Photoionization of Methane: Ionization Potential and Proton Affinity of CH4

The relative photoionization cross section of methane has been measured at room temperature and at liquid-nitrogen temperature. The data show that the ionization potential of methane is 12.615 ± 0.010 eV or lower. This result is discussed and compared with those obtained by photoelectron spectroscopy. The photoionization-efficiency curve of CH5+ was also measured and found to have the same threshold as that of CH4+. This observation sets a lower limit on the proton affinity of methane, namely PA(CH4) ≥ 5.46 eV. Consideration of other data supports the value PA(CH4) = 5.50−0.05+0.20 eV.