E. P. Bernard

ESR and X-ray Investigations of Deuterium Atoms and Molecules in Impurity-Helium Solids

Impurity-helium solids created by injecting deuterium atoms and molecules into superfluid 4He have been studied via electron spin resonance (ESR) and x-ray diffraction methods. We measured the g-factor, the hyperfine constant and the spin-lattice relaxation time of D atoms in D-D2-He solids. These measurements show that D atoms are mainly stabilized in D2 clusters. Using an x-ray method we found the size of D2 clusters to be ∼90Å in diameter and the densities of D2 molecules in the samples to be of order 2.5⋅1021 cm−3 . The highest average concentration of D atoms achieved in D-D2-He solids was ∼1.5⋅1018 cm−3 . The local concentrations of D atoms within D2 clusters is found to be large (∼2⋅1019 cm−3).

ESR investigation of hydrogen and deuterium atoms in impurity-helium solids

Impurity-helium solids (Im–He solids) are porous solids created by injecting a beam of mixed helium and impurity gases into superfluid 4He. In this work we use electron spin resonance (ESR) techniques to investigate Im–He solids containing atoms and molecules of hydrogen and/or deuterium. We have performed studies of low temperature (T∼1.35 K) tunnelling chemical reactions in which deuterium atoms replace the hydrogen atoms bound in H2 or HD molecules to produce large (up to 7.5×1017 cm−3) and relatively stable concentrations of free hydrogen atoms. The time dependence of H and D atom concentrations has been investigated for Im–He samples with different initial ratios of hydrogen and deuterium ranging from 1:20 to 1:1. The satellite ESR lines associated with the dipolar coupling of electron spins of H and D atoms to the nuclear moments of the hydrogen nuclei found in neighboring molecules have been observed in Im–He solids. The forbidden hyperfine transition of atomic hydrogen involving the mutual spin fl...

CW ESR studies of impurity-helium condensates containing krypton and hydrogen atoms

Impurity-Helium condensates (IHCs) containing krypton and hydrogen atoms have been studied in superfluid helium-4 via CW ESR techniques. The IHCs studied in this work are gel-like aggregates of nanoclusters composed of krypton and hydrogen atoms. We have found that such samples contain very high average concentrations of hydrogen atoms (~1018cm-3) as obtained by integration of the microwave absorption signal. Local concentrations (~1019cm-3) of H atoms were calculated from the ESR line width. Detailed studies of the ESR line shapes lead to the conclusion that a large fraction of the H atoms lies on the krypton cluster surfaces.

Phase transition of DNA-linked gold nanoparticles: Creation of a high concentration of atomic hydrogen in impurity-helium solids

Abstract The exchange tunneling reactions D+H2→HD+H and D+HD→D2+H were used to generate high concentrations of atomic hydrogen in impurity-helium solids. The dependence of atom concentration on the content of hydrogen in the injected gas mixture gave a maximum concentration of 7.5×10 17 cm −3 hydrogen atoms for an initial gas ratio H2:D2:He=1:4:100.

Two‐Pulse Electron Spin Echo Study of Deuterium‐Helium Solids

We measured electron spin echoes from deuterium atoms within deuterium‐helium solids with an X‐band pulse ESR spectrometer. Our two‐pulse electron spin echo envelope modulation (ESEEM) measurements are well described by a model that places 50% – 60% of the deuterium atoms at the interface between the molecular deuterium nanoclusters and the superfluid liquid helium. The remainder of the atoms lie in substitutional sites within the nanoclusters. We also report the spin‐lattice relaxation times T1 for these atoms.

Pulse Electron Spin Resonance Method For Investigation Of Atoms In Impurity‐Helium Solids

We have constructed an X‐band pulse electron spin resonance spectrometer for the investigation of hydrogen, deuterium, and nitrogen impurity‐helium solids at low temperatures. The spectrometer is of a conventional homodyne detection design incorporating a sampling oscilloscope but uses a modified CW ESR resonant cavity.

Pulse and Continuous Wave Electron Spin Resonance Investigations of H and D Atoms in Impurity‐Helium Solids

By using pulse and continuous wave (CW) X‐band ESR methods the kinetics of the tunneling exchange chemical reaction D+HD→D2+H in HD‐D2‐He solids has been studied. It was found that in the course of this reaction the number of HD molecules in the vicinity of D atoms decreased while the surroundings of H atoms did not change.

Susceptibility measurements of impurity-helium condensates containing magnetic impurities

The magnetic susceptibilities of impurity-helium condensates (IHCs), containing nanocrystals of molecular oxygen and atomic nitrogen free radicals embedded in molecular N2 have been measured via a SQUID magnetometer in the temperature range between 1.1 and 2.1 K. The susceptibilities of the samples containing nitrogen atoms followed Curie-Weiss behavior with very small Weiss temperatures ranging from 0 to -0.4 K. The behavior of samples composed of O2 nanocrystals deviated sharply from results for bulk solid. The susceptibilities of the samples were 102 larger than for bulk solid O2 and showed Curie-Weiss behavior with a Weiss temperature in the range from -4.5 K to -5 K. This result is qualitatively consistent with results obtained in other laboratories for O2 confined in restricted geometries.

Pulse ESR studies of impurity-helium condensates containing Kr, H, and D atoms

Hydrogen and deuterium atoms trapped in impurity-helium condensates (IHCs) formed by condensing krypton, hydrogen, and deuterium atoms in superfluid 4He were studied with pulse electron spin resonance. We have used electron spin echo envelope modulation (ESEEM) to probe the magnetic interactions between atoms of hydrogen isotopes and 83Kr nuclei. Modeling of the ESEEM spectra suggests that the vast majority of the hydrogen and deuterium atoms reside at the surface of the Kr clusters. This conclusion is in agreement with that from continuous wave ESR experiments with IHCs of similar composition.

ESR Investigations of Spin‐Pair Radicals in Nitrogen‐Helium Solids

We have investigated nitrogen‐helium solids by X‐band CW‐ESR. Our samples, containing molecules and atoms of either the 14N or the 15N isotope, were produced from N2:He gas mixtures with different nitrogen impurity concentrations ranging from 0.25% to 1%. Samples were formed in a helium cryostat at 1.5 K by applying a radio‐frequency discharge to a gas mixture jet that was then introduced into superfluid 4He. We achieved average concentrations of stabilized nitrogen atoms as high as 2⋅1019 atoms/cm3. We observed a strong ESR signal of stabilized nitrogen atoms at ∼3235 Oe. For the first time we also detected weak ESR signals corresponding to the ΔMS = 2 transitions of 14N⋯14N or 15N⋯15N spin‐pair radicals in nitrogen‐helium solids at ∼1617 Oe. The intensities of these signals were ∼1000 or more times smaller than those of the main atomic signals. We investigated the thermal stability of the nitrogen‐helium solids and observed the spin‐pair radical signals in dry samples at T = 3.1 K. After explosive disin...