Electron spin resonance study of atomic hydrogen stabilized in solid neon below 1 K

Abstract

We report on an electron spin resonance study of atomic hydrogen stabilized in a solid Ne matrices carried out at a high field of 4.6 T and temperatures below 1 K. The films of Ne, slowly deposited on the substrate at the temperature ∼1 K, exhibited a high degree of porosity. We found that H atoms may be trapped in two different substitutional positions in the Ne lattice as well as inside clusters of pure molecular H2 in the pores of the Ne film. The latter type of atoms was very unstable against recombination at temperatures 0.3-0.6 K. Based on the observed nearly instant decays after rapid small increase of temperature, we evaluate the lower limit of the recombination rate constant kr ≥ 5 · 10 −20 cm 3 s −1 at 0.6 K, five orders of magnitude larger than that previously found in the thin films of pure H2 at the same temperature. Such behavior assumes a very high mobility of atoms and may indicate a solid-to-liquid transition for H2 clusters of certain sizes, similar to that observed in experiments with H2 clusters inside helium droplets (Phys. Rev. Lett 101, 205301 (2008)). We found that the efficiency of dissociation of H2 in neon films is enhanced by 2 orders of magnitude compared to that in pure H2, which is instigated by a strong emission of secondary electrons.

Electrons Trapped in Solid Neon–Hydrogen Mixtures Below $$1\, \hbox {K}$$

“…In our recent study [6], we found that the solid neon with small (well below 6%) admixtures of hydrogen forms a highly porous solid with small clusters of pure H 2 embedded inside the pores. We observed an exceptionally high recombination rate of H atoms in such H 2 clusters upon raising the temperature to 0.3-0.6 K, which could be explained by a solid-liquid transition in the clusters.…”

“…In our recent study of H atoms in solid Ne:H 2 mixtures [6], we found that raising temperature from 0.1 to 0.3-0.6 K and admitting a superfluid He film into the sample Fig. 3 Spectrum of electrons trapped in the "pure" Ne Sample 1 (100 ppm H 2 ) after condensing He film into sample cell.…”

“…The fitting procedure and detailed explanation of different trapping sites of H atoms in Ne:H 2 solid mixtures is presented in Ref. [6].…”

“…A weak van-der-Waals inter-action, low atomic polarizability and a large bandgap make solid neon a particularly suitable medium for conducting optical and electron spin resonance studies of such transient species. In contrast to atomic hydrogen, the smallest among the metastable radicals, which may occupy two different positions in a solid neon lattice [4][5][6], observation of electrons in a Ne matrix is far more difficult due to the large − 1.3 eV negative electron affinity of neon atoms [7]. However, exposure of solid neon samples to an electron beam is accompanied by a large yield of secondary electrons, which is 60 for each incident 1-3 keV primary electron [8].…”

Evidence for melting of HD and D2 clusters in solid neon below 1 K

“…Clusters of a greater size can also be prepared by annealing hydrogen films quench condensed onto a cold substrate [11,12]. In our recent study [13], we observed that a large number of such small molecular hydrogen clusters can be accommodated in a porous matrix of quench condensed solid neon films. In addition, an exceptionally high recombination rate of H atoms in such H 2 clusters at temperatures 0.3-0.6 K provided evidence for a solidliquid transition in H 2 clusters predicted for solid H 2 in a restricted geometry [6].…”