Theoretical Study Of Photoassociation And Recombination Dynamics Of Cold Atoms

With the rapid development of laser cooling and magnetic trap technologies,it becomes more and more feasible to prepare cold(1 mK?1K)and ultracold(less than 1 mK)atomic gases in experiment.These cold and ultracold atomic gases,which present strong quantal features,provide platforms to explore quantum information and computation,quantum phase transition,few-body and many-body physics,etc.Compared with cold and ultracold atoms,cold and ultracold molecules have even more complex internal structures,which make it pos-sible to find and to explore the novel physical phenomena,such as quantum fluid,superfluid phase transition,and quantum simulation,etc.Furthermore,cold and ultracold molecules are also important for the measurement of fundamental physical constants,and for the in-vestigation of molecular spectroscopy and ultracold chemistry,etc.Photoassociation(PA)and recombination are two important.processes in the ultracold atomic and molecular research field.Photoassociation is a process to prepare cold molecules from cold atoms with the interaction of laser fields,which is an efficent,method to prepare cold molecular gases from cold atomic gases.Recombination processes of cold atoms is a nonradiative scattering process that several atoms collide to form relevant molecules and atoms.This process is an important loss mechanism that determines the stability of cold atomic gases,and one of the basic methods to prepare cold molecules.Cold atomic gases relevant to Na,He and H are ready to be prepared nowadays in experiment.He and H are also abundant in the interstellar space and the relevant collision process is of importance in the study of the interstellar evolution.Based on these atomic gases this dissertation studies the relevant photoassociation and the three-body recombination(TBR)processes in detail,which are summarized as follows.Firstly,based on the HeH+ system,the variation of the optimized PA probability,rel-evant to the variety of initial(relative collision)momenta of the collision pair(He+H+),is investigated.It is shown that with the increase of the initial momentum of He+H+ the optimized PA probability first increases and then decreases,and that,the above threshold dissociation gradually tends to be a dominate accompany process.At some initial momenta,the multiphoton transition process is strong.Furthermore,the relationships between the optimized laser pulse parameters and the initial momentum of He+H+ are also discussed.Secondly,based on the NaH system,the H/D isotope substitution and molecular align-ment effects on the pump-dump PA process are investigated.It is found that the isotope effects on the probability of PA are proportional to Franck-Condon factors of the relevant systems.The field-free alignment of the prepared molecules in the pump process can affect the coupling intensity between the electronic states and population transfer pathways of the dumping process.As a result,the final population transferred to the bound states of the ground electronic state varies periodically with the central time of the dumping pulse.More-over,the counterintuitive relationship between the field-free alignnment of the NaH system and the final PA probability has been well explained,and the isotope effects on the molecu-lar alignment and the molecular alignment effects on the Na2 system via a pump-dump PA scheme are also reported.Whereafter,the role of sharp avoided crossings(SACs)in recombination of the cold 4He3 system is investigated.It is found that for the J =0 partial wave,the SACs among three-body adiabatic potential curves in the short hyper-radial range play important roles in coupled-channel calculations.This may be ascribed to the attractive potential well of the(two-body)recombination channel,which leads to a strong tunneling effects in the short,hyper-radial range.For higher partial waves(J>0),the recombination channel is changed to be repulsive and makes the wave functions to be unaccessible in the short hyper-radial range.Therefore,the omission of SACs for these symmetries has no effect on the numerical results,which leads to great savings on hyper-radial grid points in the practical numerical calculations.Moreover,we present an effective method associated with the application of the consistent phase convention.At last,the TBR processes of the cold 4He4HeH-/4He4HeD-system and the distribu-tion of the product states are investigated.It is found that in the ultracold limit(E?0.1 mK),different structures of three-body potentials and nonadiabatic couplings lead to differ-ent distributions of product states.The rate of TBR into the 4HeH-molecular ion is nearly two orders of magnitude larger than that of TBR into the neutral 4He2 molecule for the 4He4HeH-system.In contrast,the yield of the neutral 4He2 molecule are 1.4 times higher than that of the 4HeD-molecular ion for the 4He4HeD-system.