Theoretical Study On Collision Dynamics Of Triatomic Systems In The Low Collision Energy

Molecular collision dynamics is a subject that studies collisional scattering from the microscopic level of atoms and molecules.The method of theoretical studying molecular collision dynamics is to simulate the collision process between atoms and molecules by computers and use the calculated results to reveal the internal mechanism of the scattering.The triatomic system is a benchmark system for studying the process of molecular collision,and its inelastic and reactive processes have important applications in the fields of astrophysics and low-temperature chemistry.However,the research on the triatomic systems is faced with some problems,such as insufficient low-temperature research,lack of potential energy surface of the system and insufficient research on the reaction of the ro-vibrationally excited molecules.To solve these problems,theoretical research of the molecular collision dynamics on three different triatomic systems was performed in this work.First,the scattering dynamics features of He with H2 and its isotopologues were studied on a new potential energy surface(PES).Then the scattering process of TiO molecules and He atoms was studied based on the constructed high-precision He-TiO PES.Finally,the dynamics of the H+HD reaction was deeply studied based on the classical CCI PES.The specific work includes the following three parts:(1)Based on the new high-precision BSP PES of the He-H2 system,the dynamics studies were performed by using the time-independent quantum method for the inelastic rotational quenching process of the rotationally excited H2 and its isotopologues due to He collision,and the dynamics results of integral scattering cross sections at the collision energy range of 10-5 cm-1 to 104 cm-1 and the rate coefficients for temperatures ranging from 10-4 K to 3000 K were obtained.The results show that the cross sections follow Wigner's threshold law in the low collision energy region,and the rate coefficients are almost constants at low temperatures.In the high collision energy region,the scattering cross sections show the typical energy transfer process,and the rate coefficients increase with increasing temperature.The van der Waals well on the BSP PES causes the scattering resonance.Compared with the previous results on the MR PES,the potential well on the BSP surface is deeper,thus the scattering resonance is stronger.The isotopic effect is obvious in the low collision energy region,the scattering resonance peak moves to the low collision energy region and the amplitude of the scattering resonance increases as the increase of reduced mass.(2)Based on high level energy points,a new accurate He-TiO PES was constructed by using the neural network method,and the dynamics of the scattering process between He atoms and TiO molecules was studied in detail.In the ab initio calculation,the small-core relativistic effective core potential ECP10MDF with the supplementary atomic natural orbital basis set was employed for Ti atom,the aug-cc-pVQZ basis set was employed for O atom,and the cc-pVQZ basis set was employed for He atom.All the ab initio energy points were calculated by using the multi-reference configuration interaction method,and higher order correlation was compensated by the Davidson correction.Based on this PES,the time-independent quantum method was used to study the rotational quenching of the rotationally excited TiO molecules with He atoms,and the quenching cross sections with collision energy ranging from 10-5 cm-1 to 104 cm-1 and the rate coefficients with temperature ranging from 10-4 K to 3000 K were obtained.Wigner's threshold law is proved to be valid in the ultralow energy regime,and the shallow well on the PES induces weak scattering resonance.The study of isotope effect shows that 4He is a more effective coolant than 3He in the transition j=1?0 of TiO molecules.(3)Based on the accurate CCI PES of the H3 system,the vibrational quenching process of the HD in the first vibrational excited state due to H collisions was studied by using timeindependent quantum method.The state-to-state integral reaction cross sections were obtained for the HD(v=1,j=0)+H?D+H2(v'=0,j')reactions in the collision energy range of 1 cm-1 to 10000 cm-1.A Feshbach resonance in the low-energy regime below the reaction barrier,near the collision energy of 85 cm-1,was observed for the first time.The Feshbach resonance is attributed to the coupling of the initial energy level v=1,j=0 with the adiabatic potential v=1,j=1 of the HD molecules,and the resonance is dominated by the contribution from a single partial wave L=1.The phase behavior of reaction resonance proves that the Feshbach resonance exists,and the resonance was also observed on the BKMP2 PES at the same collision energy.The calculation results on the PESs of CCI and BKMP2 show that the lifetimes of the resonance state both are very long.The suggestions for experimental observation of this Feshbach resonance are given based on the dynamics results obtained in this work.At last,the dynamics of HD(v=1,j>0)+H reaction was studied,and it was found that the cross section is inversely proportional to the gap of energy levels between the initial and final states of the reaction,where the collision energy is greater than the reaction barrier.In the theoretical study of the above three triatomic systems,the results of inelastic and reactive processes in low collision energy regime were obtained by time-independent quantum methods,and the ro-vibrational quenching of molecules in the systems was analyzed and discussed.In this process,we mastered the general rules of low-energy collisions of triatomic systems,explained the dynamic behavior of low-temperature scattering,and discovered the quantum phenomena in the ro-vibrational quenching of the reaction.Through the research work of this thesis,it has deepened the understanding of the low-energy collision dynamics of the triatomic systems.