States Of Helium Research Articles

Fano resonances observed in helium nanodroplets

Doubly-excited Rydberg states of helium (He) nanodroplets have been studied using synchrotron radiation. We observed Fano resonances related to the atomic N = 2,0 series as a function of droplet size. Although similar qualitatively to their atomic counterparts, the resonance lines are broader and exhibit a shift in energy which increases for the higher excited states. Furthermore, additional resonances are observed which are not seen in atomic systems. We discuss these features in terms of delocalized atomic states perturbed by the surrounding He atoms and compare to singly excited droplets.

Equation of state and transport properties of warm dense helium via quantum molecular dynamics simulations

The equation of state, self-diffusion, and viscosity coefficients of helium have been investigated by quantum molecular dynamics (QMD) simulations in the warm dense matter regime. Our simulations are validated through the comparison with the reliable experimental data. The calculated principal and reshock Hugoniots of liquid helium are in good agreement with the gas-gun data. On this basis, we revisit the issue for helium, i.e., the possibility of the instabilities predicted by chemical models at around 2000 GPa and 10 g/cm3 along the pressure isotherms of 6309, 15 849, and 31 623 K. Our calculations show no indications of instability in this pressure-temperature region, which reconfirm the predictions of previous QMD simulations. The self-diffusion and viscosity coefficients of warm dense helium have been systematically investigated by the QMD simulations. We carefully test the finite-size effects and convergences of statistics, and obtain numerically converged self-diffusion and viscosity coefficients by using the Kubo-Green formulas. The present results have been used to evaluate the existing one component plasma models. Finally, the validation of the Stokes-Einstein relationship for helium in the warm dense regime is discussed.

Rydberg states of helium in electric and magnetic fields of arbitrary relative orientation

A spectroscopic study of Rydberg states of helium (n = 30 and 45) in magnetic, electric and combined magnetic and electric fields with arbitrary relative orientations of the field vectors is presented. The emphasis is on two special cases where (i) the diamagnetic term is negligible and both paramagnetic Zeeman and Stark effects are linear (n = 30, B ≤ 120 mT and F = 0–78 V cm−1), and (ii) the diamagnetic term is dominant and the Stark effect is linear (n = 45, B = 277 mT and F = 0–8 V cm−1). Both cases correspond to regimes where the interactions induced by the electric and magnetic fields are much weaker than the Coulomb interaction, but much stronger than the spin–orbit interaction. The experimental spectra are compared to spectra calculated by determining the eigenvalues of the Hamiltonian matrix describing helium Rydberg states in the external fields. The spectra and the calculated energy-level diagrams in external fields reveal avoided crossings between levels of different ml values and pronounced ml-mixing effects at all angles between the electric- and magnetic-field vectors other than 0. These observations are discussed in the context of the development of a method to generate dense samples of cold atoms and molecules in a magnetic trap following Rydberg–Stark deceleration.

AN X-RAY SPECTRAL MODEL OF REPROCESSING BY SMOOTH AND CLUMPY MOLECULAR TORI IN ACTIVE GALACTIC NUCLEI WITH THE MONACO FRAMEWORK

ABSTRACT We construct an X-ray spectral model of reprocessing by a torus in an active galactic nucleus (AGN) with the Monte Carlo simulation framework MONACO. Two torus geometries of smooth and clumpy cases are considered and compared. In order to reproduce a Compton shoulder accurately, MONACO includes not only free electron scattering but also bound electron scattering. Raman and Rayleigh scattering are also treated, and scattering cross sections dependent on chemical states of hydrogen and helium are included. Doppler broadening by turbulence velocity can be implemented. Our model gives results consistent with other available models, such as MYTorus, except for differences due to different physical parameters and assumptions. We studied the dependence on torus parameters for a Compton shoulder, and found that a intensity ratio of a Compton shoulder to the line core mainly depends on column density, inclination angle, and metal abundance. For instance, an increase of metal abundance makes a Compton shoulder relatively weak. Also, the shape of a Compton shoulder depends on the column density. Furthermore, these dependences become different between smooth and clumpy cases. Then, we discuss the possibility of ASTRO-H/SXS spectroscopy of Compton shoulders in AGN reflection spectra.

