Theoretical Study Of Controlling The Orientation Of Diatomic Molecules In Ultra-short Laser Pulse Fields

Molecular reaction dynamics,also known as micro-reaction dynamics,is a subject of understanding the nature and regularity of chemical reactions from the microscopic properties of atoms and molecules,analyzing the motion and the interaction between molecules.It applies the advanced analytical method of modern physical chemistry to study the dynamic structure,reaction process and reaction mechanism of the elementary chemical reactions of single molecules in different states and molecular systems at the atomic and molecular level.According to the theory of quantum mechanics,light can interact with atoms and molecules through the polar distance of the galvanic,magnetic dipole moment,electric multi pole moment,magnetic multi pole moment,fine structure,hyperfine structure,polarization and magnetization,Stark effect,Zeeman effect and so on.Our thesis is based on the time-dependent quantum wave packet theory,which theoretically investigates the directional process of polar diatomic molecules under the interaction of ultra-short laser pulses(only considering the interaction between the laser field and the electric dipole moment).The main works in this thesis include these two following aspects:With the LiH molecule as an example,we propose a theoretical scheme to control molecular orientation by using few-cycle phase-jump pulse and few-cycle phase-jump pulse train.(1)First of all,a theoretical scheme with a few cycle phase jump pulse to control the molecular orientation is proposed,with the Li H molecular as an example.By accurately solving the time-dependent Schr?dinger equation,which including the rotational degrees of freedom and vibrational degrees of freedom,the influence of the two parameters-jump phase and the peak intensity of the external field pulse on the directional effect of the periodic phase jump pulse is discussed in detail.Compared with the terahertz few-cycle laser pulses,the use of few-cycle phase-jump pulses can excite more ro-vibrational transitions and hence increase the molecular orientation degree.By optimizing the field parameters,a maximum value of 0.860 for the degree of polar LiH molecular orientation is obtained.The effect of temperature on molecular orientation is also discussed in our article,the degree of molecular orientation decreases with the increase of temperature.(2)Secondly,on the basis of(1),we also put forward a theoretical scheme used for achieving an effect of Li H molecular orientation by using few-cycle phase-jump pulse train.Here,we first calculate the molecular orientation of three terahertz few-cycle laser pulses,and then compare the different effect between the terahertz few-cycle pulse train and the few-cycle phase-jump pulse train when the pulse number are 1,3,and 6 in order.Finally,we calculate the optimal number of pulses corresponding to the maximum orientated degree under different jump phase.The results show that by adjusting the parameters of the laser pulse reasonably,we can obtain relatively satisfactory results,and it can also be a good explanation for the experiment.