Electron ionization of selected cyclohexanes

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 similar to 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.