Photodissociation Dynamics of Bromine-Containing Molecular and Ions
Supervisor:chen zhao dong feng
Halogen-containing transient species(radicals,ions,excited states)in gas-phase play an important role in photochemistry.The study of its spectroscopy and chemical kinetics not only helps us to understand the microscopic mechanism of halogen chemistry in the atmosphere,the studying of molecular electronic structure and spectral properties as well as complex reaction mechanism and evolution process also deepen our understanding of intramolecular interactions.The dissertation mainly presents the photodissociation dynamics of bromine-containing molecular and ions using home?made CIT-VMI spectrometer and Supersonic Molecular Beam Ion Velocity Imaging setup.The main contents including:(1)The electron transition spectra of the isotope-resolved Br2+ molecular ions over the energy range of 720 nm-500 nm are recorded by CIT-VMI spectrometer combined with the[1+1]two-photon absorption photofragment excitation spectra.The spectrometer combines quadrupole mass spectrometry and cryogenic ion trap technology successfully prepare Br2+and its isotopic molecular ions with high concentration,low translational temperature and vibrational temperature.The 14∑u--X2Πg transition bands are observed for the first time,and the electronic state assignments and vibrational identification of the A2Πu-X2Πg band system are clarified by VMI experiments and isotope effects.By the vibrational analyses of the experimental spectra,we obtain the band origin Tt of the upper electronic state(l3∑u,3/2-;14∑u,1/2-,A2Πu,3/2,A2Πu,1/2),vibrational frequency ωe and nonharmonic constant ωeχe and other spectral constants.(2)Based on the photofragment excitation spectra of Br2+,we studied the photodissociation dynamics of[1+1]two-photon photolysis of Br2+ ions over 630 nm-500 nm energy range using velocity map imaging technology.Experimentally,the velocity images of the photo fragment Br+ ion are recorded at 13 resonance wavelengths of the 4∑u,3/2--x2Πg,3/2 transition bands,and the total translational kinetic energy spectra of the Br2+ molecular ions dissociation products Br+ are obtained.The TKER,the branching ratio of each dissociation channel,and the angular distribution are obtained,three possible photodissociation mechanisms are also discussed.The state-resolved TKER spectrum of the Br2+ molecule shows that the[Br+(3P)+Br(2P)]and[Br+(1D)+Br(2P)]channels are opened,while the[Br+(1D)+Br(2P)]channel has not been reported yet.The quantitative analysis of the relative quantum yield of each dissociation channel indicates that:under the 560 nm-540 nm range,the Br2+ molecules in the 14∑u,3/2 state will transition to the bound state,and the predissociation occurs by coupling with a repulsive state;Over the 540 nm-521 nm range,the quantum yield of each dissociation channel is basically unchanged,indicating that transition to the repulsive state occurs along the single potential energy surface;Near?520 nm the relative quantum yield of the[Br+(3P1)+Br(2P1/2)]channel is mutated,possibly due to the presence of conical intersection near the dissociation threshold.In addition,the anisotropy parameter β of each dissociation channel obtained are all positive values,indicating that the transition type of Br2+ molecular ion photodissociation is a parallel transition.(3)The UV photodissociation dynamics of CiHsBr molecules over the range of 200 nm-215 nm are studied by using the upgraded supersonic molecular beam ion velocity imaging setup.The photolysis mechanism based on the theoretical model of Cs configuration molecular potential energy surface are discussed.The total kinetic energy distribution,angular distribution and branching ratio of the two dissociation channels of O2H5Br molecular dissociation products are obtained.The translational energy distribution of the dissociated product Br(2P3/2)is consistent with the prediction of the soft radical model,indicating that the CiHsBr molecule has exhibited an instinct chain-configuration(C-C-Br).Based on the anisotropy parameter p of dissociation fragment,we obtain the proportion of parallel transition and vertical transition components at each excitation wavelength.It is found that the photodissociation of the CsHsBr molecule near the 203 nm is mainly from the transition to the 1Q1 state.The dissociation product Br(2P1/2)reach the highest relative quantum yield near 213 nm,probably due to the avoided crossing which comes from energy level splitting.(4)The photodissociation dynamics of 1-Br-2,4,6-FPh molecule over the 234 nm-267 nm range are studied by Supersonic Molecular Beam Ion Velocity Imaging setup.The photodissociation mechanism of the iLb absorption band is discussed.The yields measurement of the photolysis fragment by mass spectrometry revealed that the quantum yield of the excited Br(2P1/2)atom is very low,indicating the weak spin-orbit coupling.The total translational energy distribution and angular distribution of the 1-Br-2,4,6-FPh molecular dissociation products are obtained by ion velocity imaging technique,and combined with the calculation of the molecular potential energy surface,we consider the Br(2P3/2)fragments are coltributed to three different photodissociation mechanisms:direct dissociation mechanism of S2-At"(nσ*)along repulsive state,S1-A"(ππ*)→Si-A’(πσ*)predissociation mechanism and conical intersection between S1-A"(ππ*)and S1-A’(πσ*)potential energy surfaces.In addition,it is found that the anisotropy parameter of Br(2P3/2)dissociated from 1-Br-2,4,6-FPh molecule are mutated over the energy range of 4.62-4.76 ev,and oscillation behavior also occurred in the mutated region.The combination of the potential energy surface indicates that the orientation of the transition dipole moment is also affected by the change of the molecular nuclear configuration during molecular excitation process,especially the stretching vibration mode of the C-F bond.