Matrix Isolation of H Atoms at Low Temperatures

Abstract

The recent history of the matrix isolation of atomic free radicals at low temperatures started with a research program at the US National Bureau of Standards and continued with the important breakthrough at Chernogolovka in Russia where a jet containing atomic free radicals was directed onto the surface of superfluid ⁴He. The samples collected consisted of gel-like substances made up of molecular nanoclusters, allowing the atomic free radicals to be isolated from one another and studied at 1.3 K. More recently, techniques were developed at Turku University which have been made the region T<1 K accessible for studies of H atoms entrapped in H₂ films. Very high concentrations of H atomic free radicals (∼10¹⁸–10¹⁹ cm⁻³) have been attained using both the Turku and Chernogolovka methods. A discussion of the most recent experiments at Cornell and Turku will be given. Microwave and mm wave electron paramagnetic resonance techniques have been employed in these experiments. These techniques permitted studies of the exchange tunneling chemical reaction D+HD→H+D₂. Diffusion of H atoms through solid H₂ proceeds via the reaction H+H₂→H₂+H, leading to recombination (H+H→H₂). Quantum overlap of H atoms is thought to be responsible for exotic behavior of H atoms in solid H₂ films below 1 K, including a significant departure from the Boltzmann distribution of the relative populations of the two lowest hyperfine levels of atomic H.