Photodissociation Dynamics Of Bromofluorobenzenes

The main content of this dissertation contains two sections:the photodissociation dynamics of bromofluorobenzenes and the charge recombination dynamics of CH3NH3PbI3 single crystals.In detail,the former presents the photodissociation dynamics of 1-bromo-2,6-difluorobenzene,1-bromo-3,5-difluorobenzene and bromopentafluorobenzene studied in a supersonic-jet molecular beam,using an ultraviolet laser tunable in the 267-234 nm range and ion velocity imaging technique as a detection tool.The photodissociation mechanisms of the three molecules are determined based on the analysis of experimentally measured speed and angle distributions of Br fragments in combination with molecular potential energy surfaces.Further information on individual molecular potential energy surfaces,including dissociation barrier height and intersection-crossing point in excited states,are also derived.The second section of this dissertation describes an UV-VIS optical microscope setup that is built up to investigate the charge-recombination dynamics of solar cell materials.Using this setup,UV absorption spectra,fluorescence spectra and fluorescence lifetime of three different CH3NH3PbI3 single crystals have been studied.The three single crystal samples were synthesized by three different fabrication processes.By fitting the experimental results to a kinetic model,we can get dynamic parameters.The photodissociation dynamics of 1-bromo-2,6-difluorobenzene and 1-bromo-3,5-difluorobenzene at 267-234 nm:The photodissociation dynamics of 1-bromo-2,6-difluorobenzene and 1-bromo-3,5-difluorobenzene have been studied in the 267-234 nm ultraviolet range using our home-made time-sliced velocity mapped ion imaging apparatus.We have analyzed the experimental data for both photofragmented ground-state Br(2P3/2)(denoted as Br)and spin-orbit excited-state Br(2Pi/2)(denoted as Br*)atoms,to infer their speed and angular distributions.Our analysis indicates that the speed distributions of produced Br atoms contain three Gaussian components,corresponding to three independent dissociation pathways on the excited potential energy surfaces:the high translational energy(ET)component(denoted high-ET),the medium translational energy(ET)component(denoted medium-ET)and the low translational energy(ET)component(denoted low-ET).The speed distributions of Br*atoms are all found to consist of two Gaussian components,which correspond to two independent dissociation pathways on the excited potential energy surfaces,the high translational energy(ET)component(denoted high-ET)and the medium translational energy(ET)component(denoted medium-ET).The dissociation behaviors are similar between 1-bromo-2,6-difluorobenzene and 1-bromo-3,5-difluorobenzene.Mechanisms for individual dissociation channels are discussed in detail below.The photodissociation channels of Br:(1)high-ET component is due to direct dissociation along the repulsive state S1-B1.This channel appears in high dissociation energy area and becomes the major dissociation channel compared to other two channels.Its anisotropy parameter is close to 2,which is the main character of a fast dissociation process.(2)medium-ET component is due to predissociation from a repulsive state S1-B1 after crossing with repulsive state S1-B2,which has a small potential barrier.The weight of this channel decreases as dissociation energy increased and the anisotropy parameter changed from negative value to positive value.It has both dissociation characters from S1-B1 and S1-B2 states.The two processes compete with each other along the whole dissociation range.When dissociation energy is low,dissociation through S1-B2 state surface dominates,and anisotropy parameter value is negative.As dissociation energy increases,the portion of dissociation along S1-B1 state surface increases,anisotropy parameter becomes positive at 234 nm,but smaller than 1.(3)low-ETr component corresponds to predissociation process that arises from dissociate along T1-B1 state after conversion between S1-B2 and T1-B1 states.This channel has two significant characters:very low translational energy almost zero and its anisotropy parameter is close to zero too.The photodissociation channels of Br*:The characters of each dissociation component similar to is the corresponding channel of Br.According to the molecular potential energy surface(PES),no singlet state converging to Br*at long C-Br distance exist.The relative quantum yields of φ(Br*)is small,less than 5%.The major dissociation product is Br.Therefore,we suggest that each dissociation path of Br intersystem-crossing with repulsive T1-B1 state at long C-Br distance,yielding the production of Br*along T1-B1 while