3He Atoms Research Articles

Nuclear Magnetic Resonance Spectroscopy

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Probing the dynamics of 3He atoms adsorbed on MCM-41 with pulsed NMR

We report measurements of the nuclear spin-spin and spin-lattice relaxation times for 3He adsorbed on MCM-41 for temperatures 0.08 < T < 1.2 K. Deviations from Curie behavior are observed at low temperatures. The relaxation times exhibit a two-component behavior representing the differing dynamics of the mobile quasi-free molecules in the center of the tubes compared to the adsorbed layer on the walls. The amplitudes of the two components provide an accurate measure of the number of fluid-like molecules traveling in the center of the nanotubes.

Impurity self-energy in the strongly-correlated Bose systems

We proposed the nonperturbative scheme for the calculation of the impurity spectrum in the Bose system at zero temperature. The method is based on the path-integral formulation and describes an impurity as a zero-density ideal Fermi gas interacting with Bose system for which the action is written in terms of density fluctuations. On the example of the 3He atom immersed in the liquid helium-4 a good consistency with experimental data and results of Monte Carlo simulations is shown.

Nonperturbative theory of atom-surface interaction: corrections at short separations

The nonperturbative expressions for the free energy and force of interaction between a ground-state atom and a real-material surface at any temperature are presented. The transition to the Matsubara representation is performed, whereupon the comparison is made with the commonly used perturbative results based on the standard Lifshitz theory. It is shown that the Lifshitz formulas for the free energy and force of an atom-surface interaction follow from the nonperturbative ones in the lowest order of the small parameter. Numerical computations of the free energy and force for the atoms of He and Na interacting with a surface of an Au plate have been performed using the frequency-dependent dielectric permittivity of Au and highly accurate dynamic atomic polarizabilities in the framework of both the nonperturbative and perturbative theories. According to our results, the maximum deviations between the two theories are reached at the shortest atom-surface separations of about 1 nm. Simple analytic expressions for the atom-surface free energy are derived in the classical limit and for an ideal-metal plane. In the lowest order of the small parameter, they are found in agreement with the perturbative ones following from the standard Lifshitz theory. Possible applications of the obtained results in the theory of van der Waals adsorption are discussed.

4He adsorption on a single C40 molecule: Path integral Monte Carlo study

Path-integral Monte Carlo calculations are performed to study the adsorption of 4He atoms on two different isomers of C40: one with the D2 symmetry and the other with the D5d symmetry. Here, we employ the 4He-C40 interaction described by the sum of 4He-C interatomic pair potentials, which allows us to examine the 4He corrugations on the fullerene molecular surfaces. We first observe the layer-by-layer growth of 4He on both C40 isomers, where the first layers are located at a distance of ~ 5.7 A from the molecular centers. From the investigation of the angular 4He density profiles, we find that the 4He layers display different quantum states depending on the number of 4He adatoms N. Between N = 22 and N = 40, the first 4He layer on the D2 isomer shows several different commensurate structures whereas the corresponding 4He layer on D5d undergoes a commensurate-to-incommensurate solid transition like the first 4He layer on graphite. As N increases, the superfluid responses of the first 4He layers on both isomers exhibit reentrant behavior with superfluidity being completely quenched at structurally-ordered states. In addition, the superfluid fraction of the first 4He layer is found to be about three times as large on D2 as on D5d for the same number of 4He atoms. The different structural and superfluid properties of 4He adatoms depending on the isomeric type could provide an experimental way of selecting a specific C40 isomer.

NMR Chemical Shift of a Helium Atom as a Probe for Electronic Structure of FH, F-, (FHF)-, and FH2.

In this work, we present the first results of outer electronic shell visualization by using a 3He atom as a probe particle. As model objects we have chosen F-, FH, and FH2+ species, as well as the hydrogen-bonded complex FH···F- at various H···F- distances (3.0, 2.5, 2.0, and 1.5 Å and equilibrium at ca. 1.14 Å). The interaction energy of investigated objects with helium atom (CCSD/aug-cc-pVTZ) and helium atom chemical shift (B3LYP/pcS-2) surfaces were calculated, and their topological analysis was performed. For comparison, the results of standard quantum mechanical approaches to electronic shell visualization were presented (ESP, ELF, ED, ∇2ED). We show that the Laplacian of helium chemical shift, ∇2δHe, is sensitive to fluorine atom lone pair localization regions, and it can be used for the visualization of the outer electronic shell, which could be used to evaluate the proton accepting ability. The sensitivity of ∇2δHe to lone pairs is preserved at distances as large as 2.0-2.5 Å from the fluorine nucleus (in comparison with the distance to ESP minima, located at 1.0-1.5 Å or maxima of ELF, which are as close as 0.6 Å to the fluorine nucleus).

