Magnetic Resonance Study of Atomic Excitations in Solid Molecular Hydrogens

We study unpaired hydrogen (H,D,T) atoms in a solid molecular hydrogen lattice using EPR at 9.37 GHz. The unpaired atoms are produced by the disintegration of the triton. By studying the evolution of the EPR spectrum at fixed temperature, we are able to quantify the absolute number of atoms, the linewidth, and the spin lattice relaxation time from 1.2 K to 10 K. The observed time dependence of the atom concentration fits a tanh function plus a small linear term, but there appears to be some structure for different samples. The number of atoms increases with decreasing temperature and we have observed concentrations over 1000 ppm. The linewidth is seen to be a linear function of the atom density and nuclear spin density as predicted by Drabold and Fedders. The nuclear spin contribution to the linewidth agrees quantitatively with theory and gives insight as to the location of the various atoms. The contribution to the linewidth from the unpaired atoms indicates that there may be many more atoms than those in the narrow resonance region. The electron spin lattice relaxation T_1 is measured by a saturation recovery method and is seen to increase with increasing temperature up to 8 K. T_1 is seen to depend on the ortho concentration and linearly on the atom concentration. A diffusion coefficient is obtained from both the relaxation times and the recombination coefficient. The two diffusion coefficients differ by up to 5 orders of magnitude for T_2. The diffusion is very insensitive to the temperature characteristic of a quantum diffusion regime. Quantum tunneling has been observed for H and D atoms in HD but there is no isotope effect for D and T atoms in D-T. Finally, we have observed spontaneous and stimulated depopulation of the atomic excitations giving rise to large energy releases as studied by the sample thermometer. We characterize this phenomenon by doing a coincidence detection of the atom concentration, the thermal conductivity and NMR on the nuclei of the molecular lattice.