UBC Theses and Dissertations
Studies of valence electron densities using electron momentum spectroscopy and computational quantum chemistry Neville, John J.
The valence shell binding energy spectra and momentum profiles of acetone, dimethoxymethane and glycine have been studied by electron momentum spectroscopy at a total energy of 1200 eV. These experimental measurements present several challenges, in both data collection and analysis, that are typically not encountered in electron momentum spectroscopy studies of smaller molecules. A newly constructed energy-dispersive multichannel electron momentum spectrometer yields improvements in both sensitivity and energy resolution as compared with single-channel spectrometers used in previous studies. These instrumental improvements address several of the challenges of performing electron momentum spectroscopy studies of larger molecules and make possible the experimental measurements presented in this thesis. The studies of acetone and dimethoxymethane each encompass the approximate binding energy range of 5-60 eV, while the study of glycine is limited to the binding energy range of 6-27 eV. The current work provides the first experimental values of the inner-valence (6a)⁻¹, (4b)⁻¹ and (5a)⁻¹ ionization potentials of dimethoxymethane, which are 20.4, 22.6 and 23.9 eV, respectively. For all three molecules, many-body effects are evident in the ionization spectra at binding energies greater than 16 or 17 eV. The experimental momentum profiles of all three molecules are compared with theoretical profiles obtained from Hartree-Fock (HF) and density functional theory (DFT) calculations using the target Hartree-Fock and target Kohn-Sham approximations, respectively. The convergence of the theoretical results with basis set is investigated by performing the calculations with a range of basis sets. In the case of the DFT calculations, the sensitivity of the theoretical profiles to the choice of exchange-correlation functional is also investigated. The outermost experimental momentum profiles of acetone (i.e., 5b₂ and 2b₁) and dimethoxymethane (10b + 11a) are also compared with theoretical profiles calculated by E.R. Davidson from multi-reference single and double excitation configuration interaction (MRSD-CI) calculations of the corresponding neutral molecules and ions. The theoretical momentum profiles obtained from DFT calculations are generally in good agreement with the experimental profiles. The notable exceptions are the 5b₂ profile of acetone and the 9b profile of dimethoxymethane, for which none of the theoretical methods considered here reproduce the experimental profiles. Agreement is poorer between the HF profiles and the experimental data. This is particularly so for the outer-valence momentum profiles, where the HF calculations tend to underestimate the intensity at low momentum. There is little difference between the large-basis-set HF calculations and the MRSD-CI calculations of the outermost momentum profiles of acetone and dimethoxymethane. Convergence of the theoretical momentum profiles with respect to basis set occurs with the 6-311++G** basis set for both the HF and DFT calculations; theoretical momentum profiles calculated using smaller basis sets tend to underestimate the intensity at low momentum, particularly for the outer-valence momentum profiles. The DFT profiles are relatively insensitive to changes in the exchange-correlation functional.
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