Quantum Dynamics Simulations Of Ultrafast Electronic Flux In Molecules Induced By Laser

Electron dynamics plays an important role in many chemical reactions and biological processes.In particular,the movement of electrons in molecules cause a very important physical phenomenon:charge transfer.With the development of laser technology for more than 70 years,the time resolution of charge transfer process has been improved from nanoseconds(Ins=10-9 s)to attoseconds(1as=10-18 s).The traditional charge transfer(picoseconds,1 ps=10-12 s,to hundreds of femtoseconds,1 fs=10-15 s)is caused by the nuclear motions.In contrast,the ultrafast charge transfer(few femtoseconds to few hundred attoseconds)is caused by multi electronic superposition states.This is called "charge migration".Compared with the traditional charge transfer,charge migration is a pure quantum dynamical process with faster transfer speed.It can move between donor and acceptor sites many times.Such novel phenomena in charge migration offer new opportunities and challenges for quantum simulation methods with high accuracy and time resolution.This PhD thesis uses quantum dynamics simulations in order to discover various forms of the electronic flux during ultrafast charge migration,and it develops concepts for laser control of charge migration.For this purpose,it starts out from a simple model with fixed nuclei and a superposition of two states.Next it employs a model with a single electronic state but with moving nuclei.Finally it ends up with a model of a coherent superposition of two electronic states coupled to multi-dimensional nuclear motions.Applications are to ultrafast charge migration in the benzene,HCCI+and Na2 molecules.The increasing level of complexity of the models yields more and more realistic quantum dynamics simulations which allow the discovery of several novel phenomena.The details are as follows1.Charge migration in benzene and in HCCI+in the fixed nuclear model.In the benzene molecular model,four linearly x-and y-as well as circularly right and left polarized laser pulses are designed to prepare benzene molecules in four different degenerate superposition states.As a consequence,the pulses induce four different types of charge migration in benzene.The angular electronic flux reveals that the charge migration in benzene proceeds between two nodes with zero flux,from the "source"to the "sink".The positions of the "source" and "sink" can be driven by circularly polarized pulses.In the HCCI+molecular model,first,we develop the theory for the time evolutions of the axial electronic density and flux.Second,we design the laser pulse which excites the model HCCI+from the electronic ground state to the target superposition state that launches charge migration with maximum axial electronic flux.Third,we present quantum dynamics simulation of the generation of the axial electronic flux during charge migration.Specifically,during the laser pulse,the charge migration already exists from the very beginning,and the efficiency of the charge migration increases gradually.2.Electronic flux in a single electron state coupled to one-dimensional nuclear motion model,exemplified for Nat vibrating in the double-minimum potential of the excited(21?u+(JM?00))state.This quantum dynamics simulation reveals that the electronic flux density consists of two antagonistic components.One arises from electrons which flow essentially coherently with the nuclei.The other one,which is oppositely directed(i.e.,antagonistic)and more intense,is due to the transition of the electronic structure from "Rydberg" to "ionic" type as the nuclei cross the potential barrier between the "inner" and the "outer"potential wells.This "transition" component of the electronic flux density rises and falls sharply as the nuclei cross the barrier3.Charge migration in the model HCCI+in superposition of two states with moving nuclei.In previous theoretical work,a quantum dynamics simulation of charge migration for fixed nuclei in HCCI+has already demonstrated the good agreement with the experiment during the first few cycles.We propose a new quantum dynamics simulation of charge migration in HCCI+,taking into account the influence of the nuclei(seven vibrational degrees of freedom of HCCI+molecules).The nuclear motion can switch charge migration off(decoherence)and on(recoherence).This is a new discovery in attosecond-to-femtosecond chemistry and physics which opens new prospects for laser control over electronic dynamics via nuclear motions.In particular,we show that well-designed pump and dump laser pulses can enforce recoherences of charge migration at various different target times.