DIFFUSION OF ATOMS AND MOLECULES IN THE SOLID HYDROGENS

The ''motional averaging'' of the NMR spectra has been used to determi ne the diffusion coefficient of molecules in HD, D-T, and T-2 solids. The molecular hop frequency and diffusion coefficient are calculated f rom the measured spin-spin relaxation time and the rigid lattice secon d moment. Samples prepared by depositing streams of H-2 or D-2 gas, co ntaining atoms produced by microwave discharge, onto cold substrates, held at 2 K or below are designated ''amorphous'' while those prepared by slow cooling from the liquid state are designated ''crystalline.'' We find that the diffusion in crystalline solids (c-H-2, etc.) is con trolled by the number of vacancies in the lattice and have obtained va lues of the vacancy formation energy, E(V), the barrier height energy, E(b), and the energy of the first tunneling level in the hydrogen pot ential, E(t), for all the isotopes. The vacancy hopping rate, at the t riple point, is approximately the same for all the isotopes. Data for the various isotopes can be compared by scaling the temperature by the quantum parameter. Measurements (by others) on both radiation damaged crystalline (c-H-2) and undamaged amorphous (a-H-2) solids at the ato m recombination coefficients are used to extract the atom hop frequenc y. In c-H-2, we find that the atom and molecule hopping rates are almo st identical. Other data on crystalline solids, taken by NMR technique s on ortho to para conversion in solid T-2, yield model dependent atom hop rates. The atom and molecule hopping rates still agree even thoug h the recombination coefficients no longer follow a simple thermally a ctivated form. The recombination coefficients (and hence hopping rates ) for crystalline solids differ from those of amorphous solids.