Study On The Potential Energy Surface And Quantum Dynamics For He-LiH Complex

Study on weak-interaction systems is always an important topic of many experiments and theories in fields of molecular physics and chemical physics. Lots of attentions of astrochemists and astrophysists were devoted to the investigation of the complexes built with LiH molecule owing to the important role in astrochemistry. Since He-LiH complex is a small proto-type of weak-interaction systems, and the high computational precision can be achieved, the international scholars are paying special attentions to this system.In this thesis, we have detailedly studied on the potential energy surface and quantum dynamics of the He-LiH complex. There are five sections. In chapter one, the computational methods for the weak intermolecular interaction and the development of dynamical studies for the He-LiH system are briefly introduced.In chapter two, the basic principle of the molecular orbits ab initio and the theory of the inelastic scattering are presented.In chapter three, the intermolecular interaction energies corresponding to different He-LiH configurations have been calculated by supermolecular and CCSD(T) approach with a large basis setcontaining the bond function set 3s3p2dlf, and then the potential energy surface are obtained after the analytical function is constructed with the nonlinear least-squares fitting. As the result, the potential energy surface has two potential wells .The deeper one corresponds to the linear configuration He-Li-H, and Rm= 4.25a0, Vm=-177.75 cm-1. The shallower one corresponds to the linear configuration He-H-Li, and Rm=10.0a0, Vm=-9.95cm-1.The potential energy surface is extremely anisotropic, mainly originated from the anisotiopy of the induced force related to thelarge electronic dipole momentum (u = 5.88D) of the LiH molecule.In chapter four, the process of rotational inelastic collision of He-LiH complex has been discussed with close-coupling method based on the potential energy surface from chapter three. As a result, the transition cross sections of j = 0 - j' originated from the contributions of both the anisotropic short-range interaction ("rigid" repulsion and attraction) and the long-range "soft" repulsion, and it isn't evident for the long-range attraction. The near-linear collision of He atom from H atom side with the LiH molecules is the most efficient manner for j = 0 - j'= 1. Furthermore, the state-to-state integral cross sections display a pronounced oscillatory structure. The long-range "soft" repulsion partial cross sections have only an effect on the integral cross sections for j = 0 j= 1,2,3, and the cross sections for j > 4 are almost decided by the anisotropic short-rang partial. Our results are more close to theexperiment ones than VB.In chapter five, the rovibrational energy levels and wave functions of He-LiH system were calculated using discrete position representation method based on the CCSD(T) potential energy surface. There are ten rovibrational bound states, and zero point energy is 144.49 cm-1. The wave function of the first vibration level mostly distribute near =180,and it shows this eigenstale essentially locate at the deeper potential well at the end of Li. The second vibration level is a stretching excited one with vibration frequency 29.85cm-1, which can also be assigned to the deeper potential well, however, the shallower well has its role on the probability distributions by tunneling effect.