Harvard-MIT Center for Ultracold Atoms, Cambridge, Massachusetts 02138, USA.
J Chem Phys. 2011 Apr 14;134(14):144301. doi: 10.1063/1.3575399.
We present an ab initio study of cold (4)He + ThO((1)Σ(+)) collisions based on an accurate potential energy surface (PES) evaluated by the coupled cluster method with single, double, and noniterative triple excitations using an extended basis set augmented by bond functions. Variational calculations of rovibrational energy levels show that the (4)He-ThO van der Waals complex has a binding energy of 10.9 cm(-1) in its ground J = 0 rotational state. The calculated energy levels are used to obtain the temperature dependence of the chemical equilibrium constant for the formation of the He-ThO complex. We find that complex formation is thermodynamically favored at temperatures below 1 K and predict the maximum abundance of free ground-state ThO(v = 0, j = 0) molecules between 2 and 3 K. The calculated cross sections for momentum transfer in elastic He + ThO collisions display a rich resonance structure below 5 cm(-1) and decline monotonically above this collision energy. The cross sections for rotational relaxation accompanied by momentum transfer decline abruptly to zero at low collision energies (<0.1 cm(-1)). We find that Stark relaxation in He + ThO collisions can be enhanced by applying an external dc electric field of less than 100 kV∕cm. Finally, we present calculations of thermally averaged diffusion cross sections for ThO in He gas, and find these to be insensitive to small variations of the PES at temperatures above 1 K.
我们基于耦合簇方法（用单、双和非迭代三激发对其进行扩展基函数增强）评估的精确势能面（PES），对冷（4）He + ThO((1)Σ(+))碰撞进行了从头算研究。转动能级的变分计算表明，在其基态 J = 0 转动态下，（4）He-ThO 范德华复合物具有 10.9 cm(-1)的结合能。计算出的能级用于获得形成 He-ThO 复合物的化学平衡常数对温度的依赖关系。我们发现，在低于 1 K 的温度下，复合物的形成在热力学上是有利的，并预测在 2 到 3 K 之间自由基态 ThO(v = 0, j = 0)分子的最大丰度。在弹性 He + ThO 碰撞中，动量转移的截面在低于 5 cm(-1)的范围内呈现出丰富的共振结构，并在高于此碰撞能的情况下单调下降。伴随着动量转移的转动弛豫的截面在低碰撞能（<0.1 cm(-1)）下急剧降至零。我们发现，在 He + ThO 碰撞中可以通过施加小于 100 kV∕cm 的直流电场来增强 Stark 弛豫。最后，我们给出了 He 气体中 ThO 的热平均扩散截面的计算结果，并发现这些截面在 1 K 以上的温度下对 PES 的微小变化不敏感。
