Studies of the ionization of molecules by electron impact - II. Excited states of the molecular ions of methane and the methyl halides

Abstract
The ionization of methane and the methyl halide molecules by essentially mono-energetic electrons, produced by pulse techniques, has been studied in detail in a mass spectrometer. It has been possible to detect for the first time the production of the molecular ions of these compounds in most of their excited electronic states. In the cases of the ions of methyl bromide and iodide we have been able to resolve the components of the doublets of the ground 2 E states which arise from spin-orbital interactions in these molecular ions. The several ionization potentials of each of the molecules which refer to the formation of the ions in their different electronic excited states have been measured. These new results are of interest in that they enable the molecular-orbital theories of the electronic structures of methane and the methyl halides to be assessed. They also provide support for recent theories of the origin of the ions in the mass spectra of organic compounds. It has been demonstrated that there is a monotonic relationship between the ionization potential of electrons in the [ σα 1 ] bonding orbital localized in the C— X bond of these molecules and the corresponding bond dissociation energy.