Makrides Constantinos, Barker Daniel S, Fedchak James A, Scherschligt Julia, Eckel Stephen, Tiesinga Eite
Joint Quantum Institute, College Park, Maryland 20742, USA and Department of Physics, University of Maryland, College Park, Maryland 20742, USA.
National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
Phys Rev A (Coll Park). 2019 Apr 29;99(4). doi: https://doi.org/10.1103/PhysRevA.99.042704.
Ongoing efforts at the National Institute of Standards and Technology in creating a cold-atom vacuum standard device have prompted theoretical investigations of atom-molecule collision processes that characterize its operation. Such a device will operate as a primary standard for the ultrahigh-vacuum and extreme-high-vacuum regimes. This device operates by relating loss of ultracold 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 H. We compute the relevant Li+H Born-Oppenheimer potential energy surface, paying special attention to its uncertainty. Coupled-channel calculations are then used to obtain total rate coefficients, which include momentum-changing elastic and inelastic processes. We find that inelastic rotational quenching of H is negligible near room temperature. For a ( = 300)-K gas of H and 1.0-K gas of Li atoms prepared in a single hyperfine state, the total rate coefficients are 6.0(1) × 10 cm/s for both Li and Li isotopes, where the number in parentheses corresponds to a one-standard-deviation combined statistical and systematic uncertainty. We find that a 10-K increase in the H temperature leads to a 1.9% increase in the rate coefficients for both isotopes. For Li temperatures up to 100 K, changes are negligible. Finally, a semiclassical Born approximation significantly overestimates the rate coefficients. The difference is at least ten times the uncertainty of the coupled-channel result.
美国国家标准与技术研究院在创建冷原子真空标准装置方面的持续努力，促使人们对表征其运行的原子 - 分子碰撞过程进行了理论研究。这样一种装置将作为超高真空和极高真空区域的主要标准。该装置通过将超冷锂原子因与周围原子和分子碰撞而从保守阱中损失的情况，与背景密度相关联，并通过理想气体定律从而与压力相关联来运行。这些环境中的主要背景成分是分子氢H₂。我们计算了相关的Li + H₂ 玻恩 - 奥本海默势能面，并特别关注其不确定性。然后使用耦合通道计算来获得总速率系数，其中包括改变动量的弹性和非弹性过程。我们发现，在室温附近，H₂ 的非弹性转动猝灭可以忽略不计。对于处于单一超精细态制备的300 - K的H₂ 气体和1.0 - K的锂原子气体，锂及其两种同位素的总速率系数均为6.0(1)×10⁻⁹ cm³/s，括号中的数字对应于统计和系统不确定性合并后的一个标准偏差。我们发现，H₂ 温度升高10 K会导致两种同位素的速率系数增加1.9%。对于高达100 K的锂温度，变化可以忽略不计。最后，半经典玻恩近似显著高估了速率系数。差异至少是耦合通道结果不确定性的十倍。
