Formation of Nuclear-Polarized Phases of H Atoms Embedded in Solid H_{2} Films

Hydrogen atom quantum diffusion in solid parahydrogen: The H + N2O → cis-HNNO → trans-HNNO reaction

The diffusion and reactivity of hydrogen atoms in solid parahydrogen at temperatures between 1.5 K and 4.3 K are investigated by high-resolution infrared spectroscopy. Hydrogen atoms are produced within solid parahydrogen as the by-products of the 193 nm in situ photolysis of N₂O, which induces a two-step tunneling reaction, H + N₂O → cis-HNNO → trans-HNNO. The second-order rate constant for the first step to form cis-HNNO is found to be inversely proportional to the N₂O concentration after photolysis, indicating that the hydrogen atoms move through solid parahydrogen via quantum diffusion. This reaction only readily occurs at temperatures below 2.8 K, not due to an increased rate constant for the first reaction step at low temperatures but rather due to an increased selectivity to the reaction. The rate constant for the second step of the reaction mechanism involving unimolecular isomerization is shown to be independent of the N₂O concentration as expected. The inverse concentration dependence of the rate constant for the reaction step that involves the hydrogen atom demonstrates clearly that quantum diffusion influences the reactivity of the hydrogen atoms in solid parahydrogen, which does not have an analogy in classical reaction kinetics.

Experimental cell with a Fabry-Pérot resonator tuned in situ for magnetic resonance studies of matrix-isolated radicals at temperatures below 1 K

We describe the design and construction of an experimental cell for the study of free radicals in macroscopically thick films of solidified molecular and rare gases by 128 GHz Electron Spin Resonance (ESR) at temperatures below 1 K. The ESR resonator has an open Fabry-Pérot design, and its frequency can be tuned in situ by adjusting the spacing between the mirrors. The tuning mechanism consists of a piezo positioner and a stainless-steel edge-welded bellows, which can change the resonator frequency by at least 6 GHz. The films of solidified gases can be deposited either directly from a room temperature reservoir or by recondensing from a specially arranged chamber. The free radicals can be created in the solid films by dissociating matrix species by running an rf discharge in a helium vapor. We suggest that such a sample cell design can also be used for a broad range of low-temperature ESR experiments where sample cooling needs to be enhanced by the presence of superfluid helium.