. Such a device will operate as a primary standard for the Ultra-High- Vacuum and Extreme-High-Vacuum regimes. This device operates by relating loss of microK lithium atoms from a conservative trap by collisions with ambient atoms and molecules to the background density and thus pressure through the ideal gas law. The predominant background constituent in these environments is molecular hydrogen H2 . We compute the relevant Li+H2 Born-Oppenheimer potential energy surface, paying special attention to its uncertainty. Coupled-channels calculations are then used to obtain total rate coefficients. We find that H2 rotational quenching is negligible near room temperature. For a T = 300 K gas of H2 and 1.0 μK gas of Li atoms prepared in a single hyperfine state, the total rate coefficients are 5.91(4) × 10−9 cm3 /s and 6.00(4) × 10−9 cm3 /s for 6 Li and 7 Li isotopes, respectively, where the numbers in parenthesis corresponds to a one-standard- deviation combined statistical and systematic uncertainty. We find that a ten degree Kelvin increase in the H2 temperature leads to a 1.8% increase in the rate coefficients for both isotopes. Finally, a semi-classical Born approximation significantly overestimates the rate coefficients. The difference is at least ten times the uncertainty of the coupled-channels result.
Physical Review A (Atomic, Molecular and Optical Physics)


pressure sensor, UHV, ultracold atoms
Created April 29, 2019, Updated March 2, 2020