PFI-ZEKE photoelectron spectra of the methane cation and the dynamic Jahn-Teller effect

High resolution pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) ph otoelectron spectroscopy has been used to record the photoelectron spectra of CH4, CDH3, CD2H2 and CD4. The observed extensive progression of rotation ally resolved transitions between 100800 cm(-1) and 104100 cm(-1) reveals f or the first time the complex energy level structure of the methane cation. The high resolution enabled the determination of accurate values for the a diabatic ionization potentials of the different isotopomers. Based on a sim ple one-dimensional model for the pseudorotation in the different isotopome rs, progress has been made towards the understanding of the Jahn-Teller eff ect at low energies. The static Jahn-Teller distortion in the ion could be determined directly from the vibrationless photoelectron transition in CD2H 2. The analysis of the rotational structure in this spectrum with a rigid r otor model leads to an approximate experimental C-2v structure. The dynamic s of the other methane isotopomers near the adiabatic ionization potentials is dominated by large amplitude vibrational motions between equivalent str uctures. The corresponding ground state tunneling motions takes place on a picosecond time scale.