Xe-NH(X^3Σ^-)体系的势能面和冷碰撞动力学研究

Potential energy surface and cold collision dynamics of Xe-NH(X^3Σ^-) system

以超冷Xe原子感应冷却NH(X^3Σ^-)分子实验为背景,理论研究磁场中Xe和NH的冷碰撞动力学性质.通过从头算方法得到了解析表达的Xe-NH体系势能面,并在此基础上采用量子动力学计算方法,研究了磁场条件下NH低场追索态(n=0, m_j=1)的冷碰撞塞曼弛豫截面.结果表明超冷Xe原子感应冷却NH分子可能在实验上难以实施.

Sympathetic cooling is one of the most promising techniques for producing ultracold molecules from precooled molecules. Previous researches have shown that it is inadequate to use the ultracold alkali-metal atoms as coolant for sympathetic cooling. To explore the possibility of ultracold alkali-earth-metal atoms as coolant, in this paper a theoretical investigation is performed of the cold collision dynamics for Xe-NH（X^3Σ^-） system in magnetic fields. The interaction potential energies of Xe-NH complex are calculated respectively by using the single and double excitation coupled-cluster theory with the noniterative treatment of triple excitations [CCSD（T）] method and complete basis set limit extrapolated method. An analytic express of potential energy surface（PES） is given for the first time. A single global minimum value occurs at R = 7.14a0, θ = 102.76° with an energy of -153.54 cm^-1, and the PES has a weak anisotropy. Combine the ab initio PES with quantum scattering theory, then the cold collisional dynamics of Xe-NH system in a magnetic field will be studied. The elastic and inelastic transition cross sections and their ratios of NH molecules in the lowest low-field following state（n = 0, mj= 1） under different magnetic fields and collisional energies are calculated. The results show that the elastic cross section is independent of magnetic field, and the inelastic cross section changes with magnetic field,especially at an ultracold temperature. A common rule of thumb is that to successfully implement cooling, the ratio of elastic cross section to inelastic cross section needs to reach 100 at least. The results suggest that it is likely to be a challenging work to perform sympathetic cooling of NH molecule by ultracold Xe atom.