Tunneling splittings in vibronic energy levels of CH3F+ (X2E) studied by high resolution photoelectron spectroscopy and ab initio calculation.

The energy levels of CH3F(+) (X(2)E), which show strong vibronic coupling effect (Jahn-Teller effect), have been measured up to 3500 cm(-1) above the ground vibrational state using one-photon zero-kinetic energy photoelectron spectroscopic method. Theoretical calculations have also been performed to calculate the spin-vibronic energy levels using a diabatic model and ab initio adiabatic potential energy surfaces (APESs) including the energy gradients and derivative couplings between the APESs. The calculations showed that the tunneling splittings of the vibrational energy levels occur due to the deep potential energy wells formed by the Jahn-Teller deformation. The calculated spin-vibronic energy levels are in good agreement with the experimental data. For example, the energy splitting for the first excited vibrational energy level is calculated as 111 cm(-1) that is confirmed by the experimental value. The experimental spectrum was assigned based on the fundamental vibrational modes calculated at the energy minimum. The fundamental vibrational modes related to the H-C-F bending, H-C-H bending, C-F stretching, and C-H stretching vibrations have been observed.