Evaluation of molecular electronic wavefunctions by electron momentum spectroscopy
Creators: name: Hollebone, Bruce Paul
Publisher: name: University of British Columbia
Year: 2009

description: The valence binding energy spectra and orbital electron momentum profiles of HF, HC1 and C₂H₄ have been measured by symmetric non-coplanar electron momentum spec troscopy (EMS) at an impact energy of 1200 eV. The measured momentum profiles are compared with calculated cross-sections using a range of wavefunctions from minimal ba sis to essentially Hartree-Fock limit in quality. The effects of correlation and relaxation on the calculated momentum profiles are investigated using multi-reference singles and doubles configuration interaction (MRSD-CI) calculations of the full ion-neutral overlap distributions. The effects of using several different configuration spaces is investigated in the case of the lowest binding energy momentum profile ofC₂H₄.The experimental mo mentum resolution is accounted for in all of the calculated momentum profiles using the Gaussian-weighted planar grid procedure. The effects of vibrational averaging are also investigated in the case of the highest occupied molecular orbital (HOMO) of ethylene. For all three molecules, the effects of final state correlation on the valence binding energy spectrum are investigated using MRSD-CI calculations and the results are compared with other calculations and with experiment. The measurements for HF and HC1 are significantly different from previously reported EMS profiles for several ionization processes. There is also a significant discrepancy between SCF limit calculations and the measured HF hr momentum profile. However, with the inclusion of electronic correlation and relaxation effects using MRSD-CI methods reasonable agreement between theory and experiment is achieved. For the other HF profiles and all the valence profiles of HC1 the inclusion of many-body effects is found to provide little further improvement in the already quite good fit of theory to experiment at the near Hartree Fock limit level. The present measurements of the valence orbital momentum profiles forC₂H₄are in generally good quantitative agreement with Hartree-Fock results using moderate to large basis set as well as with much more sophisticated many-body treatments. The observed momentum profiles of ethylene, like those of acetylene, are well described at the near energy-converged Hartree-Fock level. As was observed previously in acetylene, while the consideration of many-electron interactions greatly improves both total energies and other electron properties, it has little effect on the shapes of the theoretical momentum profiles. Vibrational motion is also found not to have a large effect on the calculated shape of the 3b₃u π-system momentum distribution. New EMS measurements are also reported in this work for the outermost valence π orbitals of the two seriesC₂H₄,C₂H₃F, CH₂CF₂,C₂HF₃ andC₂F₄ and (CH₃)₂C0, CH 3CHO andH₂CO. These sample studies of substitution effects in the C=C and C=O π -systems have revealed trends in both series of compounds which are consistent with ab initio calculations. For the fluoro-ethylenes, near Hartree-Fock limit theory is found to provide a very good description of the EMS momentum profiles. The theoretical mod els of these molecules imply that increasing fluorination both removes electron density from between the carbon centres, weakening the π -bond and causes the formation of C-F anti-bonds in the HOMO. Significant differences are found between the measured and theoretical momentum profiles for the methyl substituted series of formaldehyde, acetaldehyde and acetone even at the MRSD-CI level. For these three molecules, methyl substitution is found to have a net electron-withdrawing effect (compared with hydrogen) on the C=O π -system.