Threshold Photoelectron-Photoion Coincidence Velocity Imaging And Its Application

When a molecule absorbs a vacuum ultraviolet (VUV) photon, the phenomena of photoionization and photodissociation are usually the predominant processes, which widely exist in the areas of interstellar space, atmospheric chemistry and plasma, and are also the important parts of chemical reaction dynamics. The detailed study of the phenomena at molecular scale can improve our understanding and knowledge. Photoelectron-photoion coincidence (PEPICO) detected electrons and ions in photoionization, and could provide a lot of infromation at one time, for example, internal state of reactor, speed of dissocation, branching ratio and energy distribution of products. Threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging combined the characteristics of synchrotron radiation, coincidence spectroscopy and velocity map imaging, and collected threshold photoelectrons and corresponding photoions, which was powerful to prepare and analyze the dissociation of widely state-selected ions. A double velocity map imaging was used in TPEPICO velocity imaging to control the trajectories of electrons and ions simultaneously, and could improve their collection efficiencies and energy resolutions. This dissertation mainly includes the following parts:1) Construction of TPEPICO velocity imaging spectrometer. A TPEPICO velocity imaging spectrometer, in which a set of open electron and ion lenses were utilized to map velocity images of electrons and ions simultaneously, was constructed at the U14-A beamline of National Synchrotron Radiation Laboratory. It had the ability that the full 3-Dimensional velocity distribution of products dissociated from definite state-selected ions could be obtained conveniently. In order to suppress the contribution of hot electrons in threshold photoelectron spectroscopy (TPES) and mass-selected TPEPICO spectroscopy, a repelling electric field using extra lens, instead of traditional accelerating field, was applied to magnify images of electrons. The typical energy resolution of TPES was measured to be 9 meV (FWHM) as shown on the 2P1/2 ionization of argon. The measured mass resolving power for the present TPEPICO imaging spectrometer was above 900 of M/ΔM, and the kinetic energy resolution of TPEPICO image was better than 3% ofΔE/E.2) Advancements of TPEPICO velocity imaging. The repelling electric field could suppress hot electrons more efficiently than the accelerating field, but the goal of completely filtering hot electrons in TPES was still beyond the ability of the TPEPICO imaging spectrometer. Especially in the region of Franck-Condon gap, sometimes the probability of autoionization could be much stronger than direct photoionization. So to obtain a pure TPES without the contamination of hot electrons, the subtraction method of hot electrons should be a good choice. Unlike other groups'configuration, a couple of MCPs, simply refined electron mask and anodes were used to actualize the subtraction. After some TPES experiments, it was demonstrated that the subtraction method could work properly and efficiently with the repelling field.TPEPICO velocity imaging has some different characters with the traditional laser-based imaging, for example, the representation of false coincidence events and the influence of molecular beam in TPEPICO image, which would lead the obstacle that the experimental results could not obtain directly. To overcome the obstacle, data disposure schemes for TPEPICO image were also introduced in the article. As a representative, TPEPICO 3D time-sliced image of N~+ fragment ions dissociated from state-selected NO~+(c3P, v~+=0) ions was chosen to exhibit the effects of the data disposure schemes.3) Applications of TPEPICO velocity imaging. Using synchrotron radiation as light source, some TPEPICO velocity imaging studies were performed to investigate the photoionization and dissociative photoionization processes of molecules. TPES, TPEPICO time-of-flight mass spectra, mass-selected TPEPICO spectra and TPEPICO images were obtained experimentally, and the dissociation dynamics of ions were also discussed in the dissertation.First, the dissociation of vibrational state-selected O2~+(B2Σgˉ, v~+=0~6) ions was investigated by TPEPICO velocity imaging. Both speed and angular distributions of O~+ fragment ions dissociated from individually vibronic levels of O2~+(B2Σgˉ) were obtained directly from TPEPICO 3D time-sliced images. Two channels, O~+(4S)~+O(3P) and O~+(4S)~+O(1D), were respectively observed for the dissociation of O2~+(B2Σgˉ, v~+) ions, and their branching ratios were found dramatically dependent on the vibrational states. Based on the calculated potential energy curves, a new intersection mechanism was suggested for the predissociation of O2~+( B2Σgˉ, v~+=0~6) ions.Second, the dissociative photoionization of N2O molecule via the C2Σ~+ ionic state was studied by TPEPICO velocity imaging. Four fragment ions, NO~+, N2~+, O~+ and N~+, were observed respectively, and the NO~+ and N~+ ions were always dominant in the whole excitation energy range. Subsequently, the TPEPICO 3D time-sliced images of NO~+ dissociated from vibrational state-selected N2O~+(C2Σ~+) ions were recorded. Thus kinetic and internal energy distributions of NO~+ fragments were obtained directly, suggesting that the NO~+ fragments were formed via both NO~+(X1Σ~+)~+N(2P) and NO~+(X1Σ~+)~+N(2D) dissociation channels. Almost the same vibrational population reversions were identified for the both dissociation pathways. Interestingly, the obtained branching ratios of the two channels exhibited some dependence on the excited vibrational mode of N2O~+(C2Σ~+), in which the excited asymmetrical stretching potentially promoted dissociation possibility along the NO~+(X1Σ~+)~+N(2D) pathway. In addition, the measured anisotropic parameters of NO~+ were close to 0.5, indicating that the C2Σ~+ state was fully pedissociative indeed with a tendency of parallel dissociation, and therefore the corresponding predissociation mechanisms for the N2O~+(C2Σ~+) ions were depicted.Third, as a prototype of high symmetric molecule, the dissociation of CH3Cl~+ ions was also explored by TPEPICO velocity imaging. The ground CH3Cl~+(X2E) state was stable, and both A2A1 and B2E ionic states could dissociate to form CH3~+ and CH2Cl~+ fragment ions. The kinetic energy released distribution and angular distribution of CH3~+ fragment ions were acquired directly from TPEPICO images. The experimental results showed that the CH3~+ fragment ions dissociated from CH3Cl~+(A2A1) ions exhibited with a large kinetic energy released distribution, and displayed an obvious excitation of v2 vibrational mode. However, after increased the photon energy, the image and kinetic energy released distribution of CH3~+ fragment ions dissociated from CH3Cl~+(B2E) state did not show any structure. With the help of previously calculated potential energy curve, the changeable dissociation dynamics of A2A1 and B2E states were also discussed. It was found that the dissociation of A2A1 state followed a rapid direct process, whereas the B2E state performed internal conversion to the ground X2E state and then statistically dissociated.Last, TPEPICO spectroscopy was applied to study the mixture of Xe/Ar/Ne noble gases. TPES, TPEPICO time-of-flight mass spectra and TPEPICO spectra of the mixture gases were obtained experimentally. It was found that the mass-selected TPEPICO spectra for every components of the mixture was very similar to the TPES of the corresponding pure sample, and every TPEPICO spectra did not interfere with each other. The above consequence demonstrated that the mass-selected TPEPICO spectroscopy had the power to study the components of mixture, and we hoped it could be applied to investigate clusters, free radicals, combustion chemistry, and other fields in the future.