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

The works in this thesis are based upon the time-dependent quantum wave packet method. to study the orientation and above threshold ionization dynamics of diatomic molecules in ultrashort laser field. The main works are as follows.With the LiH molecule as an example, we propose three new schemes to control the field-free molecular orientation by utilizing the characters of the external laser fields and dif-ferent combination of several fields.(1) A theoretical scenario used for achieving efficient field-free molecular orientation with a few half-cycle pulses (HCPs) is proposed, with the LiH molecule as an example. Compared with a single HCP. three HCPs can excite more rovibrational transitions and obviously en-hance the degree of molecular orientation. By optimizing the field amplitude and the delay time of laser field, a maximum value of0.798for the degree of molecular orientation is ob-tained.(2) Using the two-dimension time-dependent quantum wave packet method, we investi-gate theoretically the field-free molecular orientation driven by a modulated two-color laser pulse together with a delayed half-cycle THz pulse. In our control scheme. the half-cycle THz pulse is used to orient molecules, and the modulated two-color laser pulse is utilized to control the rovibrational excitation and enhance the molecular orientation degree. The effects of the phase of envelope (?) and the period Tp of the modulated two-color laser field on the molecular orientation and the population of the target state are discussed in detail. It is shown that more molecules are populated to the target state and more efficient molecular orientation is achieved by adjusting these parameters of the modulated two-color laser pulse. The molecular orientation is comparatively robust towards thermal averaging in the region of T<50K.(3) Considering the effect of the polarizability. we present a theoretical scheme used for achieving an efficient long-lived field-free molecular orientation by utilizing two modulated few-cycle THz pulses with an appropriate delay time. The exact numerical calculations are performed by solving the time-dependent Schrodinger equation including the vibrational and rotational degrees of freedom. The results indicate that the orientation degree and the orien- tation duration are strongly related to the period of envelope. the central frequency and the phase of envelope of the modulated few-cycle THz pulses. Taking a compromise. a long-lived and efficient field-free molecular orientation can be realized by adjusting these laser param-eters which is available in the current experiment. The effect of temperature on molecular orientation is also discussed.We also investigate theoretically the above-threshold ionization (ATI) of the polar NaK molecule exposed in few-cycle laser field. The left and right ATI spectra are calculated by integrating the ionization continuum wave function over the left and the right half spheres along the laser polarization. We find that the left and right ATI spectra are asymmetric, and this asymmetry depends strongly on the molecular orientation and the carrier-envelope phase (CEP) of the laser pulse. Moreover, we also perform the calculation for different ini-tially rotational states to study the effect of rotational temperature on the ATI danamics.