Catastrophic Recombination/Reaction of Aluminum Atoms in Cryogenic Parahydrogen Solids

We report the results from experiments testing the limits of chemical energy storage in Al-atom-doped cryogenic parahydrogen (pH₂) solids produced by codeposition of Al vapor and pH₂ gas. These results map out three regimes of behavior. As described in the immediately preceding companion manuscript, for target Al atom concentrations, [Al]_target ≲ 200 parts-per-million (ppm), the Al/pH₂ solids are optically transparent and primarily contain isolated Al atoms, with a small admixture of AlH molecules formed by Al atom recombination/reaction. For [Al]_target ≳ 500 ppm, the depositions fail immediately, producing highly optically scattering solids with evidence of extensive Al atom recombination/reaction. For intermediate [Al]_target concentrations, near-threshold metastable transparent solids are formed which suffer catastrophic recombination/reaction as they grow past some critical thickness. Analysis of these catastrophic events using a minimal two parameter model [Jackson. J. Chem. Phys. 1959, 31, 722-729] yields a critical Al atom concentration near [Al] ≈ 280 ppm and an Al atom diffusion length ≈ 100 μm. While this diffusion length appears unphysically large, it is roughly compatible with even larger 300-900 μm wavelengths of spatial inhomogeneities visible in images of a remnant postrecombination sample. This spatial pattern is evocative of Turing structures or perhaps the structures sometimes formed upon phase separation; however, the actual mechanism for morphogenesis remains undetermined. The largest achieved Al atom concentrations of [Al] ≈ 300 ppm are roughly two orders-of-magnitude lower than those required for advanced chemical propulsion applications.