Quantum Control Of Population Transfer And Orientation Of Diatomic Molecules With Femtosecond Laser Pulses

Molecular reaction dynamics is a subject investigating the reactions of molecules in microcosmic levels and explaining the microscopic chemical reaction mechanism. Molecular quantum control and molecular orientation are two important reserch topics in the field of the molecular dynamics, and also the currently international popular research topics. In this thesis, a two dimmensional time-dependent wave packet method is used to numerically solve the time-dependent Schrodinger equation, the control of population transfer process and rovibrational population distribution by femotosecond pulses is explored, and a theoretical plan controlling molecular orientation by tera-hertz pulses is proposed.In the study of coherent control of quantum states. we employ the two dimensional time-dependent quantum wave packet method including vibrational and rotational degrees of freedom and investigate the rovibrational population transfer between two electronic states of the LiH molecule driven by a train of ultrashort pulses. We find laser pulse with a lower intensity (108-1010W/cm2) has a good selectivity. Theoretical calculation shows that the manipulation of selective rovibrational quantum state is possible by utilizing a train of ultrashort pulses with lower peak intensity and the efficiency of population transfer is close to100percent. We calculate the effective values of rovibrational population under different laser pulse intensities. The result shows that the effective value increases with the increase of the intensity of a single laser pulse when the intensity is below a certain value. However. when the pulse intensity exceeds a certain value, the efficiency reduces with the increase of intensity of a single pulse. The population transfer from an initial state to a selective rovibrational state is also controllable by varying the relative phase between two adjacent pulses of a pulse train. Moreover, the effect of rotational temperature on the population transfer is also discussed. Specially, we provide the rovibrational population distributions at initial state and target state at T=20and60K. And we give the reason why the molecular population efficiency decrease with the increase of temperature. In the study of molecular orientation control. we take the diatomic molecules LiH as an example and demonstrate theoretically that an efficient field-free molecular orientation driven by the positive chirped laser pulse whose frequency is in the terahertz regime can be achieved. Exactly numerical calculations are performed by solving the time-dependent Schrodinger equation including the vibrational and rotational degrees of freedom. We compare the effective values of orientation by utilizing positive chirped pulse with that by half-cycle or negatively chirped pulses in the framework of two-dimensional wave packets. The maximum of effective value in our plan by positive chirped pulse can reach0.85. which is larger than0.75and0.70obtained by the other two pulses. at the same intensity4.78×108W/cm2. This proves that our plan with positive chirped pulse have a great advantage in improving the molecular orientation. Simultaneously, we discuss the effect of temperature on molecular orientation. The molecular orientation decreases with the increase of temperature. At the end. we have shown the time evolution of the molecular orientation <cos θ> at a temperature of20K by the positive chirped pulse, considering the contribution of different rotational states.