Jahn-Teller effect in the methane cation: Rovibronic structure and the geometric phase.

The rovibronic structure in the photoelectron spectrum of CH4 has been assigned in infrared-vacuum ultraviolet double-resonance experiments and analyzed using an effective tunneling-rotation Hamiltonian. Comparison of theoretical and experimental level structures of CH4+ reveals the effect of the geometric phase and establishes that at low energies, the structure and dynamics are governed by a large-amplitude tunneling motion connecting equivalent minima of C(2v) geometry. The rotationless ground state of CH4+ is a tunneling doublet of triply degenerate levels and the lowest level has rovibronic symmetry F2. The adiabatic ionization energy of CH4 is 101753.0(15) cm(-1).