Controlling Atomic Photoassociation Dynamics And Charge Transfer In Collision Process By Shaped Laser Pulses

Laser control of atomic photoassociation and charge transfer during the collision have become two hot topics in the field of atomic and molecular physics. This thesis aims to investigate theoretically how to control photoassociation and charge transfer by shaped laser pulse during the collision. It is summarized as follows:(1) We present a theoretical scheme to promote photoassociation probability of ultracold atoms. By solving the Schrodinger equation, it is found that the slowly-turned-on and rapidly-turned-off laser pulse can reduce the energy exchange between the molecules and pulse after the time corresponding to the peak amplitude, to prevent the photoassociated molecules in the excited electronic state from returning to the initial scattering state. The higher probability of photoassociation requires the shorter falling time. In addition, we achieve higher photoassociation probability by population accumulation effect with a train of this kind of shaped laser pulses.(2) The photoassociation dynamics of the ultracold atoms steered by a modulated laser field has been investigated theoretically. The shape of time-dependent electric field and its spectral distribution are sensitive to phase of envelope. When the period of envelope is longer than the full width at half maximum, the spectral intensity distribution can be well modified by changing the phase of envelope. The final population distributions of vibrational states involved in the PA process and the final PA efficiency are strongly influenced by the relevant Frank-Condon factors of these vibrational states, the distributions of spectral intensity, and the asymmetry of time-dependent modulated laser field.(3) We theoretically investigate the photoassociation dynamics of ultracold atoms steered by the picosecond laser pulse with cubic-phase modulation. The cubic-phase pulses can pro-long the temporal duration and increase the pulse area, and enhance the PA efficiency. When the pulse area is larger than a certain value, the cubic-phase pulses can achieve obviously higher PA efficiencies, although they have lower energies. The cubic phases can not change the range that the vibrational states of the excited electronic state are populated because they display the same spectral intensity distribution as the transform-limited pulse.(4) We investigate theoretically how to stabilize photoassociated molecules in the outer well by a pump pulse to the inner well via a vibrational state of the ground electronic state as an intermediary by utilizing an additional optimized shaped pulse, which requires its spectral width is enough to cover the energy difference between the initial state and the target state in the inner well. Nearly all molecules in the inner well of the excited electronic state can be transferred to the lower vibrational levels of the ground electronic state by a transform-limited dump pulse.(5) Optimal control of charge transfer in slow H++D collisions in the presence of laser fields is studied in the context of a two-state Hamiltonian based on an adiabatic represen-tation for diatomic molecular ions. Field-free charge transfer through nonadiabatic coupling alone produced a lower yield, while optimal control schemes achieve almost unit yield of charge transfer and significantly involve several vibrational states during the collision. Nu-merical simulations were performed using two different target schemes to obtain the desired optimal laser control fields. The adaptive target scheme was more efficient, while producing an outgoing wave packet of complex shape. The fixed Gaussian target scheme converged much more slowly, but capable of producing an outgoing wave packet in precise agreement with the designated Gaussian target.