Rotational tunneling in CH4 at temperatures below 1 K studied by inelastic neutron scattering

The temperature dependence of the tunneling states of CH4 II doped with 0.25% O2 has been studied by high resolution inelastic neutron scattering in the temperature range 0.04–5 K. The results are in good agreement with a model based on inhomogeneous broadening except that the linewidth remains finite at very low temperatures. For the first time and through slow cooling, methane has been entirely converted into the A state. With rapid cooling, however, about 4% of the molecules remain in the E state.