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A non-equilibrium plasma is created in a micro-capillary quartz tube (800 µ m of internal diameter), by a DC-pulsed micro-dielectric barrier discharge (micro-DBD) and the propagation of an ionisation wave, in mixtures of He/H2O/VOC at atmospheric pressure where the studied volatile organic compounds (VOCs) are representative of molecules belonging to different chemical families: alcohols (methanol, ethanol, isopropanol, tert-butanol), ketones (acetone), nitriles (acetonitrile), and aromatic hydrocarbons (toluene). The conversion efficiency of these VOCs is studied as a function of the applied voltage on the micro-DBD (or electrical energy deposited in the plasma) and of the initial concentration of the molecules in the range from 1 ppm up to 3000 ppm (depending on the molecule), with the help of high-resolution real-time mass spectrometry Fourier transform ion cyclotron resonance associated to chemical ionisation (CI-FTICR) using H3O+ as precursor ion. A variety of by-products resulting from the conversion of VOCs are identified and quantified, emphasising that the micro-capillary plasma is able to induce a complex chemical reactivity. A qualitative analysis of the involved kinetics, based on the existing literature, reveals that helium species (ions and metastable states) and radicals coming from the dissociation of the water molecules (O and OH) are the most probable candidates to explain the formation of all compounds detected by the CI-FTICR apparatus. Quenching processes of the metastable He(23S) by the VOCs, leading to the dissociation of the molecules, are suggested to explain some of the experimental results. [ABSTRACT FROM AUTHOR]

We developed a radio frequency discharge apparatus for He gas to investigate the spin states of metastable helium (He*) interacting with solid-state surfaces. Our apparatus consisted of a stainless steel vacuum chamber, in which a coil produced He* by discharging introduced He gas. The spin states of the He* were detected using optical pumping and probing techniques. The chamber was designed to accommodate various solid-state samples. We measured the He* polarization produced at a dielectric prism surface by total internal reflection of the circularly polarized pumping light. Our apparatus can be used to investigate possible spin transfer from various solid surfaces to He* atoms. [ABSTRACT FROM AUTHOR]

The search for low-energy antideuterons in cosmic rays allows the addressing of fundamental physics problems testing for the presence of primordial antimatter and the nature of Dark Matter. The PHeSCAMI (Pressurized Helium Scintillating Calorimeter for AntiMatter Identification) project aims to exploit the long-living metastable states of the helium target for the identification of low-energy antideuterons in cosmic rays. A space-based pressurized helium calorimeter would provide a characteristic identification signature based on the coincident detection of a prompt scintillation signal emitted by the antideuteron energy loss during the slowing-down phase in the gas, and the (≈µs) delayed scintillation signal provided by the charged pions produced in the subsequent annihilation. The performance of a high-pressure (200-bar) helium scintillator prototype, tested in the INFN-TIFPA laboratory, will be summarized. [ABSTRACT FROM AUTHOR]

• Unveiled Fe segregation at HEA phase boundaries under high helium doses. • Demonstrated novel insights into radiation-enhanced diffusion mechanisms. • Employed advanced STEM and EDS to elucidate microstructural changes. • Revealed the segregation dynamics in FeCrMn with metastable FCC structure. This study investigates the radiation-induced Fe segregation in the dual-phase FeCrNiMnAl high-entropy alloy (HEA) under the helium (He) ion irradiation with a dose of 5 × 1017 ions/cm2. The superior bubble swelling resistance of the chemical-ordered NiAlMn phase with B2 structure is observed compared to the FCC FeCrMn matrix, which can be attributed to their differences in atomic packing factors. Fe segregation at phase boundaries (PBs) is detected after irradiation, as evidenced by a higher concentration of Fe at PBs than in the matrix by about 10 at.%. This phenomenon can be ascribed to the migration of Fe interstitials induced by irradiation cascades and the punching mechanism during bubble growth. In essence, the Fe segregation in this study is driven by the radiation-enhanced diffusion and metastable state of FCC-structured FeCrMn with supersaturated Fe atoms. These insights provide theoretical foundations and innovative perspectives for the radiation-induced segregation (RIS) in HEAs. [ABSTRACT FROM AUTHOR]

Microwave resonator-driven microplasmas are a promising technology for generating the high density of rare-gas metastable states required for optically pumped rare gas laser systems. We measure the density of argon 1s5 states (Paschen notation) in argon-helium plasmas between 100 Torr and atmospheric pressure using diode laser absorption. The metastable state density is observed to rise with helium mole fraction at lower pressures but to instead fall slightly when tested near atmospheric pressure. A 0-D model of the discharge suggests that these distinct behaviors result from the discharge being diffusion-controlled at lower pressures, but with losses occurring primarily through dissociative recombination at high pressures. In all cases, the argon metastable density falls sharply when the neutral argon gas fraction is reduced below approximately 2%. [ABSTRACT FROM AUTHOR]

