Diffusive motion of hydrogen in mordenite studied by quasi-elastic neutron scattering

Neutron time-of-flight experiments have been done with hydrogen adsorbed in Na-mordenite at temperatures between 35 and 130 K and densities betweenn = 43 andn = 13 H₂ molecules in the unit cell of the zeolite lattice. The quasi-elastically scattered neutrons give rise to a peak that is not broadened at half-height with respect to the experimental energy resolution of 90 μeV (46 ns). A Lorentzian broadening at the bottom of the quasi-elastic peak has been interpreted in terms of a diffusion model yielding a rotational diffusion constantD_r = 2 · 10^{11rad s?1} and a translational diffusion constantD_t = 2 · 10^?3cm² s^?1 atT = 130K andn = 35 H₂ molecules/unit cell.