Neutron radiography of a static density gradient of 3He gas at cryogenic temperatures

We demonstrate a stationary helium gas density gradient which is needed for a proposed novel low-energy μ+ beam line. In a closed system with constant pressure the corresponding density gradient is only a function of the temperature. In a neutron radiography experiment two gas cells with different geometries were filled with 3He gas at constant pressures of about 10mbar. Temperatures in the range from 6K to 40K were applied and density distributions with a maximum to minimum density ratio of larger than 3 were realized. The distribution was investigated employing the strongly neutron absorbing isotope 3He. A simple one-dimensional approach derived from Fourier׳s law describes the obtained gas density with a deviation <2%.

Electron emission in collisions of fast highly charged bare ions with helium atoms

We have studied the electron emission from ground state helium atom in collision with fast bare heavy ions at intermediate and high incident energies. In the present study, we have applied the present three-body formalism of the three Coulomb wave (3C-3B) model and the previously adopted four-body formalism of the three Coulomb wave (3C-4B). To represent the active electron in the helium atom in the 3C-3B model, the initial bound state wavefunction is chosen to be hydrogenic with an effective nuclear charge. The wavefunction for the ejected electron in the exit channel has been approximated to be a Coulomb continuum wavefunction with same effective nuclear charge. Effectively the continuum-continuum correlation effect has been considered in the present investigation. Here we have calculated the energy and angular distribution of double differential cross sections (DDCS) at low and high energy electron emission from helium atom. The large forward-backward asymmetry is observed in the angular distribution which is explained in terms of the two-center effect (TCE). Our theoretical results are compared with available experimental results as well as other theoretical calculations based on the plain wave Born approximation (PWBA), continuum-distorted wave (CDW) approximation, continuum-distorted wave eikonal-initial state (CDW-EIS) approximation, and the corresponding values obtained from the 3C-4B model [S. Jana, R. Samanta, M. Purkait, Phys. Scr. 88, 055301 (2013)] respectively. It is observed that the four-body version of the present investigation produces results which are in better agreement with experimental observations for all cases.

Analytical investigation of process pipe failure in cryo-line; impact of different crack sizes

Safety is important in cryogenic field dealing with liquid or supercritical state helium. Cryolines carrying helium at 4 K temperature level and equipped with thermal compensation devices, like bellows, are prone todevelopment of cracksand its propagation after a number of expansion and contraction. Even after careful design, appropriate material selection and a strict quality control during manufacturing, probability of crack development in the Cryo-line cannot be avoided. An investigative study of failure of process pipe due to the development of crack has been carried out for a long Cryo-line laid in a confined space considering the space between two vacuum barriers is 10 meter. Study shows that minimum temperature of the Outer Vacuum Jacket (OVJ)goes down to 251 K in 540th second andthe minimum temperature of the confined space reaches to 73 K in 37th second. As the crack size reduces, the OVJ pipe temperature approaches to the minimum value in longer time and confined space final temperature increases.

First-principles modeling of three-body interactions in highly compressed solid helium

We present a set of three-body interaction models based on the Slater-Kirkwood (SK) potential that are suitable for the study of the energy, structural, and elastic properties of solid $^{4}\mathrm{He}$ at high pressure. Our effective three-body potentials are obtained from the fit to total energies and atomic forces computed with the van der Waals density functional theory method due to Grimme, and represent an improvement with respect to previously reported three-body interaction models. In particular, we show that some of the introduced SK three-body potentials reproduce closely the experimental equation of state and bulk modulus of solid helium up to a pressure of $\ensuremath{\sim}60$ GPa, when used in combination with standard pairwise interaction models in diffusion Monte Carlo simulations. Importantly, we find that recent predictions reporting a surprisingly small variation of the kinetic energy and Lindeman ratio on quantum crystals under increasing pressure are likely to be artifacts deriving from the use of incomplete interaction models. Also, we show that the experimental variation of the shear modulus, ${C}_{44}$, at pressures $0\ensuremath{\le}P\ensuremath{\le}25$ GPa can be quantitatively described by our set of SK three-body potentials. At higher compression, however, the agreement between our ${C}_{44}$ calculations and experiments deteriorates and thus we argue that higher order many-body terms in the expansion of the atomic interactions probably are necessary in order to better describe elasticity in very dense solid $^{4}\mathrm{He}$.