keep the same dissociation features as Br.It should be noted that anisotropy parameters of Br medium-ET component vary significantly between 264.8 nm(4.682 eV)and 258.7 nm(4.793 eV).In this dissociation range,the anisotropy parameters changes from positive to negative,and then back to positive value.The most likely reason is that this dissociation energy region is around the potential barrier of the S1-B2 state.When dissociation energy is lower than the potential barrier,dissociation along S1-B1 surface is dominant and anisotropy parameters is positive.As dissociation energy increases to slightly higher than the potential barrier of the S1-B2 state,dissociation along S1-B2 surface becomes dominant and anisotropy parameters is negative.When dissociation energy is higher enough that dissociation along S1-B1 surface becomes dominant again and anisotropy parameters becomes positive as we have observed.In summary,our experiment results of photodissociation dynamics of 1-bromo-2,6-difluorobenzene and 1-bromo-3,5-difluorobenzene at 267-234 nm show that the location of crossing point of states S1-B1 and S1-B2 should be higher than 267 nm because fast dissociation channels exist in the whole dissociation range.And high-ET component becomes more evident in high dissociation energy,indicating that the potential barrier of the S1-B2 state is around 4.7 eV.The photodissociation dynamics of bromopentafluorobenzene at 267-234 nm:The speed and angular distributions have been analyzed for both the ground-state Br(2P3/2)atom(denoted as Br)and the spin-orbit excited-state Br(2P1/2)atom(denoted as Br*).The speed distributions of Br atoms are all found to consist of three Gaussian components,corresponding to three independent dissociation pathways on the excited potential energy surfaces:the high translational energy(ET)component(denoted high-ET),the medium translational energy(ET)component(denoted medium-ET)and the low translational energy(ET)component(denoted low-ET).The speed distributions of Br*atoms are all found to consist of two Gaussian components,which correspond to two independent dissociation pathways on the excited potential energy surfaces,the high translational energy(ET)component(denoted high-ET)and the medium translational energy(ET)component(denoted medium-ET).Because of its large photodissociation wavelength range,dissociation channels varied a lot along with different dissociation energy.A preliminary analysis indicates that,the high-ET component comes from direct dissociation along repulsive state,and the medium-ET component is a predissociation process which originates from curving crossing between a repulsive state and a bound state.The low-ET component dissociates via conversion process between excited states.The relative quantum yields of Br*and Br changed significant between 267 nm and 234 nm,as well as the observed irregular angular distributions from our experiment,hinting that the ultraviolet photodissociation dynamics of bromopentafluorobenzene may involve quantum interference between excited states.Further analysis on the present experimental results requires an accurately ab-initio calculated molecular potential surface that is not available at this moment.Construction of UV-VIS optical microscopy setup:A UV-VIS microscopy setup is constructed to study the perovskite solar cell.This setup consists of light source(laser and white light)system,microscopy system,detection system and signal collecting system.It can be used to measure UV absorption spectra,fluorescence spectra,fluorescence lifetime of nanometer size crystal via pump-probe approach,as well as other large molecular systems.Characterization of the optical properties of CH3NH3PbI3 crystals:Using our home-made UV-VIS microscopy setup,we have measured UV absorption spectra of the CH3NH3PbI3 crystal with a white light sources,from which the bandgap has been determined.A 400 nm,2.5 MHz laser beam is chosen as the pump light source to excite the CH3NH3PbI3 crystal.The resulting time-resolved fluorescence spectra are recorded under conditions of different excitation light flux.Three different size crystals,obtained through three different fabrication processes,are studied in this work.Due to crystal size and fabrication process,we name them as CH3NH3PbI3 crystal,CH3NH3Pbl3/Si,CH3NH3PbI3 big crystal,respectively.The experimentally recorded fluorescence decays are analyzed,based on the charge-recombination dynamics model,to determine the key dynamical parameters.Our experimental results also demonstrate that,the CH3NH3PbI3 crystal has the smallest trap state density,and therefore,for the potential application in solar cell,CH3NH3Pbl3 crystal has the highest performance among the three crystals.