激光冷却OH分子的理论研究

采用高精度的多参考组态相互作用方法计算OH分子基态和第一激发态的势能曲线.为获得更精确的计算结果,在计算过程中考虑Davidson修正、标量相对论效应、核价相关效应和自旋轨道耦合效应.基于计算的Λ-S和?态的势能曲线,对一维径向薛定谔方程进行数值求解,得到各个电子态的光谱数据,与已报道的实验值和理论值相符合.获得OH分子的永久偶极矩、跃迁偶极矩、振动能级、Franck-Condon因子及辐射寿命,结果表明,A~2Σ~+→X~2Π跃迁具有高度对角化的Franck-Condon因子(0.9053)和短的辐射寿命(5.8363×10~(-7)s),符合激光冷却分子的条件.制定了激光冷却OH分子的具体方案,计算得到激光冷却跃迁A~2Σ~+→X~2Π所需的三束激光波长,主光束波长为307.1532 nm,两束重抽运激光波长为344.9163和349.7659 nm.计算结果为超冷OH分子的实验制备提供重要的理论依据.

Laser cooling of OH molecules in theoretical approach

Ultracold molecules have wonderfully potential applications in quantum system, precision measurement, and chemical dynamics, and so on. Thus, people have a strong desire for investigating the potential cooling candidates. Feasibility of laser cooled OH molecules is investigated by ab initio quantum chemistry. Potential energy curves for the ground state X²Π and low-lying excited state A²Σ⁺ of OH molecules are calculated by multi-reference configuration interaction method to develop an applicable cooling transition. In order to obtain more accurate results, the calculations involve Davidson corrections, scalar relativistic corrections, core-valence correlation, and spin-orbit coupling effects. Based on the obtained potential energy curves of Λ-S and Ω states, spectroscopic parameters are determined by solving the one-dimensional radial Schrödinger equation, which are in good agreement with available theoretical and experimental values. The permanent dipole moments, transition dipole moments, vibrational levels, Franck-Condon factors and radiative lifetimes of OH molecules are also calculated. The results indicate that the OH molecule has a highly diagonally distributed Franck-Condon factor (f_{00}=0.9053) for the A²Σ⁺ (ν'=0} ight) → X²Π (ν"=0} ight) transition and short radiative lifetime (τ₀₀=5.8363×10^-7 s) for the A²Σ⁺ state. It means that the OH molecule meets the criteria as a promising candidate for direct laser cooling, which can ensure rapid and efficient laser cooling. Finally, a specific scheme for laser cooling of OH molecules is proposed, and the scheme for the A²Σ⁺ → X²Π transition requires three laser wavelengths, i.e., main pump laser with λ₀₀=307.1532 nm, two repumping lasers, with λ₁₀=344.9163 nm and λ₂₁=349.7659 nm, respectively. The data imply the probability of laser cooling OH molecules with three electronic levels. In addition, the calculated results also indicate that spin-orbit splitting of X²Π is much less than vibrational level, which leads to the conclusion that spin-orbit coupling has no effect on laser cooling scheme of OH molecules. The results above will provide an important theoretical basis for preparing ultracold OH molecule.