Thermal mobility of atomic hydrogen in solid argon and krypton matrices

Decay patterns of atomic hydrogen trapped in argon and krypton matrices are followed by electron paramagnetic resonance (EPR). Hydrogen atoms are gene rated by uv-photolysis of HBr and HCl precursor molecules. The EPR signals due to interstitially trapped hydrogen atoms in octahedral sites disappear near 16 and 24 K in Ar and Kr, respectively. Substitutionally trapped H ato ms are thermally stable up to evaporation temperature of the solids. The fa te of thermally released H atoms in Ar is exclusively due to geminate recom bination of the parent molecule. The observed kinetics is well fitted with double exponential decay. The kinetic behavior reflects short-range dissoci ation and recombination dynamics in Ar. In the Kr matrix, a change from fir st-order to second-order kinetics is observed at higher concentrations as f ormation of molecular hydrogen becomes important. From bimolecular decay ki netics, a diffusion constant of 4 X 10(-15) cm(2) s(-1) is deduced for H-at om diffusion in Kr at 26.9 K. The obtained activation energies, 6-7 kJ/mol in Ar and 9-14 kJ/mol in Kr, are measures of thermally activated cage dynam ics and show only weak dependence on the hydrogen isotope. (C) 1999 America n Institute of Physics. [S0021-9606(99)01004-1].