Electron ionization of methane: The dissociation of the methane monocation and dication

Time-of-flight mass spectrometry and two-dimensional coincidence techniques have been used to determine, for the first time, the relative precursor-specific partial ionization cross sections following electron-methane collisions. Precursor-specific partial ionization cross sections quantify the contribution of single, double, and higher levels of ionization to the partial ionization cross section for forming a specific ion (e.g. CH+) following electron ionization of methane. Cross sections are presented for the formation of H+, H-2(+), C+, CH+, CH2+, and CH3+, relative to CH4+, at ionizing electron energies from 30 to 200 eV. We can also reduce our dataset to derive the relative partial ionization cross sections for the electron ionization of methane, for comparison with earlier measurements. These relative partial ionization cross sections are in good agreement with recent determinations. However, we find that there is significant disagreement between our partial ionization cross sections and those derived from earlier studies. Inspection of the values of our precursor-specific partial ionization cross sections shows that this disagreement is due to the inefficient collection of energetic fragment ions in the earlier work. Our coincidence experiments also show that the lower energy electronic states of CH42+ populated by electron double ionization of CH4 at 55 eV are the same (ground T-3(1), first excited E-1(1)) as those populated by 40.8 eV photoionization. The 3T1 state dissociating to form CH3+ + H+ and CH2+ + H-2(+) and the E-1(1) to form CH2+ + H+ and CH+ + H+. At this electron energy, we also observe population of the first excited triplet state of CH42+ (T-3(2)) which dissociates to both CH2+ + H+ + H and CH+ + H+ + H-2. (C) 2011 American Institute of Physics. [doi:10.1063/1.3519636]