Tunnelling of hydrogen in alkali metal intercalation compounds

Rotational tunnelling spectroscopy for hydrogen absorbed in the second stage graphite intercalation compounds C24M(H2) x where M = Rb and Cs, 0·5 < x < 2 is reported in the energy range 100 μeV < ΔE < 1400 μeV (0·8 cm-1 ∼ 11 cm-1). Two distinct sites, A and B, for the hydrogen uptake can be deduced from the spectra which are consistent with site symmetries A, tetrahedral; B octahedral. The spectra of HD calculated using the crystal field parameters for H2 agree well with measurements and indicate molecular motion about the molecular centre of mass rather than the bond centre. The sites can be preferentially blocked by co-absorbing deuterium or methane. The tunnelling spectra disappear in the temperature range 90 K < T < 150 K becoming broader with the simultaneous appearance of quasielastic scattering.