Photochemistry of Methane in the Photoionization Region.

ABSTRACT

Methane was irradiated with microwave operated helium (21.2 eV) and neon (16.7-16.8 eV) resonance lamps which were separated from the reaction vessel by an aluminum window. The quantum yields of the stable end products have been determined at methane pressures ranging from 1 to 20 torr. Over this pressure range the abundances of the primary ions, determined through various diagnostic experiments, are within experimental error the same at 21.2 eV as at 16.7-16.8 eV ( CH 4 + ≃ CH 3 + ≃ 0.5 , CH 2 + ⩽ 0.02 ), and are in good agreement with the primary mass spectra obtained in a photoionization mass spectrometer under collision free conditions (P < 10-5 torr). The C 2 H 5 + which is formed by the reaction CH 3 + + CH 4 → C 2 H 5 + + H 2 loses a proton by an undetermined mechanism to give C2H4 as a product. There is no evidence for the formation of neutral fragments such as H2, C, CH or CH2 at 16.7-16.8 eV. The fact that the ionization quantum is equal to unity in this energy range accounts for the absence of these intermediates. At 21.2 eV where (Φionization = 0.95) there is concrete evidence for the formation of carbon atoms (Φ(C) ⩾ 0.002). In an attempt to demonstrate the usefulness of enclosed neon and helium resonance light sources in the ion-molecule kinetic studies, the relative probabilities of transfer of H- over D- from various partially deuterium labeled hydrocarbons to C 2 H 5 + (or C 2 D 5 + ) has been determined. The results presented in this article resolve the existing disagreements between previous helium resonance photolysis studies on CH4.