Dissociative Photoionization Of Several Molecules By Threshold Photoelectron Photoion Coincidence

In this dissertation,we used two threshold photoelectron-photoion coincidence velocity map imaging apparatus to investigate the dissociative phtoionization dynamics of vinyl chloride,methyl fluoride and dimethyl carbonate,based on photoionization with synchrotron radiation.Two photoelectron-photoion coincidence instruments are placed at National Synchrotron Radiation Laboratory in Hefei and Swiss Light Source,respectively.That at Hefei Light Source is home-built by ourselves,in which electrons and ions are collected simultaneously with double velocity map imaging.Using a repelling electric field,the image of photoelectrons is amplified,and the contamination of hot electrons is effectively suppressed in TPES and thus the energy resolution is improved.The kinetic energy resolving power,E/? E,is better than 3%,that helps us to measure the kinetic energy release and angular distribution of fragment ions.Only the velocity map image of photoelectrons is obtained with an energy resolution of 3 meV in the coincidence instrument(iPEPICO)at Swiss Light Source,while a typical Wiley-Mclaren time of flight(TOF)spectrometer is applied for photoions.It is suitable to study the isomerization and dissociation processes of small organic molecules,and determine the accurate appearance energy and the enthalpy of formation of fragments.To investigate the dissociative photoionization of vinyl chloride,the dissociation dynamics of internal energy-selected C2H3Cl+ ion were performed.In the photon energy range of 11.0-14.2 eV,three electronic excited states,A2A',B2A",and C2A',of the C2H3Cl cation were prepared.A unique fragment ion,C2H3+,was observed within the excitation energy range.Through 3-dimensional time-sliced velocity map images of C2H3+,the kinetic energy released distributions(KERD)and angular distributions of fragment ions were obtained.At B2A" state,the total KERB showed a bimodal distributions of a statistical dissociation(Boltzmann distribution)and a non-statistical dissociation(Gaussian-type).An additional non-statisticalcomponent was found at the lower kinetic energy in the KERD when the excited energy was above the C2A' state.The overall dissociative photoionization mechanisms of C2H3Cl+in the B2A" and C2A' states are proposed,with the aid of the new time-dependent density functional theory calculations of the Cl-loss potential energy curves.Similarly,dissociations of the internal energy-selected methyl fluoride cation along the C-H bond and C-F bond were investigated in the photon energy of 12.40-19.90eV.In the lower energy(X2E state),CH2F+was the unique fragment ion,while the CH3+ ions became dominate at both the A2A,and B2E ionic states.The isotropic distribution was found in dissociation of CH3F-at X2E state,and the total KERD showed a Maxwell-Boltzmann profile,which are consistent with its predissociation characteristic.A perpendicular distribution of CH3+ along the polarization vector of VUV photon was observed for dissociation of CH3F+ ion at A2A1 and B2E states.With the aid of the calculated H-loss and F-loss potential energy curves of CH3F+,the dissociation mechanisms were proposed:CH2F+ is produced from the I2A' state of X2E due to the Jahn-Teller effect,a C-F bond breaking occurs rapidly along the A2A1 state,while the internal conversion followed by rapid dissociation is expected for dissociation of CH3f+ at the B2E state.Using a VUV photon in the energy range of 10.30-12.50 eV,the dissociative photoionization of dimethyl carbonate occurs.A series vibrational bands were observed near threshold ionization in the threshold photoelectron spectrum,and the O-C-O bend mode combination with the C=o stretch mode were identified to be active by the Franck-Condon simulation.The ionization energy of dimethyl carbonate was determined to be 10.47±0.01 eV.With the increase of excitation energy,more isomerization and dissociation pathways were opened.Through reproducing the breakdown diagram,the 0 K appearance energies of main fragments were determined.The quantum chemical calculations of dissociation pathways illuminated the observed dissociation processes.Using these values,the enthalpy of formation of dimethyl carbonate was determined to be ?fH0K(DMC(g))=-548.3 ± 1.5 kJ/mol,and the heat of formation of its cation is ? fHOK(DMC+(g))= 461.9 ± 1.8 kJ/mol.