We report on an extensive study of interatomic Coulombic decay (ICD) widths in helium trimer computed using a fully ab initio method based on the Fano theory of resonances. Algebraic diagrammatic construction for one-particle Green's function is utilized for the solution of the many-electron problem. An advanced and universal approach to partitioning of the configuration space into discrete states and continuum subspaces is described and employed. Total decay widths are presented for all ICD-active states of the trimer characterized by one-site ionization and additional excitation of an electron into the second shell. Selected partial decay widths are analyzed in detail, showing how three-body effects can qualitatively change the character of certain relaxation transitions. Previously unreported type of three-electron decay processes is identified in one class of the metastable states. [ABSTRACT FROM AUTHOR]

• This paper focus more on the effect of different helium storage rate on the collapsing configuration. In the simulation, the sufficient quantity of helium can get the SWCNTs rid of collapsing. • Two key storage rates are introduced to classify the final nanostructures of (5 n , 5 n) SWCNTs. The energy analysis shows that the dumbbell-like type and the circular cross-section are the steady state, while the peanut-like type is the metastable state. • This method provides a basis for the manufacture of CNTs with different physical properties in future nanodevices. The analysis method that introduce helium atoms into the cavity of SWCNTs could also be extended to the applications in other kinds of molecules. In this paper, we propose a method to control the collapsed nanostructurings of single-walled carbon nanotubes (SWCNTs) by helium storage and the key mechanism is elaborated from the potential energy aspect. The molecular dynamics simulation shows that enough helium atoms could restore the collapsing configuration of (40, 40) carbon nanotubes back to the circular cross section. The length of the flat double carbon atoms layer in the dumbbell type structure and the diameter of nearly circular edges can be precisely altered by adjusting the helium atom numbers. Two key storage rates are introduced to classify the final nanostructures of (5 n , 5 n) SWCNTs. The relationship between helium numbers and the final configuration is investigated; the analysis method that introduces helium atoms into the cavity of SWCNTs can also be extended to other kinds of molecules. [ABSTRACT FROM AUTHOR]

Solid layers of helium on graphite surface are known to go into a highly mobile state (HMS) once the solid layer is enforced to slip on the substrate. The HMS collapses to the stable inert state with the lifetime extending over $$10^4$$ s. In this paper, we suggest that the HMS is a structural glass by showing the similarities between the present system and an organic conductor named BEDT-TTF. The latter was recently discovered to be an electronic glass if the cooling rate is rapid enough across the freezing temperature (Wigner crystal formation temperature). We then address a novel annealing process promoted by a local condensate which should be seen for the present He-4 case as the condensation fraction grows in the liquid overlayer. [ABSTRACT FROM AUTHOR]

We observed the rotational spectral lines of the He* excimer within the range of 910-930 nm in the corona discharge in normal liquid He at the temperature of 4.2 K and the pressure of 1 atm. The spectral range is filled with the rotational lines of the C Σ ⇒ A Σ singlet and the c Σ ⇒ a Σ triplet transitions. These transitions end at the rotational levels of the lowest metastable terms, A Σ and a Σ of the He* excimer. Then, the population of the rotational levels with the K' number of the upper C Σ and c Σ terms (the quantity of the molecules with the rotational moment of K' in the excited molecule ensemble in the discharge) is proportional to the intensity of the rotational lines marked K' of the C Σ ⇒ A Σ singlet and the c Σ ⇒ a Σ triplet. The populations might be calculated according to the experimental intensities of the rotational spectral lines. The emitting corona plasma in the liquid He is nonequilibrium and the rotational level populations do not correspond to the Boltzmann distribution. The efficient rotational temperature exceeds the liquid He temperature, 4.2 K. [ABSTRACT FROM AUTHOR]

[Display omitted] • Helium atmospheric pressure plasma jet spectra were enhanced by the LIP. • A grounded metallic target extended lifetime of plasma radiation in APPJ. • Population density of He 2 3 S state increased to ∼50 times by LIP. • Relative spatial distribution of the 2 3 S state was deduced along helium flow. • 2 3 S state played a key role in the enhancement of He spectra. A helium (He) atmospheric pressure plasma jet (APPJ) system was constructed, and two guided streamers in He APPJ were produced. The propagation of the guided streamer was observed using ICCD camera when a grounded metallic target was employed. The target extended the lifetime of the plasma radiation of the guided streamer. A ns pulsed laser was irradiated at He APPJ with the target and optical emission spectra were measured. The interaction between the APPJ and pulsed laser enhanced various spectra of He APPJ. The enhancement of the spectra, the line integrated population density of He 2 3 S metastable state, electron temperature, and electron density were measured as the time interval between the pulsed laser and the guided streamer changed. The relative spatial distribution of He 2 3 S metastable state in the guided streamer in the absence of the pulsed laser was deduced along He flow. The role of He metastable state was discussed when the pulsed laser interacted with He APPJ. [ABSTRACT FROM AUTHOR]

We apply the variational method in Hylleraas coordinates to solve the energy eigenvalue problem for antiprotonic helium molecular systems including p ¯ 3 He + and p ¯ 4 He + . The numerical accuracy on the nonrelativistic energies is shown to reach 10 −17 , thus the precision of our results is only limited by the width of the metastable states. Expectation values of the Dirac delta operators for these states are also calculated. [ABSTRACT FROM AUTHOR]

