Theoretical Study Of Ionization And Dissociation Dynamics Of Diatomic Molecules In Femtosecond Laser Fields

Along with the development of the laser pulse technology,the ultra-fast and ultra-intense laser pulses are used to control the photochemical reactions more and more extensively.Studying the interaction between ultra-fast and ultra-intense laser fields and atoms or molecules in theory is very necessary.For the solution of few-body problems such as interactions of atoms or diatomic molecules with laser fields,the time-dependent quantum wave packet method which is based on the solution of the time-dependent SchrSdinger equation,is an efficient way.Thus,it is widely used in the study of molecular reaction dynamics.The works in this thesis are based upon the time-dependent quantum wave packet method to study the dynamics of diatomic molecules in femtosecond laser fields,in order to make use of the femtosecond laser pulse to control the chemical reaction.The main works are as follows.(1) Using the one-dimension model,the effect of the nonadiabatic coupling between the Rydberg state C²Î and the valence state B²Î on the C²Î â†X²Î absorption spectrum of the NO molecule is calculated and discussed.From the evolution of the excited wave packet,it can be seen that one part of the excited wave packet travels on the Rydberg state C²Î ,and the other part is trapped in the valence state B²Î .The coupling between C²Î and B²Î can affect the absorption spectrum of C²Î â†X²Î in spectrum intensities and peak locations.The peaks ofν′= 0,1,2 are shifted to the red and those ofν′= 3,4,5 are shifted to the blue.(2) The feasibility of steering molecular population transfer via ionization continuum is studied using the Na₂ molecule as an example.The effects of the intensity, delay and detuning of the laser pulse on the population transfer are discussed in detail based on the one-dimension model.The effect of molecular rotation and alignment on the population transfer is studied by comparing the one-dimension model with the two-dimension one.It is shown that although the ionization and the molecular rotation can decrease the population transfer efficiency to some extent,a large part of population transfer via ionization continuum can still be achieved by properly choosing the laser parameters.(3) An approach of controlling the dissociation product branching ratio of the Br₂ molecule is proposed based on the dissociating wave packet interference.Two pump pulses create dissociating wave packets interfering with each other.Using twodimension model,the radial distributions of dissociation products in coordinate and momentum space and the angular distribution are calculated and analyzed.It is shown that by varying the phase difference and delay time between the two pump pulses,the interference pattern of dissociating wave packets can be controlled and the dissociation probabilities in different dissociation channels can be changed to different degrees.(4) The nonadiabatic photodissociation and the following photoionization processes have been studied,using the NaI molecule as an example.Dissociation probabilities of different dissociation,channels and the branching ratio can be controlled by using the first order nonresonant nonperturbative dynamic Stark effect(DSE).Using two-dimension model,the effects of the delay time,intensity,frequency and phase of the Stark pulse on the nonadiabatic dissociation dynamics and the dissociation probabihties are calculated.It is shown that although in this first order nonresonant nonperturbative DSE case,the Stark shift changes direction twice every laser period, i.e.,the energy difference between the two diabats can be enlarged and reduced backward and forward at the laser pulse carrier frequency,the nonadiabatic dissociation dynamics can be controlled by the first order nonresonant nonperturbative DSE.By choosing proper parameters of the Stark pulse,the dissociation product branching ratio can be efficiently Controlled,which can be reflected from the final photoelectron kinetic energy distributions.