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Experimental quantum chemistry by binary (e,2e) spectroscopy Leung, Kam Tong


The notion that chemists might benefit by looking at molecular orbitals and chemical bonding phenomena from the complementary momentum-space perspective was first suggested by Coulson and Duncanson some forty years ago. With the development of binary (e,2e) spectroscopy in the last decade, experimental momentum densities of individual orbitals can now be measured directly and this has provided the first "real" look at molecular bonding in the laboratory. Binary (e,2e) spectroscopy measures the binding energy spectrum and the spherically averaged momentum distribution using high energy electron impact ionization and coincidence detection techniques. The experimental orbital momentum distributions not only have helped to identify the symmetry (s-type or p-type), order and nature of the characteristic orbital involved in the ionization process, but also have made it possible to stringently evaluate the quality of ab-initio self-consistent-field wavefunctions. The valence-shell binding energy spectra and momentum distributions of the noble gases and a number of small molecules including H₂, CO₂, CS₂, OCS and CF₄ have been measured using a state-of-the-art binary (e,2e) spectrometer. An existing spectrometer has been modified to provide high momentum and timing resolutions as well as sufficient energy resolution for resolving most of the structures of the species reported. New and definitive results on the valence- shell electronic structure and orbital bonding pattern of these species have been obtained. Possible chemical trends in the electronic structure and orbital densities in the noble gas group and in the valence isoelectronic linear triatomic group: CO₂, CS₂, and OCS have been investigated. Computer generated density contour maps and three-dimensional orbital density visualization of theoretical wavefunctions in both position and momentum space are used to facilitate interpretation of the experimental momentum distributions. This density topographical approach is instrumental in extending the present understanding of momentum-space chemical properties. In particular, such an approach has provided a new and complementary picture of the covalent bond in molecular hydrogen in momentum-space. The first experimental estimation of the spherically averaged bond density in momentum-space by binary (e,2e) spectroscopy is also attained.

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