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

Abstract

High resolution pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectroscopy has been used to record the photoelectron spectra of CH₄ , CDH₃, CD₂H₂ and CD₄. The observed extensive progression of rotationally resolved transitions between 100800 cm⁻¹ and 104100 cm⁻¹ reveals for the first time the complex energy level structure of the methane cation. The high resolution enabled the determination of accurate values for the adiabatic ionization potentials of the different isotopomers. Based on a simple one-dimensional model for the pseudorotation in the different isotopomers, progress has been made towards the understanding of the Jahn–Teller effect at low energies. The static Jahn–Teller distortion in the ion could be determined directly from the vibrationless photoelectron transition in CD₂H₂. The analysis of the rotational structure in this spectrum with a rigid rotor model leads to an approximate experimental C_2v structure. The dynamics of the other methane isotopomers near the adiabatic ionization potentials is dominated by large amplitude vibrational motions between equivalent structures. The corresponding ground state tunneling motions takes place on a picosecond time scale.