The Field-Free Orientation Of CO Molecule Controlled By Ultrafast Laser Pulses

Molecular orientation is an important subject for physics and chemistry,which has been widely used in the fields of photoelectron angular distribution and high-order harmonic generation.In this paper,the orientation of CO molecule under the rigid rotor model is theoretically calculated controlled by ultrashort laser pulses.The main tasks are as follows:(1)The effects of two-color femtosecond pulse and positive chirped pulse with delay time on the field-free orientation of CO molecule are theoretically calculated.Compared with single femtosecond or positive chirped laser,the combination of two laser pulses can greatly improve the molecular orientation.It is found that with the increase of electric field intensity,both positive and negative orientation of CO molecule increases first and then decreases.In addition,the delay time between the two pulses can significantly improve the molecular orientation.Moreover,by changing the carrier frequency and chirp value of the positive chirped laser pulse,the molecular orientation can be changed in a certain range.Molecular orientation can also be manipulated by adjusting the relative amplitude of the second harmonic field and the fundamental field of the two-color femtosecond pulse.At the same time,the effect of rotation temperature is also considered.Molecular orientation decreases rapidly with the increase of rotation temperature.(2)Field-free orientation of CO molecule is studied theoretically by combining two dual-color shaped laser pulses with low intensity of THz laser pulse.The result shows that the molecular orientation is improved by using two dual-color shaped laser pulses and low-intensity of THz laser pulse compared with single THz laser pulse with high intensity,and the field intensity of THz laser pulse is reduced by using this method,which meets the experimental requirements.Firstly,the effects of the rise time of the shaped laser pulse and the electric field intensity on the field-free orientation of CO molecule are considered.In addition,the molecular orientation can be controlled to some extent by changing the two delay time between the three pulses.Moreover,the increase or decrease of molecular orientation can be controlled coherently by changing the central frequency and carrier envelope phase of the THz laser pulse.Finally,the effect of initial rotation temperature on orientation is considered,and it is found that the orientation decreases rapidly with the increase of temperature.