Controlling Photoassociation,Photodissociation And Orientation Dynamics By Ultrafast Laser Pulses

Controlling the reactions of molecules and atoms with laser field has been a popular topic which attracts the interest of researchers both in physics and chemistry. In this paper, we focus on three themes. In the first work, the formation of ultracold Cs2molecules via photoassociation (PA) with a spectral π-jump laser pulse is theoretically investigated by solving the time-dependent Schrodinger equation using the mapped Fourier grid method. Numerical calculations show that the population distributions of associated molecules mainly result from the off-resonant excitation of the π-jump laser pulse. Compared with the transform-limited pulse, an efficient population accumulation in the PA process is achieved with the π-jump pulse. The PA efficiency is related to the Rabi angle of the laser pulse. A π-jump pulse can be used to enhance more effectively the PA efficiency than a transform-limited pulse with the same Rabi angle.In the second work, the field-free orientation induced by a single-cycle terahertz (THz) laser pulse is studied for two "heavy" molecules, NaI and IBr. Two methods are used and compared in the calculations:One is to solve the exact time-dependent Schrodinger equation (ETDSE) by considering the full-rovibrational degrees of freedom, and the other is to invoke the rigid-rotor approximation (RRA). Calculations are performed at the central frequency varying from0.05to1.0THz and at the peak intensity taken to be5×107,2×108and5×108W/cm2, respectively. The degree of field-free orientation is strongly dependent on the central frequency and the peak intensity of the single-cycle THz pulse. The molecular orientation obtained by the RRA calculations is in good agreement with that obtained by the ETDSE in the given parameter region.In the third work, the orientation and predissociation dynamics of the Nal molecule are studied by using a time-dependent wavepacket method. The Nal molecule is first pre-oriented by a single-cycle pulse (SCP) in the terahertz (THz) region and then predissociated by a femtosecond pump pulse. The influence of the molecular field-free orientation on the predissociation dynamics is studied in details. We calculate the radial and the angular distributions, the molecular orientation degrees, and the time-dependent populations for both the ground and excited electronic states. It is found that the pre-orientation affects the angular distributions significantly, and that it has weak influence on the radial distributions. By changing the delay time between the THz SCP and the pump pulse, the angular distribution of the fragments from the predissociation can be manipulated. Last, we calculate the Nal molecular field-free orientation degrees under different envelope phases of the sine squared pulse. The effect of the pulse phase to the molecular field-free orientation is discussed.