PHOTOIONIZATION OF CH3 - HEAT OF FORMATION OF CH2

Methyl radicals produced by pyrolysis at various temperatures have been photoionized by photons in a 1.6-Å bandwidth over the energy range 9.5-16.5 eV with mass-spectrometric detection. The curve of photoionization cross section for the parent ion shows that direct ionization predominates in the threshold region, and that the geometrical form of the positive ion in its ground state is very similar to that of the neutral radical. Hot bands and other vibrational features observed near threshold are only partly explained. The data are compared with absorption spectra and electron-impact curves. The ionization efficiency curve for production of CH2+ from CH3 is measured at several pyrolysis temperatures and the threshold for 0°K is determined to be 15.09±0.03 eV. This yields the value 91.9±1.0 kcal/mole for the heat of formation of the CH2 free radical.