Electron impact ionisation and partial ionisation cross-sections of iso-octane

Electron impact ionisation of iso-octane is studied using mass spectrometry. Cross-sections for the formation of fragment ions are measured between 17 and 79 eV. C4H9+ is the most abundant ion, C8H18+ is completely absent. A total ionisation cross-section of 2×10−15 cm2 is found at 79 eV. Ab initio calculations show that the principal ionic species observed, C4H9+, C4H8+, C7H15+, C3H6+ and C3H11+, may be issued directly from iso-octane by simple C–C bond split processes or with H atom rearrangements. Ionisation of the two major neutral species issued from bond split, C4H9 and C4H10, can lead to the principal ions observed.