High-resolution C 1s photoelectron spectra of methane, ethene, propene, and 2-methylpropene

Vibrational fine structure in the C 1s photoelectron spectra of methane, ethene, propene, and 2-methylpropene has been observed using high-resolution 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 accounted for almost quantitatively with the assumption that the primary vibrational excitation is stretching of the CH bond attached to the core-ionized carbon atom, using the results from methane to establish the intensities of the CH stretching vibrations in the other molecules. Ab initio calculations of the geometrical changes accompanying C 1s core ionization support this picture. The high resolution in these experiments makes it possible to determine the core-ionization energies of the inequivalent carbons in propene and 2-methylpropene, as well as the 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 determined experimentally.