Quantum diffusion of vacancies and impurities in solid hydrogen

The proton nuclear transverse relaxation T₂ of HD impurities and ortho-H₂ impurities in solid parahydrogen is observed to be essentially temperature independent below 8 K, passes through a minimum near 9.2 K on warming, and then increases exponentially on approaching the melting point. The low-temperature values of T₂ are pressure dependent and a factor of 5.4 larger than the rigid lattice values. This ``line narrowing'' is attributed to quantum tunneling of the HD impurities at low temperatures. The minimum in T₂ is attributed to the breakup of coherent tunneling due to scattering by phonons and vacancies.