A Study Of The Wave Packet Dynamics Of Photoassociation Controlled By The Ultrashort Laser Pulse

The time-dependent quantum wave packet method has been widely applied to the physical and chemical areas to study the interaction dynamics of molecules with laser fields. Especially, with the emergence and development of femtosecond laser techniques, the molecular dynamics can be detected and investigated on a faster time scale. Nowadays, many methods have been proposed for controlling chemical reactions including photoassociation reaction with laser pulse.By making use of the laser field to implement the molecular state-selectivity is a significant project. The essence of this research is to excitated the molecule to the target state by adjusting the parameters of the laser pulse. Consequently, the control of state-selectivity in and between molecules can be carried out efficiently. The molecular state-selectivity has been widely used for studying collision dynamics, spectrum of molecules and laser chemistry.In this thesis, the time-dependent quantum wave packet theory was mainly used to stimulate the basic dynamics of molecules, HF and HD~+, photoassociation reaction controlled by laser pulse. First, the relationship between the initial collision momentum and the association probability together with the state-selectivity is studied for the product HF. Then the effect of the laser parameters on the vibrational state-selectivity is discussed in detail. By optimizing the laser parameters, the control of the product molecule HF in a selective vibrational state can be achieved effectively. Meanwhile, the impact of the laser pulse intensity is analyzed particularly. It is shown that a stronger laser pulse can transfer the population of the continuum state to the lower vibrational state. However, with the increasing of the laser energy, the multiphoton transition and dissociation accompany the association process. The resonant transition condition can be obtained from the calculation. The wave packet dynamics in the momentum space representation is studied, and the peaks of the wave packets can be observed at different times. In the calculation, we find that the energy difference between two neighboring peaks equals one-photon energy. This indicates that the product HF of association reaction may be dissociated by absorbing more photons. From the photoassociation study of HD~+, we conclude that the two-color laser pulses can more efficiently lower the dissociation probability and therefore enhance the yield ratio of the product than the one-color laser pulse, and the populations of the products can be controlled with the laser pulse. There are two channels in the photoassociation reactions, and the reaction probability through each channel can be calculated.Although the model employed is simple, the results may serve as a guideline for controlling the product of photoassociation reaction.