Mass-Spectrometric Study of the Photoionization of Methane

Photoionization efficiency curves for the parent and fragment ions of CH₄ have been measured with photon bandwidths of 1.6 and 0.4 Å. For the fragment ions, the effect of thermal rotational energy at room temperature is apparent in the threshold region and was taken into account to obtain accurate thresholds. The bond energy ${\mathrm{D°}}_0({\mathrm{CH}}_3\mathrm{\hspace{0.17em}-\hspace{0.17em}}\mathrm{H})$ was determined to be 4.477 ± 0.005 eV and the limit ${\mathrm{\Delta H°}}_{\text{f0}}({\mathrm{CH}}_2\text{)\hspace{0.17em}≤\hspace{0.17em}4.10}\mathrm{eV}$ was found. Ion‐pair formation yielding CH₃⁺ and H⁻ was found to occur with low probability.