HIGH-RESOLUTION C 1S PHOTOELECTRON-SPECTRA OF METHANE, ETHENE, PROPENE, AND 2-METHYLPROPENE

Vibrational fine structure in the C Is photoelectron spectra of methan e, ethene, propene, and 2-methylpropene has been observed using high-r esolution synchrotron radiation. The degree of vibrational excitation is found to increase with the number of hydrogens attached to the core -ionized carbon atom, and this observation can be rationalized using a linear coupling intensity model. The vibrational structure can be acc ounted for almost quantitatively with the assumption that the primary vibrational excitation is stretching of the CH bond attached to the co re-ionized carbon atom, using the results from methane to establish th e intensities of the CH stretching vibrations in the other molecules. Ab initio calculations of the geometrical changes accompanying C Is co re ionization support this picture. The high resolution in these exper iments makes it possible to determine the core-ionization energies of the inequivalent carbons in propene and 2-methylpropene, as well as th e difference between the adiabatic and vertical ionization energies in all four molecules. Ab initio calculations of vertical binding-energy shifts using hole-state calculations show good agreement with those d etermined experimentally.