Electron momentum spectroscopy (EMS) provides experimental atomic and molecular electronic structure information in terms of the binding energy spectrum and the experimental momentum profile (XMP), which is a direct probe of the electron momentum distribution in specific molecular orbitals. The measured XMPs permit a detailed quantitative evaluation of theoretical ab initio wavefunctions in quantum chemistry and also provide a means to investigate traditional concepts in chemical reactivity at the fundamental electronic level. This thesis reports high momentum resolution EMS measurements of the valence orbitals of H₂0, D₂0, NH₃ and H₂CO obtained using an EMS spectrometer of the symmetric, non-coplanar type operated at an impact energy of 1200eV. The measured experimental momentum profiles for the valence orbitals of each molecule have been placed on a common intensity scale, which has allowed a stringent quantitative comparison between experiment and theory. These studies now confirm earlier preliminary investigations that suggested serious discrepancies between experimental and theoretical momentum distributions. Exhaustive consideration of possible rationalizations of these discrepancies indicate that double zeta quality and even near Hartree-Fock quality wavefunctions are insufficient in describing the outermost valence orbitals of H₂0 and NH₃. Preliminary results for H₂CO also indicate that near Hartree-Fock wavefunctions are incapable of describing the outermost 2b₂ orbital. Interactive and collaborative theoretical efforts have therefore led to the development of new Hartree-Fock limit and also highly correlated (CI) wavefunctions for H₂0, NH₃ and H₂CO. It is found that highly extended basis sets including diffuse functions and the adequate inclusion of correlation and relaxation effects are necessary in the accurate prediction of experimental momentum profiles as measured by electron momentum spectroscopy. New EMS measurements are also reported for the outermost valence orbitals of NF₃, NH₂CH₃, NH (CH₃)₂, N (CH₃)₃ and para-dichlorobenzene. These exploratory studies have illustrated useful chemical applications of EMS. In particular, EMS measurements of the outermost orbitals of the methylated amines have revealed chemical trends which are consistent with molecular orbital calculations. These calculations suggest extensive electron density derealization of the so-called nitrogen 'lone pair' in the methylated amines in comparison to the 'lone pair' in NH₃. EMS measurements of the non-degenerate π₃ and π₂ orbitals of para-dichlorobenzene show different experimental momentum profiles consistent with arguments based on inductive and resonance effects. These experimental trends, both in the case of the amines and para-dichlorobenzene, were qualitatively predicted by molecular orbital calculations using double zeta quality wavefunctions. However more accurate prediction of the experimental momentum profiles of these molecules will need more extended basis sets and the inclusion of correlation and relaxation effects as suggested by the studies based on the smaller molecules. An integrated computer package (HEMS) for momentum space calculations has also been developed based on improvements to existing programs. Development studies testing a new prototype multichannel (in the ɸ plane) EMS spectrometer are described. / Science, Faculty of / Chemistry, Department of / Graduate

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Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

This thesis reports laser spectroscopic studies of gas-phase transition metal dimers using laser ablation/reaction with free jet expansion and laser-induced fluorescence (LIF) spectroscopy technique. Themolecules studied in this work are palladium dimer (Pd2) and vanadium dimer (V2). Many compounds formed from these transition metals are important and functional catalysts in chemical reactions. Therefore, it is of great significance to start from the fundamental level to understand the properties and characteristics of the metal bonding and also the behavior of these metals when reacting with other chemicals. The electronic transitions of Pd2and V2in the visible region were studied. Gas-phase Pd2and V2moleculeswereproduced by laser ablation of palladium and vanadium metal rod, respectively. For the Pd2molecule, eleven vibrational bands were recorded and analyzed, and have been assigned to the 〖[17.1]〗^3 □_g□ X^3 □_u^+ transition system. The bond length and vibrational frequency of the ground X^3 □_u^+ state were determined to be 2.47 Å and 211.38 cm-1, respectively. This is the first experimental investigation of the electronic transitions of Pd2.For the V2molecule,six vibrational bands were observed and assigned to a new 〖[19.6] 〗^3 □_u^□□ X^3 □_g^□ transition system. Molecular constants for the 〖[19.6] 〗^3 □_u^□ excited state were obtained from high-resolution LIF spectra. The electronic structure of the Pd2andV2molecules was discussed in detail using molecular orbital energy level diagrams, which is important for understanding the nature of chemical bonding in these dimers. Comparison of the transition metal dimers studied in this work with other dimers is also presented. / published_or_final_version / Chemistry / Master / Master of Philosophy

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published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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Thesis (Ph. D.)--University of Hong Kong, 1997. / Includes bibliographical references.