Scattering length calculations via Faddeev approach

The results of calculation of the scattering length for the collision of a 4He atom with a 4He7Li dimer are presented. These investigations are based on the hard-core version of the Faddeev differential equations.

Kinetic energy and momentum distribution of isotopic liquid helium mixtures.

The momentum distribution and atomic kinetic energy of the two isotopes of helium in a liquid mixture at temperature T = 2 K are computed by quantum Monte Carlo simulations. Quantum statistics is fully included for 4He, whereas 3He atoms are treated as distinguishable. Comparison of theoretical estimates with a collection of the most recent experimental measurements shows reasonable agreement for the energetics of 4He and pure 3He. On the other hand, a significant discrepancy (already observed in previous studies) is reported between computed and measured values of the 3He kinetic energy in the mixture, especially in the limit of low 3He concentration. We assess quantitatively the importance of Fermi statistics and find it to be negligible for a 3He concentration ≲20%. Our results for the momentum distribution lend support to what was already hypothesized by other authors, namely, that the discrepancy is likely due to underestimation of the 3He kinetic energy contribution associated with the tail of the experimentally measured momentum distribution.

Helium Droplet-Mediated Deposition and Aggregation of Nanoscale Silver Clusters on Carbon Surfaces

We present experiments and calculations of the deposition and aggregation of silver clusters embedded in helium droplets onto an amorphous carbon surface at room temperature. Calculations were also performed for deposition onto a graphene surface. They involve potentials for the interaction of carbon atoms with silver and helium atoms, provided by ab initio calculations. The numerical simulations were performed for few-nanometer-sized silver clusters including up to 5000 Ag atoms and He droplets with up to 105 4He atoms. The fluid nature of the 4He droplet is accounted for by the renormalization of the He–He interaction potential. The numerical results are consistent with deposition experiments with an average number of 3000 Ag atoms per 4He droplet and indicate that the aggregation of the silver clusters on the carbon surface is mediated by secondary droplet impacts. They also reveal nontrivial dynamics of the Ag clusters within the carrier droplet, showing a tendency to drift toward the droplet surface....

21aCE-1 ^ Xe/^3He同時メーザー発振のためのポンピングレーザーシステムの開発

21aCE-1 ^ Xe/^3He同時メーザー発振のためのポンピングレーザーシステムの開発

Observation of dynamic atom-atom correlation in liquid helium in real space

Liquid 4He becomes superfluid and flows without resistance below temperature 2.17 K. Superfluidity has been a subject of intense studies and notable advances were made in elucidating the phenomenon by experiment and theory. Nevertheless, details of the microscopic state, including dynamic atom–atom correlations in the superfluid state, are not fully understood. Here using a technique of neutron dynamic pair-density function (DPDF) analysis we show that 4He atoms in the Bose–Einstein condensate have environment significantly different from uncondensed atoms, with the interatomic distance larger than the average by about 10%, whereas the average structure changes little through the superfluid transition. DPDF peak not seen in the snap-shot pair-density function is found at 2.3 Å, and is interpreted in terms of atomic tunnelling. The real space picture of dynamic atom–atom correlations presented here reveal characteristics of atomic dynamics not recognized so far, compelling yet another look at the phenomenon.

An ultracold, optically trapped mixture of 87Rb and metastable 4He atoms

We report on the realization of an ultracold (<25 μK) mixture of rubidium (87Rb) and metastable triplet helium (4He) in an optical dipole trap. Our scheme involves laser cooling in a dual-species magneto-optical trap, simultaneous MW- and RF-induced forced evaporative cooling in a quadrupole magnetic trap, and transfer to a single-beam optical dipole trap. We observe long trapping lifetimes for the doubly spin-stretched spin-state mixture and measure much shorter lifetimes for other spin-state combinations. We discuss prospects for realizing quantum degenerate mixtures of alkali-metal and metastable helium atoms.Graphical abstract

Quantum Nuclear Motion of Helium and Molecular Nitrogen Clusters in Carbon Nanotubes