Hyperspherical three-body model calculation for the bound 3,1S-states of Coulombic systems

In this paper, hyperspherical three-body model formalism has been applied for the calculation of energies of the low-lying bound [Formula: see text]-states of neutral helium and helium like Coulombic three-body systems having nuclear charge (z) in the range [Formula: see text]. Energies of [Formula: see text]-states are also calculated for those having nuclear charge in the range [Formula: see text]. The calculation of the coupling potential matrix elements of the two-body potentials has been simplified by the use of Raynal–Revai Coefficients (RRC). The three-body wave function in the Schrödinger equation when expanded in terms of hyperspherical harmonics (HH), leads to an infinite set of coupled differential equation (CDE) which for practical purposes is truncated to a finite set and the truncated set of CDE’s are solved by renormalized Numerov method (RNM) to get the energy (E). The calculated energy is compared with the ones of the literature.

Calculating helium atomic excited states in coordinate space

Two coupled Schrödinger equations are used to calculate excited states of atomic helium. Using product state functions for the two-electron state, the shooting method is used to numerically determine the energies of the allowed singlet and triplet levels. The calculations agree well with the data, and the coordinate-space basis yields Schrödinger equations for helium that are familiar to students who have used similar methods for the hydrogen atom.

Leading term of the He-p¯He+long-range interaction

The long range interaction between an antiprotonic helium atom $\bar{p}$He$^+$ and helium atom in its ground state is studied. We calculate the dispersion coefficients $C_6$ using the Complex Coordinate Rotation (CCR) formalism in order to comply with the resonant nature of metastable states of the antiprotonic helium. We present as well numerical data on static dipole polarizabilities of antiprotonic helium states. The obtained coefficients $C_6$ may be used to estimate the collisional shift and broadening of transition lines in a low density precision spectroscopy of the antiprotonic helium.

Experimental study on the interference of autoionizing states of He

The interference of the low-lying doubly excited autoionizing states of helium (2s2(1S),2s2p(3P),2p2(1D) and 2s2p(1P)) is studied. In this paper we concentrate on the interference of the direct and indirect ionization (He+1s-1) in the neighborhood of the critical energy (93.15eV) of the possible state-to-state interference of the 2s2(1S) and 2p2(1D) resonances. The spectra of autoionizing and scattered electrons measured in three angular ranges at around 87–97eV primary energy are shown with the computer fitted parameters of the peaks.

Implementation of the thermodynamic and phase transition equations of superfluid helium in CFD software

The cryogenic design of the next generation of superconducting accelerator magnets depends on our ability to simulate the helium heat and mass transfer in the internal structure of these magnets. For that matter accurate tools must be developed such as numerical codes integrating the thermodynamic behavior and phase transition in superfluid helium. We have implemented in 2D and 3D, the He II conservation equations in Fluent© CFD software corresponding to a simplified two-fluid model. It consists of a conventional continuity equation, a modified momentum equation for the total fluid and an energy equation including the Gorter-Mellink internal convection term modeling the turbulence regime. The code is mainly suited to simulate transient and steady-state flow configurations. In addition, a new method has been developed to simulate the He II / He I transition in 2D based on a modified Volume Of Fluid method (VOF). The interface between the two states of liquid helium has been locally recreated in the corresponding cells to properly mimic the second order phase transition (no latent heat). Both steady and unsteady numerical simulation have been performed and compared with different experimental results.

Polarization control of absorption of virtual dressed states in helium

The extreme ultraviolet absorption spectrum of an atom is strongly modified in the presence of a synchronized intense infrared field. In this work we demonstrate control of the absorption properties of helium atoms dressed by an infrared pulse by changing the relative polarization of the infrared and extreme ultraviolet fields. Light-induced features associated with the dressed $1s2s, 1s3s$, and $1s3d$ states, referred to as $2{s}^{+}, 3{s}^{\ifmmode\pm\else\textpm\fi{}}$, and $3{d}^{\ifmmode\pm\else\textpm\fi{}}$ light-induced states, are shown to be strongly modified or even eliminated when the relative polarization is rotated. The experimental results agree well with calculations based on the solution of the time-dependent Schr\"odinger equation using a restricted excitation model that allows efficient treatment of the three-dimensional problem. We also present an analysis of the light-induced states based on Floquet theory, which allows for a simple explanation of their properties. Our results open a new route to creating controllable superpositions of dipole allowed and nondipole allowed states in atoms and molecules.

