Abstract

Electron momentum distributions (MDs) of selected valence-shell ionic states of cis- and trans-butene have been obtained using symmetric noncoplanar (e, 2e) spectroscopy. The measured MDs are compared with ab initio calculations using self-consistent-field wavefunctions of 4-31G, 6-31G and 6-31 + +G ** basis sets. For the highest occupied molecular orbitals (HOMOs), only MDs generated by the more diffuse 6-31 + +G ** basis set appear to adequately reproduce the experimental results. More pronounced discrepancies between experiment and calculations in the low-momentum region are found for the next HOMOs of both isomers. The level of agreement generally indicates the importance of and the need for more diffuse wavefunctions for the outer-valence orbitals. On the other hand, generally good agreement between experiment and calculations has been observed in the MDs of the two prominent inner-valence ionic states between 17 and 22 eV, which provides strong support for their assignments for the first time. Furthermore, experimental MDs measured at higher energies in the inner-valence region provide further evidence for the observation that the many-body states above 22 eV may be ascribed predominantly to ionization of the innermost valence orbitals. Finally, despite the similarity in the calculated orbital energies for the corresponding orbitals between the cis and trans isomers, their calculated MDs are found to be qualitatively different, particularly in the case of the outer-valence orbitals. This sensitivity of MDs to the physical structures of the isomers is demonstrated.

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