The Molecular Field-Free Orientation And Multi-Photon Ionization Dynamics Controlled By Ultrafast Laser Field

Molecular reaction dynamics is a subject investigating chemical reactions in a microcosmic level,and revealing the microscopic mechanism of chemical reactions.With the rapid development of the laser pulse technology,the ultra-short and ultra-intense laser pulses are frequently used to control the photochemical reactions.It is of great significance to study theoretically the interaction between the ultra-fast and ultra-intense laser pulses and molecules.It has been a popular research topic in the field of molecular reaction dynamics to study the field-free orientation and multi-photon ionization of diatomic molecules.Rapid developments in ultra-fast and ultra-intense laser pulse technology promote the investigation of both the molecular orientation and photoionization dynamics.Several schemes are presented to control theoretically or experimentally the orientation and photoionization.In recent years,the influence of molecular alignment and orientation on photoionization dynamics has excited researchers' interest.The main works in this thesis are as follows:(1)We employ the density matrix theory to study the field-free orientation of the diatomic molecule within the rigid rotor approximation.We investigate theoretically the orientation of CO molecules steered by a single-cycle pulse(SCP)train in terahertz(THz)region.The time evolutions of the density matrix elements are acquired by solving the quantum Liouville equation based on the multilevel Bloch model,and the time-dependent molecular orientation is also obtained.It is shown that the molecular orientation can be obviously enhanced by applying the pulse train.We take the laser intensity in the range of 0E(28)[0.1,10.1] MV/cm and pulse number in the range of N(28)[1,20] to investigate the effects of laser intensity and pulse number on molecular orientation.Furthermore,the optimal pulse numbers corresponding to the maximum orientation for different laser intensity are calculated.(2)We employ the time-dependent quantum wave-packet method to investigate the influence of the molecular pre-orientation on the resonance-enhanced multi-photon ionization dynamics,taking the LiH molecule for example.The LiH molecule is first pre-oriented by a THz SCP,then excited and ionized by the time delayed femtosecond pump-probe pulses.The time evolution of the wave function is obtained by solving the time-dependent Schr?dinger equation.We focus on the impact of the pre-orientation on the ionization probability,energyand angle-resolved photoelectron spectra and photoelectron angular distribution(PAD).It is found that the ionization probability and peak intensity of energy-resolved photoelectron spectra are significantly affected by the molecular orientation.The angle-resolved photoelectron spectra are related to molecular orientation.The PAD can be changed by varying the delay time between the THz SCP and pump pulse.