We study the quantum nuclear motion of N 4He atoms or N N2 molecules (N < 4) confined in carbon nanotubes using an ad hoc-developed nuclear wave function-based approach. Density functional theory (DFT)-based symmetry-adapted perturbation theory is used to derive parameters for a new pairwise potential model describing the gas adsorption to carbon materials. The predicted nuclear motion of He atoms inside a confining potential is directly compared to probability densities obtained by orbital-free He-DFT theory. The interaction of small clusters of adsorbates is also studied via a combination of both the discrete atomic and the continuous density approaches. Our results agree well with previously reported experimental and theoretical studies and provide new physical insights into the very different quantum confinement effects depending on the diameter of the carbon nanotubes and the impact of quantum phenomena on the gas storage capabilities at low temperatures.

Novel concept for neutron detection: proportional counter filled with 10B nanoparticle aerosol

The high neutron detection efficiency, good gamma-ray discrimination and non-toxicity of 3He made of proportional counters filled with this gas the obvious choice for neutron detection, particularly in radiation portal monitors (RPM), used to control the illicit transport of nuclear material, of which neutron detectors are key components. 3He is very rare and during the last decade this gas has become increasingly difficult to acquire. With the exception of BF3, which is toxic, no other gas can be used for neutron detection in proportional counters. We present an alternative where the 3He atoms are replaced by nanoparticles made of another neutron sensitive material, 10B. The particles are dispersed in a gaseous volume, forming an aerosol with neutron sensitive properties. A proportional counter filled with such aerosol was exposed to a thermal neutron beam and the recorded response indicates that the neutrons have interacted with the particles in the aerosol. This original technique, which transforms a standard proportional gas mixture into a neutron sensitive aerosol, is a breakthrough in the field of radiation detection and has the potential to become an alternative to the use of 3He in proportional counters.

Comparative investigation of pure and mixed rare gas atoms on coronene molecules.

Clusters formed by the combination of rare gas (RG) atoms of He, Ne, Ar, and Kr on coronene have been investigated by means of a basin-hopping algorithm and path integral Monte Carlo calculations at T = 2 K. Energies and geometries have been obtained and the role played by the specific RG-RG and RG-coronene interactions on the final results is analysed in detail. Signatures of diffuse behavior of the He atoms on the surface of the coronene are in contrast with the localization of the heavier species, Ar and Kr. The observed coexistence of various geometries for Ne suggests the motion of the RG atoms on the multi-well potential energy surface landscape offered by the coronene. Therefore, the investigation of different clusters enables a comparative analysis of localized versus non-localized features. Mixed Ar-He-coronene clusters have also been considered and the competition of the RG atoms to occupy the docking sites on the molecule is discussed. All the obtained information is crucial to assess the behavior of coronene, a prototypical polycyclic aromatic hydrocarbon clustering with RG atoms at a temperature close to that of interstellar medium, which arises from the critical balance of the interactions involved.

Kinetic processes in solid helium involving impurities and vacancies (Review)

A brief review is given of the kinetic behavior of impurities and vacancies in solid helium, which Andreev and Lifshitz predicted should be delocalized and converted into unique quasiparticles. Primary attention is devoted to the unusual diffusion processes in solid 3He-4He solutions as they undergo phase separation. Because mechanical stresses develop in the crystal during separation, the diffusive flow is substantially reduced and the effective diffusion coefficient becomes smaller than the coherent quantum diffusion coefficient. During the inverse transition from a separated mixture into the homogeneous state, anomalously rapid mass transfer is observed which can be explained qualitatively in terms of a model in which 3He inclusions are dissolved in three stages. Experimental data on the kinetics of phase separation are compared with a diffusive description of the process that takes into account the difference between diffusion processes outside and inside a nucleus of the new phase. Good agreement is obtained between a theoretical calculation and the experimental data. A homogeneous nucleation model is used to estimate the concentration of nuclei. For the first time, the coefficient of mass diffusion is estimated over the entire range of the concentration of the solutions. The behavior of delocalized vacancies in 4He and 3He solid solutions is studied near the separation temperature. The observed features of the pressure in this kind of system during repeated temperature cycling are explained by the formation of pure 4He vacancy clusters. Although the crystal itself has no strict periodicity owing to the random separation of 3He and 4He atoms at the lattice sites, a periodic structure is realized within a cluster and vacancies become delocalized.

