Studies On Ultrafast Nonadiabatic Dynamics Of Molecular Excited States In P-fluorotoluene And Carbonyl Sulfide

Interaction of light with matter has always been important in the field of natural science.Particularly,the ultrafast radiationless relaxation induced by UV light of molecular electronic excited states accompanied by ultrafast energy transfer plays an important role in the natural photophysical,photochemical and biological reactions.Generally,the molecular electronic excited state can deactivate through a variety of decay channels,including dissociation,isomerization,internal conversion,intersysterm crossing,vibrational energy redistribution,autoionization.This complexity of relaxation channels makes a wide variety of deactivation mechanisms.Choosing the p-Fluorotoluene(pFT)molecule of aromatic type and the linear triatomic molecule of carbonyl sulfur(OCS)as the research targets,related electronic excited state radiationless relaxation processes have been inveatigated using the combination of femtosecond time-resolved method and supersonic molecular beam technique,presenting two different electronic radiationless relaxation mechanisms of significantly different.The two main parts of this dissertation are listed below:(1)Cascaded electronic relaxation processes following two photoexcitation of 400 nm in p-Fluorotoluene(pFT).Cascaded electronic relaxation processes started from the electronically excited S2 state have been directly imaged in real time and well characterized by two distinct time constants of?85 ± 10 fs and 2.4 ± 0.3 ps.The rapid component corresponds to the lifetime of the initially excited S2 state,including the structure relaxation from the Franck-Condon region to the conical intersection of S2/S1 and the subsequent internal conversion to the highly excited S1 state.Whilst,the slower relaxation constant has been attributed to the further internal conversion to the high levels of So from the secondarily populated S1 locating in the channel three region.Moreover,photoelectron kinetic energy and angular distributions reveal the feature of accidental resonances with low-lying Rydberg states(the 3p,4s and 4p states)during the multi-photon ionization process,providing totally unexpected but very interesting information in pFT.In addition,dynamical differences with benzene and toluene of analogous structures,including,specifically,the slightly slower relaxation rate of S2 and the evidently faster decay of S1,have also presented and tentatively interpreted as the substituent effects.(2)Competing electronic relaxation processes of superexcited rydberg states B2?+nl? in OCS.Superexcited rydberg states B2?+ nl? in OCS prepared by three-photon UV excitation and their ensuing competing relaxation processes has been investigated by time-resolved ion yield spectroscopy.The time profiles of the S+ ions,which encode molecular fragmentation mechanisms,are only observable at relatively high pump intensities,thus providing unique experimental identification of the neutral predissociation channel producing S*atoms.Benefiting from this feature and by comparison with the time behavior of the OCS+ ions,three competing relaxation channels are identified:autoionization associated with both X2? and A2? ionic states;neutral predissociation producing neutral S*atoms in a picosecond timescale,appearing as a non-decaying plateau in the time profiles of the S+ ions;internal conversion to isoenergetic RA states the deactivation of which manifesting itself as a picosecond decay in the time profile OCS+ ions.