Abstract
An analysis of the Jahn–Teller effect in the photoelectron spectrum of methane was performed by the use of the ab initio self-consistent field and configuration interaction calculations. The molecular structures of the CH+4 ion, which have a local energy minimum, are found to be in the C2v and C3v symmetry. The vertical excitation energy is calculated to be 14.23 eV and the adiabatic ionization potentials of the CH+4 ion including zero-point vibrational energy are estimated to be 12.25 and 12.68 eV for the C2v and C3v symmetry, respectively. We calculated the normal modes of vibration and the Franck–Condon factor in the harmonic oscillator approximation, and assigned the vibrational band below 13.10 eV of the CH+4 ion.

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