| When a system is ionized or excited by light,its state changes and it is in a superposition state of different electronic states,and the charge in the system will be migrated.The coherent superposition state will evolve along time.During this ultrafast process,the electron density of the system will oscillate along time.Such charge migration may last for a few femtoseconds before the moving of the nucleus.Because these geometries of the excited and ground states are different,these geometries of the molecule must also evolve along time.In the limit where the electron motion is not coupled with the nuclear motion,there is coherent charge migration.In fact,the motion of the nucleus in the molecule can affect the charge distribution.When the vibration of the nucleus is considered,the decoherence or even partial recoherence of charge migration will be caused,which has been proved in small molecules.The study of de-and recoherence of charge migration can promote the development of various dynamic simulation methods and reveal that nuclear motion plays an important role in the on-off control of charge migration.By numerical simulation of quantum dynamics,we propose a general method for studying ultrafine laser-induced charge migration in molecules,including electron dynamics and nuclear motion.It can be used to study relatively large systems.In this work,we studied the detailed process of charge migration in pyrene and the de-and recoherence of charge migration.The main work completed in this thesis is as follows:Firstly,the molecular structure of pyrene was optimized by density functional theory(DFT)with def2-TZVP basis set,and obtained the minimal geometry.By analyzing the charge density and electron flux along time,we studied the characteristic of charge migration of pyrene with fixed nucleus.Second,we studied the effect of charge migration when consider nuclear vibration in pyrene.In this case,we need to analyze the normal mode,and calculate the frequency of the ground state,normal coordinates and the geometry of the excited state through quantum chemistry software.By means of formula derivation,some quantities in the formula are concretized,and the electron density and electron flux density evolve along time are obtained by numerical calculation.Decoherence and recoherences of charge migration in pyrene are happened at 11 fs and 22 fs respectively.Third,we explained the decoherence or even partial recoherence of charge migration in terms of nuclear motions.The result confirms the underlying hypothesis which has been motivated by experimental spectra.By analyzing this model,more general criteria for decoherence and recoherences of charge migration in polyatomic molecules as large(or even larger)as pyrene are also derived. |
