Loreau J, van der Avoird A
Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles (ULB) CP 160/09, 50 av. F.D. Roosevelt, 1050 Brussels, Belgium.
Theoretical Chemistry, Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.
J Chem Phys. 2015 Nov 14;143(18):184303. doi: 10.1063/1.4935259.
We present a theoretical study of elastic and rotationally inelastic collisions of NH3 and ND3 with rare gas atoms (He, Ne, Ar, Kr, Xe) at low energy. Quantum close-coupling calculations have been performed for energies between 0.001 and 300 cm(-1). We focus on collisions in which NH3 is initially in the upper state of the inversion doublet with j = 1, k = 1, which is the most relevant in an experimental context as it can be trapped electrostatically and Stark-decelerated. We discuss the presence of resonances in the elastic and inelastic cross sections, as well as the trends in the inelastic cross sections along the rare gas series and the differences between NH3 and ND3 as a colliding partner. We also demonstrate the importance of explicitly taking into account the umbrella (inversion) motion of NH3 in order to obtain accurate scattering cross sections at low collision energy. Finally, we investigate the possibility of sympathetic cooling of ammonia using cold or ultracold rare gas atoms. We show that some systems exhibit a large ratio of elastic to inelastic cross sections in the cold regime, which is promising for sympathetic cooling experiments. The close-coupling calculations are based on previously reported ab initio potential energy surfaces for NH3-He and NH3-Ar, as well as on new, four-dimensional, potential energy surfaces for the interaction of ammonia with Ne, Kr, and Xe, which were computed using the coupled-cluster method and large basis sets. We compare the properties of the potential energy surfaces corresponding to the interaction of ammonia with the various rare gas atoms.
我们对低能情况下NH₃和ND₃与稀有气体原子（He、Ne、Ar、Kr、Xe）的弹性碰撞和转动非弹性碰撞进行了理论研究。对0.001至300 cm⁻¹的能量范围进行了量子密耦计算。我们关注的碰撞情况是，NH₃最初处于反转双重态的上能级，j = 1，k = 1，这在实验背景下最为相关，因为它可以通过静电捕获和斯塔克减速。我们讨论了弹性和非弹性截面中共振的存在，以及沿着稀有气体序列非弹性截面的趋势，以及作为碰撞伙伴的NH₃和ND₃之间的差异。我们还证明了为了在低碰撞能量下获得准确的散射截面，明确考虑NH₃的伞形（反转）运动的重要性。最后，我们研究了使用冷或超冷稀有气体原子对氨进行交感冷却的可能性。我们表明，一些系统在低温 regime下表现出较大的弹性与非弹性截面比，这对交感冷却实验很有前景。密耦计算基于先前报道的NH₃-He和NH₃-Ar的从头算势能面，以及使用耦合簇方法和大基组计算得到的氨与Ne、Kr和Xe相互作用的新的四维势能面。我们比较了对应于氨与各种稀有气体原子相互作用的势能面的性质。
