Influence of Competitive Decomposition Processes on Ionization Efficiency Curves from Electron Impact

The energy dependence of ionization efficiency by electron impact for polyatomic molecules is influenced by the entrance of decomposition processes. The observed yield of parent ion drops sharply at the energy onset of a high-yield decomposition process. Ionization efficiency curves for products from other decomposition processes are similarly effected. These effects are particularly apparent for decomposition processes with energy thresholds some volts above the parent ion appearance potential, and this is illustrated by data for the ionization efficiencies from the molecules NH3, C2H4, and C2F4. Perturbation from such competitive processes can result in erroneously high measured appearance potentials for the product ions.