Motional averaging of nuclear magnetic resonance spectra was used to determine the diffusion coefficients of molecules in solid HD, D-T, and T2 (figure 9). The molecular hopping frequency and the diffusion coefficient were deduced from the measured spin-spin relaxation time and the rigid-lattice second moment. Samples which were prepared by depositing streams of gaseous H2 or D2 (containing atoms that were produced by microwave discharge), onto substrates at 2K or below, were called amorphous. Those which were prepared by slow cooling from the liquid state were called crystalline. It was found that diffusion in crystalline solids was controlled by the number of vacancies in the lattice, and values were obtained for the vacancy formation energy, the barrier height energy, and the energy of the first tunnelling level in the H potential, for all the isotopes. The vacancy hopping rate at the triple point was approximately the same for all of the isotopes. Data for the various isotopes could be compared by scaling the temperature with the quantum parameter. Published results on the atomic recombination coefficients of radiation-damaged crystalline H2 and undamaged amorphous H2 were used to estimate the atomic hopping frequency. In the case of crystalline H2, the atomic and molecular hopping rates were almost identical. Data on an ortho to para conversion in solid T2 yielded model-dependent atomic hopping rates. The atomic and molecular hopping rates still agreed, even though the recombination coefficients no longer obeyed a simple thermally activated form. The recombination coefficients (and hence the hopping rates) for crystalline solids differed from those for amorphous solids.

J.R.Gaines, P.A.Fedders, G.W.Collins, J.D.Sater, P.C.Souers: Physical Review B, 1995, 52[10], 7243-51