Theoretical Study Of Molecular Orientation With Shaping Laser Pulses

In recent years,molecular orientation ana alignment nave been paia mucn attention by researchers at home and abroad.In the prepare of oriented target molecules,it is undoubtedly an effective method to induce molecular orientation and alignment by laser pulse.According to the theoretical knowledge of quantum mechanics,we know that light interacts with atoms or molecules through magnetic dipole moment,electric dipole distance,electric multipole moment,Zeeman effect,Stark effect and so on,which can significantly change its microstructure and properties,leading to high order harmonics,orientation,alignment,subthreshold ionization,dissociation and so on.In this paper,the orientation dynamics of diatomic molecules induced by shaping laser pulse is studied by using the time-dependent quantum wave packet method.We take LiH polar molecule as an example,and use the characteristics of laser field itself or the combination of laser field to control field-free molecular orientation.The main work is as follows:(1)Firstly,a theoretical scheme is presented to realize effective molecular orientation driven by two super-Gaussian pulses(SGPs).It is shown that the efficient molecular orientation can be achieved by two SGPs,and the maximal orientation degree is 0.823,which is obviously better than those by the Gaussian pulse(GP)and a single SGP.The orientation degree and orientation duration time are dependent on the laser frequency,field intensity and delay time between two SGPs.The pulse shaped parameter N characterizing the SGP has effects on molecular orientation and rotational population.Furthermore,the molecular orientation degree and orientation duration time decrease with the increase of temperature.A long-lived molecular orientation duration time and high orientation degree can be achieved in the range of temperature from 0 to 100K.(2)Secondly,we propose a theoretical scheme to control the orientation of polar LiH molecules by using an 800nm jump-phase pulse.The time-dependent Schrodinger equation including rotational and vibrational degrees of freedom is solved precisely,then the two important parameters of jump phase and jump phase time on the orientation effect is discussed in detail.Compared with pulse without jump phase,we can observe that more rovibrational states are excited by using jump-phase pulses when jump phase ?-? and jump phase time t0=0.25T,resulting in a significantly improved molecular orientation.And when jump phase ?=?,the change of jump phase time can control the molecular orientation obviously.We discuss the effects of the intense field and full width at half maximum of jump-phase pulse on the orientation,the orientation effect is good with intense field and full width at half maximum 5fs.The calculation results show that the jump-phase pulse can effectively control the orientation of polar LiH molecules.