ESR studies of nitrogen atoms stabilized in aggregates of krypton–nitrogen nanoclusters immersed in superfluid helium
S. Mao1, R.E. Boltnev2, V.V. Khmelenko1, and D.M. Lee1
1Department of Physics and Astronomy, Texas A&M University, College Station TX 77843, USA
2Branch of Institute of Energy Problems of Chemical Physics, Chernogolovka, Moscow Region 142432, Russia
Received June 12, 2012
Impurity–helium condensates (IHCs) containing nitrogen and krypton atoms immersed in superfluid 4He
have been studied via a CW electron spin resonance (ESR) technique. The IHCs are gel-like aggregates of nanoclusters composed of impurity species. It was found that the addition of krypton atoms to the nitrogen–helium
gas mixture used for preparation of IHCs increases efficiency of stabilization of nitrogen atoms. We have
achieved high average (5·1019 cm–3) and local (2·1021 cm–3) concentrations of nitrogen atoms in kryptonnitrogen–helium condensates. The analysis of ESR lines shows that three different sites exist for stabilization of nitrogen atoms in krypton-nitrogen nanoclusters. Nitrogen atoms are stabilized in the krypton core of nanoclusters, in the nitrogen molecular layer which covers the Kr core and on the surface of the nanoclusters. High concentrations of nitrogen atoms achieved in IHCs provide an important step in the search for magnetic ordering effects at low temperatures.
PACS: 67.80.–s Quantum solids; PACS: 61.46–w Structure of nanoscale materials; PACS: 76.30.Rn Free radicals; PACS: 76.30.–v Electron paramagnetic resonance and relaxation.
Key words: impurity–helium condensates, electron spin resonance technique, nanoclusters.