The Ionization Potentials of the Deuterated Methanes

The ionization potentials of methane, and of the four deuterated methanes, have been determined by electron impact using two 90° mass spectrometers with different types of sources. In addition, the ionization potentials of acetylene, deuterated acetylene, ethylene, and deuterated ethylene have been measured. A simple empirical method is used to obtain values which are reproducible to ±0.02 volt. For the methanes the ionization potential is found to increase with the number of deuterium atoms in the molecule, with a difference of 0.18 volt between the value for CH₄ and that for CD₄. Although this difference is found to be the same on each of the mass spectrometers, there is a difference of 0.11 volt between the absolute values of the ionization potential as determined for CH₄ on the two instruments. In order to explain the difference between the ionization potentials of CH₄ and CD₄ on the basis of zero point energy differences, it is estimated that the force constants in the ionic state would need to have only about ¼ of their value in the ground state. Since this seems unreasonably low, it is suggested that the difference between the ionization potentials of the methanes, as measured by electron impact, may be caused, in part, by excess energy required for a ``vertical'' transition.