The HeI vacuum ultraviolet photoelectron spectrum of the methyl radical produced by pyrolysis of azomethane has been investigated. Three ionization potentials, corresponding to the formation of the ¹A′₁, ³E′ and ¹E′ ionic states, have been observed with vertical ionization potentials of 9.84, 14.76 and 16.10 eV respectively. The observed band positions have been interpreted with the aid of both ab initio and semiempirical molecular orbital calculations.

The band associated with the first ionization potential is the only one for which vibrational structure could be resolved. This has been investigated in both CH₃ and CD₃ using Hel and NeI radiation. From the Franck–Condon envelope of this band and the observed shifts on deuteration, it is concluded that the methyl radical is essentially planar in its ground electronic state. The factors which control the values of the out-of-plane deformation frequencies in the ground state of CH₃ and CH⁺₃ are discussed in detail. Jahn–Teller splitting has been detected in the band associated with the second ionization potential of CH₃.