Hydrogen atoms in solid xenon: trapping site structure, distribution, and stability as revealed by EPR studies in monoisotopic and isotopically enriched xenon matrices.

Trapping and decay of hydrogen atoms generated by fast electron irradiation of solid xenon doped with small hydrogen-containing molecules (acetylene, water) were studied by EPR using monoisotopic (136)Xe matrix (I = 0) and highly isotopically enriched (129)Xe matrix (I = 12). It was found that more than 99% of H atoms observed by EPR are initially trapped in the octahedral interstitial trapping sites, whereas initial population of the substitutional trapping sites is very small (less than 1%). The (129)Xe hyperfine coupling tensor parameters for major trapping site were determined from direct measurements in a (136)Xe matrix doped with small amount of (129)Xe: A(0) ((129)Xe) = -92.1 MHz and B((129)Xe) = -22 MHz. Final proof for the trapping site structure was obtained from comparison between experiment and simulation for the highly enriched (129)Xe matrix. The mean interspin distance of approximately 4 nm was estimated from the EPR signal linewidth in a (136)Xe matrix, the hydrogen atom loss upon irradiation being negligible at low doses. Decay of trapped H atoms occurring at 38-45 K leads to population (or creation) of metastable traps of lower symmetry.