ELECTRON-MOMENTUM-SPECIFIC VALENCE-SHELL ELECTRONIC-STRUCTURES OF CIS-BUTENE, TRANS-BUTENE, AND ISOBUTENE BY SYMMETRICAL NONCOPLANAR (E,2E) SPECTROSCOPY

Electron-momentum-specific ionization energy (IE) spectra of cis-, trans-, and iso-butene were obtained from 8 to 44 eV ionization energy using noncoplanar symmetric (e,2e) spectroscopy and compared with the corresponding photoelectron spectra obtained using monochromated synchrotron radiation at 100 eV photon energy. In the independent particle picture, the intensity variation of a particular ionic state as a function of the electron momentum in an (e,2e) IE spectrum is characteristic of electron momentum density of the ionized orbital. Under favorable conditions, the (e,2e) IE spectra measured at two relative azimuthal angles (0-degrees and 8-degrees) can be used to provide a unique characterization of the dominant orbital composition of a particular ionic state. Together with ab initio calculations, these spectra have been used to clarify the nature of some of the ionic states found in the photoelectron spectroscopic studies. In particular, the presence of prominent many-body structures in the inner-valence region (> 20 eV) for all three butene isomers indicates the general breakdown of Koopmans' ionization picture in this region. The nature of these many-body states is inferred from their electron momentum dependence in the (e,2e) IE spectra and from recently published configuration-interaction calculations.