Dynamics Of The Laser Induced Molecular Alignment And Orientation

The interaction between laser and molecule is an important research project in the atom and molecular physics.In this paper,the laser induced molecular alignment and orientation based on a rigid-rotor model are studied by solving the time-dependent Liouville equation.The alignment and orientation of linear molecule driven by super-Gaussian laser pulse and elliptically polarized laser pulses are discussed,respectively.We also study the orientation of symmetric-top molecules by a shaped chirped laser pulse.In chapter 1,we briefly introduce the research significance and progress of the molecular alignment and orientation.In chapter 2,we mainly introduce the knowledge of the laser pulse.Then,we introduce the definition of the molecular orientation and alignment,the properties of density matrix theory and the two main theoretical calculation methods of molecular orientation and alignment.In chapter 3,we proposed a scheme for achieving the molecular orientation by using the super-Gaussian laser pulse.We show that compared with the molecular orientation driven by Gaussian laser pulses,the molecular orientation degree can be enhanced by using the super-Gaussian laser pulse under the same parameters.By comparing the maximal orientation degree with different pulse durations,we find it is hard for super-Gaussian laser-induced orientation to reach the purely adiabatic regime,so using the super-Gaussian laser pulse can give us more pulse durations to choose from for field-free molecular orientation.We find that increasing the shape parameter N and choosing an appropriate electric field intensity can also be beneficial to enhance the orientation degree.In addition,increasing temperature has an inhibitory effect on molecular orientation for both laser pulses,but the super-Gaussian laser pulse is more feasible in an ensemble of higher temperature.In chapter 4,we proposed a scheme for achieving the molecular alignment and orientation by using the two elliptically polarized laser pulses.It is shown that the field-free molecular alignment can be achieved in a three-dimensional?3D?case,while the field-free molecular orientation is only along the x and y directions,and that the field-free alignment and orientation along different axes are related to the populations of the rotational states.We find that both in-pulse and post-pulse molecular alignment and orientation can be controlled by changing elliptic parameters.In addition,the effect of the delay time on field-free molecular alignment and orientation is discussed.In chapter 5,we proposed a scheme for achieving the molecular orientation of CH₃Cl by using a shaped chirped laser pulse with terahertz?THz?frequency.It is found that the field-free orientation degree can be enhanced with this scheme compared with the few-cycle THz laser pulse under the same conditions.In order to obtain a higher orientation degree,the effects of chirp values and shape parameters on molecular orientation are discussed.We can deduce that the maximal field-free orientation degree can be enhanced by choosing appropriate chirp values and shape parameters.It is demonstrated that a higher orientation degree is related to the larger asymmetry of the pulse energy.We also find that the shaped chirped laser induced orientation cannot reach the purely adiabatic regime due to the sudden change of the carrier envelope phase?CEP?.In addition,the initial rotational state with definite KM values also has a significant effect on molecular orientation.In chapter 6,we conclude the three research work of this paper and look forward to the next work.