Theoretical And Experimental Studies On Ultrafast Dynamics Of Molecular Excited States Based On Conical Intersections

To realize the observation of molecular motions in real time has always been a dream for science researchers.With the rapid development of femtosecond(10-15 s)laser technology,exploring the dynamics of molecule systems interacting with femtosecond laser with a shorter temporal duration as well as a higher spatial resolution has gradually been a hot research recently.Meanwhile,more and more advanced probing techinques have been successfully used for realizing the so called"Molecular Movies",such as two-dimensional spectroscopy technique,time-resolved and angle-resolved photoelectron spectroscopy technique,femtosecond X-ray Diffraction and eletron diffraction techniques,etc.As a very important topic in physical chemistry field,exploring the ultrafast dynamics of electronic and structural systems behind the nonadiabatic photophysical and photochemical phenomena based on conical intersections(Cls)has attracted tremendous interests of researchers,including the wave-packet dynamics in the vicinity of a CI in Pyrazine(C4H4N2)during internal conversion and the singlet exciton fission(SEF)process in Pentacene(C22H14)and its derivatives,and so on.In this thesis,based on the internal conversion process in Pyrazine,we will first theorectically simulate the vibrational coherence dynamics of a wave packet in the vicinity of a CI with introducing a quantum dissipative system.Then we will compare the difference of wave-packet dynamics between potential energy surface(PES)configurations of CIs and avoided crossings(ACs).Finally,we will experimentally study the structural dynamics of lattices during the SEF process in single crystalline 6,13-Bis(triisopropylsilylethynyl)Pentacene(C44H54Si2),i.e.TIPS-Pentacene,with femtosecond electron diffraction(FED)technique.(1)A theoretical model with three PESs and two vibrational modes based on Pyrazine is proposed,and the vibrational coherence dynamics of a wave packet in the vicinity of the CI between two excited states with different parameters is explored by using HEOM method.In conclusion,the process influences the quantum yield and transfer rate of photoisomerization directly.(2)The model proposed in previous section is reduced to a two-state two-mode model to simulate the wave-packet dynamics around a CI configuration with TNL master equation method based on the second perturbative approximation,and the results are compared with them in the vicinity of an AC.It is found that the vibrational coherence keeps a longer time around the CI than that of AC.In addition,berry phase is observed with increase of the electronic coupling strength between PESs.(3)A compact designed femtosecond electron diffraction(FED)setup equipped with 200 fs resolution in time and hundreds of milli-Angstrom(10-10 m,A)resolution in space is employed to study TIPS-Pentacene,which is a promising organic photovoltaic material.On one hand,the thermal diffuse scattering effect induced by atomic motions is explored by using static electron diffraction technique at 105 K.On the other hand,the singlet exciton fission(SEF)process is induced by femtosecond laser,and the related atomic structural dynamics is observed directly in ultrashort time scales.