Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford, OX1 3TA, UK.
Phys Chem Chem Phys. 2013 Sep 14;15(34):14270-81. doi: 10.1039/c3cp51839f. Epub 2013 Jul 22.
Ensembles of ultra-cold atoms, molecules and ions (both atomic and molecular) can be held in traps for increasingly long periods of time. While these trapped species remain translationally cold, for molecules the absorption of ambient black-body radiation can result in rapid thermalisation of the rotational (and vibrational) degrees of freedom. At 300 K, internal state purity is lost typically on the order of tens of seconds, inhibiting the study of quantum state selected reactions. In this paper a theoretical model is used to investigate laser-driven, blackbody-mediated, rotational cooling schemes for several (1)Σ and (2)Π diatomic species. The rotational cooling is particularly effective for DCl(+) and HCl(+), for which 92% and >99% (respectively) of the population can be driven into the rovibrational ground state. For the other systems a broadband optical pumping source (simultaneously exciting up to four transitions) is found to enhance the population that can be accumulated in the rovibrational ground state by up to 29% over that achieved when exciting a single transition. The influence of the rotational constant, dipole moments and electronic state of the diatomics on the rotational cooling achievable is also considered. An extension to polyatomic species is discussed and a combination of cold trap environments (at 77 K) and optical pumping schemes is proposed.
超冷原子、分子和离子（原子和分子）的集合可以在陷阱中被捕获，时间越来越长。虽然这些被捕获的物质仍然保持着平移冷却，但对于分子来说，环境黑体辐射的吸收会导致旋转（和振动）自由度的快速热化。在 300 K 下，内部状态纯度通常会在几十秒内丧失，从而抑制了对量子态选择反应的研究。在本文中，使用理论模型研究了几种（1）Σ 和（2）Π 双原子物种的激光驱动、黑体介导的旋转冷却方案。旋转冷却对 DCl(+) 和 HCl(+)特别有效，其中 92%和>99%（分别）的分子可以被驱动到转动振动基态。对于其他系统，发现宽带光学泵浦源（同时激发多达四个跃迁）可以将可积累在转动振动基态中的分子数增加 29%，超过了激发单个跃迁时的积累量。还考虑了双原子的转动常数、偶极矩和电子态对可实现的转动冷却的影响。讨论了对多原子物种的扩展，并提出了冷阱环境（在 77 K）和光学泵浦方案的组合。
