Thermal mobility of atomic hydrogen in solid argon and krypton matrices

Abstract

Decay patterns of atomic hydrogen trapped in argon and krypton matrices are followed by electron paramagnetic resonance (EPR). Hydrogen atoms are generated 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 atoms are thermally stable up to evaporation temperature of the solids. The fate of thermally released H atoms in Ar is exclusively due to geminate recombination of the parent molecule. The observed kinetics is well fitted with double exponential decay. The kinetic behavior reflects short-range dissociation and recombination dynamics in Ar. In the Kr matrix, a change from first-order to second-order kinetics is observed at higher concentrations as formation of molecular hydrogen becomes important. From bimolecular decay kinetics, a diffusion constant of 4×10−15 cm2 s−1 is deduced for H-atom 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 dynamics and show only weak dependence on the hydrogen isotope.