激光冷却双原子分子中的超精细结构及其Zeeman效应的理论研究

Laser cooling of diatomic molecule has a big challenge due to the complex internal level structure, the primary problem in laser cooling is the selection of appropriate diatomic molecules. A preliminary theoretical study on the electronic structure of the alkaline-earth-metal monohydrides (MH, M = Be、Mg、Ca、Sr、Ba) shows that these molecules meet certain criteria to be potential laser cooling candidates. Briefly, these criteria include (1) a highly-diagonal distributed Franck-Condon factors, (2) a shorter excited-state lifetime, and (3) no intervening electronic states to which the upper state could terminate the cycling transition. The hyperfine structure and the Zeeman effect in an external magnetic field, which are essential for laser cooling and magneto-optical trapping (MOT) of the alkaline-earth-metal monohydrides. This project will focus on the hyperfine structure and the Zeeman effect of lower ro-vibrational levels of ground state X 2Σ+ and first excited state A 2Π1/2 of MH molecules. The hyperfine structure and Zeeman effect can be evaluated numerically, which can be computed via ab initio and a computer program. According to the selection rule of the transitions, we will propose an optical cycling scheme for laser cooling of the MH molecules. The hyperfine splitting and Zeeman shift can be obtained. This project will lay the foundation for the process of laser cooling of these diatomic molecules in a MOT.

分子的复杂内部能级结构使激光冷却双原子分子存在巨大挑战，选择合适的候选分子是激光冷却的一个首要问题。前期理论研究表明碱土金属氢化物双原子分子(MH, M = Be、Mg、Ca、Sr、Ba)初步满足激光冷却条件：1)高对角分布的Franck-Condon因子；2)较短的激发态辐射寿命；3)没有中间电子态终止冷却循环。为进一步研究激光冷却MH分子的可行性，超精细结构及其Zeeman效应对磁光阱中分子的冷却和陷俘必不可少。本项目拟以MH分子基态X2Σ+和第一激发态A2Π1/2较低振转能级的超精细结构及其Zeeman效应为研究内容。利用从头算和编写程序计算MH分子X2Σ+和A2Π1/2的超精细结构及其Zeeman效应。根据跃迁选择定则，确定可能的冷却循环能级，提出冷却方案。通过本项目的研究，揭示MH分子的超精细能级分裂及其随磁场强度变化的定量关系，为磁光阱中MH分子的冷却过程提供理论上的支持。

超冷极性分子可广泛应用于电偶极矩的精确测量、检验基本物理常数是否随时间变化、偶极-偶极相互作用、超冷化学和超高分辨率分子光谱等问题的研究。本课题主要研究了碱土金属氢化物双原子分子（MH）基态和第一激发态较低振转能级的超精细结构及其在外场中的Zeeman效应。理论计算结果表明MH分子基态和第一激发态之间具有高对角分布的FC因子，且没有中间态的影响，具有ns量级的辐射寿命。通过矩阵对角化方法计算了碱土金属氢化物基态的超精细结构。同时，我们计算了与电偶极矩相应的转动跃迁分支比，给驱动特定跃迁的激光强度分布提供了理论参考。在宇称和转动态跃迁选择定则的限制下，实现A 2Π1/2 (Jʹ = 1/2, vʹ = 0) → X 2Σ1/2(N = 1, v = 0) 转动态循环跃迁的闭合性。为了将基态中的四个超精细能级同时激发到 A (vʹ = 0，Nʹ= 0，Jʹ=1/2，+)能级上去，泵浦激光通过电光调制器获得较宽的调制频率，实现与四个超精细能级的匹配。我们的研究结果给激光冷却碱土金属氢化物双原子分子的实施提供了理论指导。