X-Ray Photoelectron Spectroscopy of Simple Hydrocarbons

Abstract

Carbon $\text{1s}$ ionization potentials for methane (290.8 eV), ethane (290.6), ethylene (290.7), acetylene (291.2), cyclohexane (290.3), benzene (290.4), 2,2‐dimethylpropane (290.4), and fluoroform (299.1), together with the fluorine $\text{1s}$ ionization potential for fluoroform (694.1) and ionization potentials for the ${\text{2a}}_g$ and ${\text{2a}}_u$ orbitals of ethylene (24.5 ± 1 and 19.5 ± 1) and for the ${\text{2σ}}_g$ and ${\text{2σ}}_u$ orbitals of acetylene (23.5 and 18.5), have been measured. These values, together with those from other measurements, are compared with orbital energies obtained by use of Koopmans' theorem. The comparison indicates that this method of calculation gives orbital energies that are about 5% larger than the experimental ionization potentials for carbon $\text{1s}$ electrons and 10%–15% greater for the outer electrons. These differences are consistent with calculations of the ionization potentials of carbon atoms. The carbon $\text{1s}$ binding energy decreases with hydrogenation and when a hydrogen is replaced by an alkyl group. The first of these results is consistent with theoretical calculations and with other chemical evidence. There is an apparent discrepancy between the second result and the result of theory or other chemical evidence. A possible source of this discrepancy is discussed.