Mass-Spectrometric Study of Photoionization. III. Methane and Methane-d4

Abstract

Photoionization‐efficiency curves for the molecule and fragment ions of CH₄ and CD₄ are obtained in the wavelength region 1000 to 600 Å. The electronic structure of the molecule ion is related qualitatively to the shape of the parent‐ionization‐efficiency curve and the implications of the complicated vibronic behavior of the CH₄⁺ ion are discussed. A qualitative explanation is given for the observed isotope effect on the parent‐ionization‐efficiency curve and on the fragment‐ion threshold energies. New upper bounds for the ionization energies I(CH₄) = 12.71 eV and I(CD₄) = 12.87 eV are obtained. Other thermochemical values deduced from the study are: D(CH₃–H) = 4.41 eV, D(CD₃−D) = 4.55 eV, D(CH₂–H) = 4.87 eV, and ΔH_f (CH₂) = 4.07 eV. Zero‐point differences for methyl and methylene ions and neutrals are also estimated.