Electron ionization of selected cyclohexanes

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Absolute cross sections for electron ionization of cyclohexane (C6H12) and ethylcyclohexane (C8H16) are measured as functions of the electron energy in a range of 10–200 eV. Combined with earlier measurements on methylcyclohexane (C7H14), a comparison of the electron ionization of these three molecules is made. The total cross sections are very similar among the three molecules at low electron energies (below ∼25 eV). A trend of increasing maximum total cross section and its corresponding electron energy with the molecular size is observed. For dissociative ionization, loss of a C2H4 from the ring is a common fragmentation feature for these molecules. In the case of C6H12, which has no side chain, the C2H4 loss resulting in C4H8+ is the most important fragmentation channel throughout the energy range studied. In the cases of C7H14 and C8H16, loss of the side chain plus an additional H atom forming C6H10+ is the most abundant channel at low energies. Loss of the side chain alone forming C6H11+ prevails at elevated energies. At further elevated energies, additional loss of C2H4 from C6H11+ to form C4H7+ becomes the dominant channel.