Quantum rotations in natural methane-clathrates from the Pacific seafloor

We report inelastic neutron scattering experiments from natural methane hyd rates sampled from the Pacific sea-floor during an expedition of the resear ch ship SONNE. The experiments verify directly the content of methane by me asuring the rotational spectrum of the CH4 molecules. The existence of almo st free rotational levels excludes off-center positions of the molecules in the cages. In addition we could assign the observed peaks in the energy re gime up to 5 meV to rotational excitations only. The observed relative shar p excitations prove a surprisingly good crystallinity of the geological sam ple. The finite width is attributed to frozen-in hydrogen disorder of the i ce cages.