Quantum delocalization of molecular hydrogen in alkali-graphite intercalates

The adsorption of molecular hydrogen (H2) in the graphite intercalation compound KC24 is studied both experimentally and theoretically. High-resolution inelastic neutron data show spectral features consistent with a strong pinning of H2 along a single axis. First-principles calculations provide novel insight into the nature of H2 binding in intercalates but fail to account for the symmetry of the H2 orientational potential deduced from experiment. The above discrepancy disappears once the H2 center of mass is allowed to delocalize in the quantum-mechanical sense across three vicinal adsorption sites, naturally leading to the well-known saturation coverage of approximately 2H_2 per metal atom in this material. Our results demonstrate that H2 storage in metal-doped carbon substrates can be severely affected by hitherto unexplored quantum-mechanical effects.