Photoelectron spectroscopic studies of unstable molecular species

• Main Author: Lau, Woon Ming
• Language: English
• Published: 2010
• Subjects: Molecular structure
• Online Access: http://hdl.handle.net/2429/23615

A photoelectron (PE) spectrometer has been modified to study unstable molecules. Reconstruction of the ionization chamber has made the ionization region more easily accessible, and a quadrupole mass spectrometer has been added in order to provide mass spectral identification data under the same conditions as the PE experiment. The system is controlled by a LSI 11/03 microcomputer with suitable interfacing hardware. A real-time operating system program has been developed for data handling. Light sources such as the HLc (Hydrogen Lyman a line, l0.2eV) and HL[sub=αβƴ] (a mixture of Hydrogen Lyman α, β and ƴ lines) radiations were used to reduce ion-fragmentation. Pure S₄N₄, S₄N₂, S₃N₃ and S₂N₂ were synthesized and studied with this system. The interrelationship between the gas phase reactivities of these compounds has been established. The study of CH₃NO, its trans and cis dimers, and its isomer, CH₂NOH, has clarified some mistakes in species identification in previous PE spectroscopic work on CH₃NO and its dimers. These two studies illustrate our ability to identify unstable species even in a very complex mixture with this system. A cryopump was constructed and may be positioned close to the ionization region opposite to the sample inlet, which may be a fine nozzle. This fast pumping nozzle system has been used to produce nearly pure N₂0, and a charge-transfer complex (CH₃)₂0-BF₃, and excellent PE spectra of these species were obtained. A library of computer programs has been established which provides a wide variety of quantum mechanical computations applicable to PE band assignments. These programs, such as CNDO/2, MINDO/3, MNDO, HAM/3, GAUSSIAN 70 and 76, and RSPT (for perturbation corrections to Koopmans' theorem), were used throughout this work and their accuracy and efficiency assessed. Koopmans' theorem has been shown to break down if applied to the ionization of CH₃NO and N₂O₄. Moreover, shake-up processes in the Hel region have been studied for these two molecules and S₄N₂. Several of the molecules, such as S₄N₂, S₃N₃ and (CH₃)₂0-BF₃, have not been investigated by PE spectroscopy before. === Science, Faculty of === Chemistry, Department of === Graduate