An Orbital‐Free Quantum Hypernetted Chain Model Based Perturbation Theory for Equation of State of Hydrogen and Helium in Warm Dense Regime

AbstractWe present in this work, a thermodynamic perturbation theory for equation of state of hydrogen and helium in the warm dense regime. The system is modeled as a mixture of classical point ions and quantum electrons. A perturbation series for Helmholtz free energy and correlation functions of the ions and electrons as a function of density and temperature is proposed. Combining the classical thermodynamic perturbation theory and the orbitial‐free quantum hyper‐netted chain theory, a systematic procedure to obtain the terms of the perturbation series is developed. The ion‐ion correlations are treated within the hyper‐netted chain approximation and the ion‐electron correlations are treated within the Thomas‐Fermi‐Dirac‐Weizsäcker approximation. The method has been applied to obtain isotherms of hydrogen and helium in the warm dense regime. The isotherms are compared with available ab‐initio data and the results are analyzed. A good agreement with ab‐initio data has been observed for pressures greater than one Mbar. Advantages and limitations of the present method are discussed along with possible future improvements. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Thermal desorption behavior of helium in aged titanium tritide films

The desorption behavior of helium in TiT(1.5∼1.8)−x3Hex film samples (x = 0.0022–0.22) was investigated by thermal desorption technique in vacuum condition in this paper. The thermal helium desorption spectrometry (THDS) of aging titanium tritide films prepared by electron beam evaporation revealed that, depending on the decayed 3He concentration in the samples, there are more than four states of helium existing in the films. The divided four zones in THDS based on helium states represent respectively: (1) the mobile single helium atoms with low activation energy in all aging samples resulted from the interstitial sites or dissociated from interstitial clusters, loops and dislocations, (2) helium bubbles inside the grain lattices, (3) helium bubbles in the grain boundaries and interconnected networks of dislocations in the helium concentration of 3Hegen/Ti > 0.0094, and (4) helium bubbles near or linked to the film surface by interconnected channel for later aging stage with 3Hegen/Ti > 0.18. The proportion of helium desorption in each zone was estimated, and dissociated energies of helium for different trapping states were given.

Initiation of arcing on tungsten surface exposed to steady state He plasmas

Arcing was initiated in steady state helium plasmas by negatively biasing a tungsten electrode to around −500 V. On the tungsten electrode, nanostructures were grown by the plasma irradiation. In this study, we characterized the property of the initiated arcing by measuring the temporal evolutions of the electrode potential and the arc current. The ignition frequency and duration of arcing were presented from the potential measurements; the arc duration was in the range of <10 ms and the distribution altered when changing the biasing voltage. The behavior of arc spots was observed with a fast framing camera. It was shown that the spots split frequently, and sometimes, they run on the surface independently. From the fluctuation of the arc current, the fractal feature of arcing was revealed.

Dressing Effects in the Attosecond Transient Absorption Spectra of Doubly-Excited States in Helium

SynopsisStrong-field manipulation of autoionizing states is a crucial aspect of electronic quantum control. Recent measurements of the attosecond transient absorption spectrum (ATAS) of helium dressed by a few-cycle visible pulse [C. Ott et al, Nature 516, 374 (2014)] provide evidence of the inversion of Fano profiles. With the support of accurate ab-initio calculations that reproduce the results of the latter experiment, here we investigate the new physics that arise from ATAS when the laser intensity is increased. In particular, we show that (i) previously unnoticed signatures of the dark 2p2 1S doubly excited state are observed in the experimental spectrum, (ii) inversion of Fano profiles is predicted to be periodic in the laser intensity, and (iii) the ac-Stark shift of the higher terms in the sp+2, n autoionizing series exceeds the ponderomotive energy, which is the result of a genuine two-electron contribution to the polarization of the excited atom.

Excited state distribution and spin-effects in strong-field excitation of neutral Helium

SynopsisWe investigated the principal quantum number n distribution of excited states resulting from the interaction of Helium with strong, short laser pulses. We find excellent agreement with predictions of the semiclassical frustrated tunneling ionization (FTI) model [1] as well as fully quantum mechanical calculations. Furthermore, the excitation process directly populates triplet excited states due to the breakdown of the Russel-Saunders coupling scheme for high orbital angular momentum l states of Helium, which are predominantly populated in the strong laser field.

(e,2e) coincidence study on the coherence of the 2s2(1S) and 2p2(1D) autoionizing states of He

SynopsisThe electron impact excitation of the autoionizing states of helium, their subsequent decay into the same He+1s-1 final ionic state, and the interference of these processes have been studied. A constant ionic state (e,2e) experiment (CIS) is performed, isolating the final state by keeping the sum of the energies of the scattered- ejected electron pairs constant. The focus of this work is on the exchange interference of the 2s2(1S) and 2p2(1D) autoionizing states of helium. At around the critical energy fast variations were observed in the CIS spectra, which indicates the presence of exchange interference effects.