Hydrocarbon Ion Appearance Potentials

Abstract

Discussion of the effect of impact parameter, distribution of electron energy and transition efficiency upon the appearance potential for ions from hydrocarbons in the mass spectrometer leads to the conclusion that initial ion current onset is a correct measure of the appearance potential. In order to calibrate the appearance potential voltage scale, the spectroscopic ionization potentials for seven gases were correlated with their apparent appearance potentials to give a calibration curve. Heretofore only one gas had been used to give a simple additive constant. Appearance potentials were determined for ions from six paraffins and four olefins. For methane, ethane and propane the agreement with previous determinations is excellent. For the other hydrocarbons the values reported here are higher than those reported by others. It is concluded that dissociation energies computed from these appearance potentials are greater than true bond energies because electron collision processes give rise to excited fragments. The excitation energy appears to be greater for larger molecules.