Tritium decay helium-3 effects in tungsten

Tritium (T) implanted by plasmas diffuses into bulk material, especially rapidly at elevated temperatures, and becomes trapped in neutron radiation-induced defects in materials that act as trapping sites for the tritium. The trapped tritium atoms will decay to produce helium-3 (3He) atoms at a half-life of 12.3 years. 3He has a large cross section for absorbing thermal neutrons, which after absorbing a neutron produces hydrogen (H) and tritium ions with a combined kinetic energy of 0.76 MeV through the 3He(n,H)T nuclear reaction. The purpose of this paper is to quantify the 3He produced in tungsten by tritium decay compared to the neutron-induced helium-4 (4He) produced in tungsten. This is important given the fact that helium in materials not only creates microstructural damage in the bulk of the material but alters surface morphology of the material effecting plasma-surface interaction process (e.g. material evolution, erosion and tritium behavior) of plasma-facing component materials. Effects of tritium decay 3He in tungsten are investigated here with a simple model that predicts quantity of 3He produced in a fusion DEMO FW based on a neutron energy spectrum found in literature. This study reveals that: (1) helium-3 concentration was equilibrated to ∼6% of initial/trapped tritium concentration, (2) tritium concentration remained approximately constant (94% of initial tritium concentration), and (3) displacement damage from 3He(n,H)T nuclear reaction became >1 dpa/year in DEMO FW.

Theoretical study of the buffer-gas cooling and trapping of CrH(X6Σ+) by 3He atoms.

We present a theoretical study of the Zeeman relaxation of the magnetically trappable lowest field seeking state of CrH(X6Σ+) in collisions with 3He. A two dimensional potential energy surface (PES) was calculated with the partially spin-restricted coupled cluster singles, doubles, and non-iterative triples [RCCSD(T)] method. The global minimum was found for the collinear He⋯Cr-H geometry with the well depth of 1143.84 cm-1 at Re = 4.15 a0. Since the RCCSD(T) calculations revealed a multireference character in the region of the global minimum, we performed additional calculations with the internally contracted multireference configuration interaction with the Davidson correction (ic-MRCISD+Q) method. The resulting PES is similar to the RCCSD(T) PES except for the region of the global minimum, where the well depth is 3032 cm-1 at Re = 3.8 a0. An insight into the character of the complex was gained by means of symmetry-adapted perturbation theory based on unrestricted Kohn-Sham description of the monomers. Close coupling calculations of the Zeeman relaxation show that although the ΔMJ=MJ'-MJ = -1 and -2 transitions are the dominant contributions to the collisional Zeeman relaxation, ΔMJ<-2 transitions cannot be neglected due to the large value of CrH spin-spin constant. The calculated elastic to inelastic cross section ratio is 1600 for the RCCSD(T) PES and 500 for the MRCISD+Q PES, while the estimate from the buffer-gas cooling and magnetic trapping experiment is 9000.

Effect of nuclear structure on the electronic spectrum of the helium atom

An exact diagonalization method is used to calculate corrections to the ground state of the 4He atom related to the finite size and finite mass of its nucleus, which is interpreted as a bound state of two deuterons in the shell model. The experimentally determined quadrupole moment of the deuteron (Q0 = 2.74 × 10−27 cm2) makes it possible to estimate the size of an individual deuteron and, thereby, the size of the 4He nucleus (d ∼ 1.5 × 10−13 cm). As a result, the 4He nucleus is made up of four nucleons in the ground state (1s)4 in a spherically symmetric shell of diameter d, along which the complete charge (+2e) of the nucleus is uniformly distributed. This distribution, in turn, can be interpreted as the ground state of a rigid rotator with charges of magnitude +e attached to its ends. In this model the nucleus is a charged spherical shell of radius d/2, within which the potential is constant and finite, equal to 4e/d, and outside of which the potential falls off in accordance with the conventional Coul...

Direct measurement of the γHe / γXe ratio at ultralow magnetic field

A co-located 3He and 129Xe nuclear spin free precession measurement at sub-μT magnetic field was carried out in a magnetically shielded environment. The uncorrected quotient of the gyromagnetic ratios between neutral 3He and 129Xe atoms is determined to be 2.754 082 81(07), accounting for only statistical error. Our measurement shows that this ratio has a stability of 1.4×10-5/ √τ, demonstrating the ability to reach the current precision limit of the quotient in a 10000 s of averaging time τ. This precision is enough for the next-generation EDM search in neutral 129Xe atoms based on a similar comagnetometer scheme.