The rovibronic structure in the photoelectron spectrum of ${\mathrm{CH}}_4$ 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 ${\mathrm{CH}}_4^{+}$ 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 ${\mathrm{CH}}_4^{+}$ is a tunneling doublet of triply degenerate levels and the lowest level has rovibronic symmetry $F_2$. The adiabatic ionization energy of ${\mathrm{CH}}_4$ is $101753.0(15){\mathrm{cm}}^{-1}$.

• Received 15 August 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.173003