Theoretical Study Of Ionization And Orientation Dynamics Of Diatomic Molecules In Ultrafast Laser Fields

The ionization and orientation dynamics of diatomic molecules in ultrashort laser fields are theoretically studied in this thesis. The theoretical method is based on the numerical solution of the time-dependent Schrodinger equation. The main works are as follows.(1) The Autler-Townes (A-T) splitting in photoelectron spectrum of Li2 molecule is theoretically investigated using one-dimension time-dependent quantum wave packet method. With femtosecond laser pulses, three peaks of the A-T splitting can be ob-served in the photoelectron spectrum. The splitting results from rapid Rabi oscillations of different electronic states caused by intense ultrashort laser pluses. The effects of laser parameters on the molecular ionization dynamics are also studied. The obvious A-T splitting can be observed by choosing proper laser parameters.(2) Using two-dimension time-dependent quantum wave packet method, three-peak A-T splitting in the resonant multiphoton ionization photoelectron spectrum for a rotating Li2 molecular system in femtosecond pulse laser fields is studied. Due to the effects of molecular rotation and alignment, the Rabi oscillation in the population distribution will be damped in a certain degree. The three-peak A-T splitting can only be observed for a strongly aligned molecule with rapid Rabi oscillation. The three-peak A-T splitting dynamics can be affected by intensity, duration, temporal profile of laser pulse and initial molecular rotational temperature. The conditions to observe the A-T splitting are discussed in detail.(3) With HF and LiH as examples, we show that field-free molecular orientation induced by a terahertz half-cycle pulse can be considerably enhanced by rovibrational selective excitation. The rovibrational selective excitation process is controlled by infrared laser pulses. The calculated results are obtained by solving the full Schrodinger equation, where the coupling between the vibration and rotation is included. The extension of our scenario to consider a finite rotational temperature is presented. The effects of the molecular properties on the field-free orientation are also discussed. (4) With HF and LiH as examples, we demonstrate theoretically that an enhanced field-free molecular orientation can be achieved by a combination of terahertz few-cycle pulses and rovibrational pre-excitation. The calculations are performed by solving exactly the full time-dependent Schrodinger equation. Compared to the results using the half-cycle pulse scheme, a more efficient field-free orientation and a more obvious enhancement can be achieved by using our scenario. The molecular orientation exhibits a dependence on the peak electric-field amplitude, and the saturation of 0.75 can not be overcome under the action of a single terahertz few-cycle pulses. This scenario can be realized at a finite rotational temperature, and sustainable field-free orientation can be achieved even at room temperatures. The realization of this scenario is feasible since the THz pulses with large peak electric-field amplitudes are available under the present experimental condition.(5) We demonstrate theoretically that the long-lived and efficient field-free molec-ular orientation can be realized by utilizing two terahertz few-cycle pulses (TFCPs) appropriately delayed in time at a finite temperature. The calculations are performed by solving the time-dependent Schrodinger equation including the vibrational and the rotational degrees of freedom, with NaH, LiCl and NaI as examples. For the NaH molecule, we obtain that orientation efficiency and relative duration are 0.76 and 16.5% at 5 K, respectively. While for the LiCl molecule, orientation efficiency and relative duration are 0.75 and 22.2% at 1 K, respectively.