Paramagnetic Attraction of Impurity-Helium Solids

Impurity-helium solids are formed when a mixture of impurity and helium gases enters a volume of superfluid helium. Typical choices of impurity gas are hydrogen deuteride, deuterium, nitrogen, neon and argon, or a mixture of these. These solids consist of individual impurity atoms and molecules as well as clusters of impurity atoms and molecules covered with layers of solidified helium. The clusters have an imperfect crystalline structure and diameters ranging up to 90 angstroms, depending somewhat on the choice of impurity. Immediately following formation the clusters aggregate into loosely connected porous solids that are submerged in and completely permeated by the liquid helium. Im-He solids are extremely effective at stabilizing high concentrations of free radicals, which can be introduced by applying a high power RF dis- charge to the impurity gas mixture just before it strikes the super fluid helium. Average concentrations of 10(exp 19) nitrogen atoms/cc and 5 x 10(exp 18) deuterium atoms/cc can be achieved this way. It shows a typical sample formed from a mixture of atomic and molecular hydrogen and deuterium. It shows typical sample formed from atomic and molecular nitrogen. Much of the stability of Im-He solids is attributed to their very large surface area to volume ratio and their permeation by super fluid helium. Heat resulting from a chance meeting and recombination of free radicals is quickly dissipated by the super fluid helium instead of thermally promoting the diffusion of other nearby free radicals

On charged impurity structures in liquid helium

The thermoluminescence spectra of impurity-helium condensates (IHC) submerged in superfluid helium have been observed for the first time. Thermoluminescence of impurity-helium condensates submerged in superfluid helium is explained by neutralization reactions occurring in impurity nanoclusters. Optical spectra of excited products of neutralization reactions between nitrogen cations and thermoactivated electrons were rather different from the spectra observed at higher temperatures, when the luminescence due to nitrogen atom recombination dominates. New results on current detection during the IHC destruction are presented. Two different mechanisms of nanocluster charging are proposed to describe the phenomena observed during preparation and warmup of IHC samples in bulk superfluid helium, and destruction of IHC samples out of liquid helium

COOPERATIVE EFFECTS IN OPTICAL AND ESR SPECTROSCOPY OF NITROGEN ATOMS ISOLATED BY SOLIDIFICATED HELIUM

1. E.B. Gordon, V.V. Khmelenko, A.A. Pelmenev, E.A. Popov and O.P. Pugachev, Chem. Phys. Lett. 155(3), 301-304 (1989). 2. R.E. Boltnev, E.B. Gordon, V.V. Khmelenko, A.A. Pelmenev, I.N. Kusliniskaya, M.V. Martynenko, E.A. Popov and A.V. Shestakov, Chem. Phys. 189(2), 367-382 (1994). 3. R.E. Boltnev, E.B. Gordon, V.V. Khmelenko, M.B. Martynenko, A.A. Pelmenev, E.A. Popov and A.F. Shestakov, J. Chim. Phys. 92(2), 362-383 (1995).Author Institution: Institute for Energy Problems of Chemical Physics (branch)The heavy guest particles embedded to superfluid helium can cause its $solidification^{1}$. The so-called Impurity Helium Solid Phase (IHSP) being stable then up T = 7K shows the regular arrangement of the impurities with their reliable isolation by helium atoms. The feasibility of previously excited species capture to IHSP may be achieved. So metastable N($^{2}$D) atoms display extremely long-lived, more than $10^{4}$ s, luminescence. Their radiative decay turns out to be caused solely by excimer-like state formation with accidentally neighbouring heavy $particle^{2}$. That was proved for N($^{2}$D)-Rg pairs (Rg = Ne, Ar, Kr, Xe) by both spectra shapes and emission lifetimes observed. For N($^{2}$D)-$N_{2}$ state the comparison of atomic $N(^{2}D-^{4}S)$ and rovibronic $N(^{2}D)-N_{2}(\nu = 0) \rightarrow N(^{4}S)-N_{2}(\nu = 1)$ spectra evidences their excimer nature as $well^{3}$. The distances between neighbour N atoms in IHSP, 1 mm, are small enough for cooperative bulk magnetic effects appearances. ESR experiments with N($^{4}$S) atoms show the effects of either magnetic alignment or spin-exchange narrowing

Nonmonotonic distribution of population of the a(3)Sigma(+)(u) triplet state rotational levels in corona discharge in cryogenic helium gas

WOS:00040417340000

ODMR OF ATOMS TRAPPED IN IMPURITY-HELIUM SOLID.aSOLID.^{a}

$^{a}$Supported by RFBR Projects 98-03-33095, 98-03-32283, 99-03-33261Author Institution: Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region, Russia; A.F. Ioffe Physico-Technical Institute, St.Petersburg, 194021, Russia; Institute of Applied Physics, Bonn University, Bonn, GermanyMetastable $N(^{2} D)$ atoms are stabilized in an aerogel-like medium, soaked by superfluid helium (HeII), and called Impurity. Helium Solid (IHS), showing strong thermoluminescence in the rage of 1.4 to 4.0 K on the $^{2} D,^{-4}S$ transition (523 nm). Even slight increase in temperature (less than 100 mK) leads to significant rise in luminescence. We used IHS as a specific optical bolometer for monitoring of magnetic resonance (ODMR) of paramagnetic atoms, trapped in IHS and detected for the first time ODMR of ground state $N(^{4}S)$ atoms upon CW microwave incident on the sample and slow sweep of magnetic field. On passing through resonance the sample absorbed microwave radiation and, as a result of spin-lattice relaxation was heated large enough for excitation of luminescence and optical detection of magnetic resonance. Recently we have managed to excite blue luminescence of Kr- and Ar- IHS samples, containing diluted amounts of atomic nitrogen by applying a short heat pulses to the sample directly in Hell. The observed luminescence was found to decay at $\lambda 427$ nm with characteristic time $\tau$ less than 10 msec. We have been improving the sensitivity of this ODMR approach by employing a pulsed microwave radiation with subsequent synchronous detection of luminescence. The method proposed is expected to be universal for optical monitoring of magnetic resonance of any paramagnetic species, trapped in IHS due to non-specific nature of excitation of luminescence

Stabilization of H and D atoms in krypton–helium nanocondensates

Impurity–helium condensates formed by krypton nanoclusters containing atoms and molecules of hydrogen isotopes have been studied via an electron spin resonance (ESR) technique. Analysis of the ESR spectra has shown that most of the H and D atoms reside on the surfaces of Kr nanoclusters. Very large average concentrations have been obtained for H atoms (1.2·10¹⁹ cm⁻³) and D atoms (3.3·10¹⁹ cm⁻³) in these experiments. For the highest concentration of D atoms stabilized in the Kr–He sample, line narrowing has been observed. Exchange tunneling reactions have been studied in Kr–He sample containing H and D atoms

On the formation mechanism of impurity–helium solids: evidence for extensive clustering

Optical emission studies on a discharged nitrogen-helium gas jet injected into superfluid helium near 1.5 K are described. The analysis of atomic (a-group) and molecular Vegard-Kaplan transitions clearly indicates that the emitting species are embedded in the nitrogen clusters. The formation of the clusters is most efficient in the crater formed on the liquid surface. The model calculations based on the classical bubble model and density functional theory suggest that under the experimental conditions only clusters consisting of more than 1000 molecules have a kinetic energy sufficient for the stable cavity formation inside liquid helium. The results obtained suggest that the formation of impurity-helium solids is a consequence of extensive clustering in the gas jet