Tunneling states in methane at high pressures

Rotational tunneling states in phases II and III of solid CH4 were studied by inelastic neutron scattering techniques at 8 K and pressures up to 850 bar. The tunnel energies were found to depend strongly on pressure, while the rotational peak of the disordered molecules shifts little with pressure up to the II–III phase transition. In phase III, additional intensity was found at about 0.18 meV, which we interpret as tunneling states for those molecules that are disordered in phase II.