Recently, high-resolution spectroscopy of slow beams of metastable helium molecules He 2 ∗ generated by multistage Zeeman deceleration was used in combination with Rydberg-series extrapolation techniques to obtain the lowest rotational interval in the molecular helium ion at a precision of 18 MHz (Jansen et al., 2015), limited by the temporal width of the Fourier-transform-limited laser pulses used to record the spectra. We present here an extension of these measurements in which we have (1) measured higher rotational intervals of He 2 + , (2) replaced the pulsed UV laser by a cw UV laser and improved the resolution of the spectra by a factor of more than five, and (3) studied M J redistribution processes in regions of low magnetic fields of the Zeeman decelerator and shown how these processes can be exploited to assign transitions originating from specific spin–rotational levels ( N ″ , J ″ ) of He 2 ∗ . [ABSTRACT FROM AUTHOR]

On the light of a previous work and its recommendations by Matobaet al.[J. Phys. B41, 145205 (2008)], this paper proposes to look at the mobility of ground and metastable-excited states of C+ions moving into a helium buffer gas. The calculations, based on the three-temperature theory for solving the Boltzmann kinetic equation, are accomplished with quantum-mechanical transport cross sections at the low temperatures 4.3 and 77 K. The obtained mobility results are contrasted with the available theoretical data and experimental measurements, which show acceptable agreements. [ABSTRACT FROM PUBLISHER]

Recently, a metastable state of solid $$^4$$ He has been produced by locally lowering the density of the solid below the melting density using a focused acoustic wave pulse. An unexpected instability of the solid has been found about 4 bar below the melting line. This paper reports on experiments precisely localizing in time the instability birth within the acoustic pulse. It is found that, as expected, the instability always appears during a depression swing of the wave. However a metastability pressure limit does not emerge clearly. Total stress duration seems also to play a role in the instability triggering, suggesting a fatigue effect. [ABSTRACT FROM AUTHOR]

The formation of dianions in helium nanodroplets is reported for the first time. The fullerene cluster dianions (C60) n2− and (C70) n2− were observed by mass spectrometry for n≥5 when helium droplets containing the appropriate fullerene were subjected to electron impact at approximately 22 eV. A new mechanism for dianion formation is described, which involves a two-electron transfer from the metastable He− ion. As well as the prospect of studying other dianions at low temperature using helium nanodroplets, this work opens up the possibility of a wider investigation of the chemistry of He−, a new electron-donating reagent. [ABSTRACT FROM AUTHOR]

Depopulation of metastable He(2 1 S) by radiative association with hydrogen and lithium ions resulting in the formation of the HeH + and HeLi + molecular ions is investigated. Energy dependent cross-sections for spontaneous and stimulated processes on the spin-singlet manifold are calculated using a fully quantal approach and considering the association to rotational–vibrational states of the lowest singlet electronic states X Σ + 1 , A Σ + 1 , and B Σ + 1 from the continuum states of the B Σ + 1 electronic state. Evaluation of the cross-sections is based on highly accurate quantum calculations taking into account all possible state-to-state transitions at thermal energies (for spontaneous association) or at higher background temperatures (stimulated association). The corresponding rate coefficients are then presented as functions of temperature. As expected calculations show that the rate coefficients for radiative association to the X and A states are several orders of magnitude larger than the one for the B state formation. On the other hand, stimulation of the radiative association by black-body background radiation has a noticeable effect only on the B → B process. [ABSTRACT FROM AUTHOR]

The absolute number densities of helium atoms in the 2s 3 S 1 metastable state were determined in four plasma-based ambient desorption/ionization sources by atomic absorption spectroscopy. The plasmas included a high-frequency dielectric barrier discharge (HF-DBD), a low temperature plasma (LTP), and two atmospheric-pressure glow discharges, one with AC excitation and the other with DC excitation. Peak densities in the luminous plumes downstream from the discharge capillaries of the HF-DBD and the LTP were 1.39 × 10 12 cm − 3 and 0.011 × 10 12 cm − 3 , respectively. Neither glow discharge produced a visible afterglow, and no metastable atoms were detected downstream from the capillary exits. However, densities of 0.58 × 10 12 cm − 3 and 0.97 × 10 12 cm − 3 were measured in the interelectrode regions of the AC and DC glow discharges, respectively. Time-resolved measurements of metastable atom densities revealed significant random variations in the timing of pulsed absorption signals with respect to the voltage waveforms applied to the discharges. [ABSTRACT FROM AUTHOR]

We examine the metastable and unstable regimes of condensation of superfluid helium and parahydrogen on spheres and cylinders at finite temperatures, employing finite range density functionals. The goal is to compare calculations of sizes and spreads of films at the onset of metastability and of instability with the predictions of a simple phenomenological model that contemplates the curvature of the substrate. We have focused on two cases, helium on nanospheres and nanocylinders of different materials, and hydrogen on fullerenes. We are able to locate the onset of metastability and of spinodal instability in the adsorption isotherms of every sample and to extract the width of the condensed fluid. It is shown that the predictions of the so-called simple model agree surprisingly well with the more elaborate calculations. [ABSTRACT FROM AUTHOR]