Electron impact ionization of C3H8: appearance energies and temperature effects

The threshold behavior of the cross-section for positive ions formed via electron impact ionization of propane (C3H8) has been studied in a crossed electron/neutral beams apparatus with mass spectroscopic analysis of the ions. Appearance energies (AEs) for two states (Cs and C2v) of the parent ion C 3 H 8 + / C 3 H 8 (AE(Cs) = 10.51 ± 0.05 eV and AE(C2v) = 10.96 ± 0.05 eV) and for six additional fragment ions have been determined. Moreover, for the four most abundant cations appearance energies have been measured at two different temperatures (290 and 690 K) showing a decrease of the appearance energies with increasing gas temperature. The present experimental results are compared with appearance energies calculated using high level quantum chemical methods (G3, G3B3 and CBS-QB3).