Helium Atoms Research Articles

Interstellar Conditions Deduced from Interstellar Neutral Helium Observed by IBEX and Global Heliosphere Modeling

In situ observations of interstellar neutral (ISN) helium atoms by the IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX) mission are used to determine the velocity and temperature of the pristine very local interstellar medium (VLISM). Most ISN helium atoms penetrating the heliosphere, known as the primary population, originate in the pristine VLISM. As the primary atoms travel through the outer heliosheath, they charge exchange with He+ ions in slowed and compressed plasma, creating the secondary population. With more than 2.4 million ISN helium atoms being sampled by IBEX during ISN seasons 2009–2020, we compare the observations with the predictions of a parameterized model of ISN helium transport in the heliosphere. We account for the filtration of ISN helium atoms at the heliospheric boundaries by charge-exchange and elastic collisions. We examine the sensitivity of the ISN helium fluxes to the interstellar conditions described by the pristine VLISM velocity, temperature, magnetic field, and composition. We show that comprehensive modeling of the filtration processes is critical for interpreting ISN helium observations, as the change in the derived VLISM conditions exceeds the statistical uncertainties when accounting for these effects. The pristine VLISM parameters found by this analysis are the flow speed (26.6 km s−1), inflow direction in ecliptic coordinates (255.°7, 5.°04), temperature (7350 K), and B − V plane inclination to the ecliptic plane (53.°7). The derived pristine VLISM He+ density is 9.7 × 10−3 cm−3. Additionally, we show a strong correlation between the interstellar plasma density and magnetic field strength deduced from these observations.

Exponentially Correlated Hylleraas–Configuration Interaction Studies of Atomic Systems. III. Upper and Lower Bounds to He-Sequence Oscillator Strengths for the Resonance 1S→1P Transition

The exponentially correlated Hylleraas–configuration interaction method (E-Hy-CI) is a generalization of the Hylleraas–configuration interaction method (Hy-CI) in which the single rij of an Hy-CI wave function is generalized to a form of the generic type rijνije−ωijrij. This work continues the exploration, begun in the first two papers in this series (on the helium atom and on ground and excited S states of Li II), of whether wave functions containing both linear and exponential rij factors converge more rapidly than either one alone. In the present study, we examined not only 1s2 1S states but 1s2p 1P states for the He I, Li II, Be III, C V and O VII members of the He isoelectronic sequence as well. All 1P energies except He I are better than previous results. The wave functions obtained were used to calculate oscillator strengths, including upper and lower bounds, for the He-sequence lowest (resonance) 1S→1P transition. Interpolation techniques were used to make a graphical study of the oscillator strength behavior along the isoelectronic sequence. Comparisons were made with previous experimental and theoretical results. The results of this study are oscillator strengths for the 1s2 1S→ 1s2p1P He isoelectronic sequence with rigorous non-relativistic quantum mechanical upper and lower bounds of (0.001–0.003)% and probable precision ≤ 0.0000003, and were obtained by extending the previously developed E-Hy-CI formalism to include the calculation of transition moments (oscillator strengths).

Helium role in Fe9Cr1.5W0.4Si F/M steel during Fe++He+ dual-beam irradiation

In materials exposed to neutron irradiation, helium atoms produced by transmutation reactions are prevalent and play a significant role in the evolution of dislocation loops, causing mechanical property degradation. However, systematic investigation on the helium role is lack and possible origin of helium effects is unclear. Here, by performing 400 keV Fe ion and 30 keV He in-situ irradiation, this study found that about 9.2×1012 cm−3 loops nucleated for every increase of 1 appm He at 0.9 dpa in Fe++He+ dual beam irradiation and 87% loop density was produced with the help of helium at 2262 appm/dpa. The different loop evolution in single He irradiation and Fe++He+ irradiation indicated the presence of high-density TEM-invisible (≤ 2 nm) small loops after Fe+ irradiation at high temperatures. The large number of loops produced by helium irradiation were possibly related with the small bubbles.

Gaussian functions with odd power of r produced by the free complement theory.

We investigate, in this paper, the Gaussian (G) function with odd powers of r, rxaybzcexp(-αr2), called the r-Gaussian or simply the rG function. The reason we investigate this function here is that it is generated as the elements of the complement functions (cf's) when we apply the free complement (FC) theory for solving the Schrödinger equation to the initial functions composed of the Gaussian functions. This means that without the rG functions, the Gaussian set of functions cannot produce the exact solutions of the Schrödinger equation, showing the absolute importance of the rG functions in quantum chemistry. Actually, the rG functions drastically improve the wave function near the cusp region. This was shown by the applications of the present theory to the hydrogen and helium atoms. When we use the FC-sij theory, in which the inter-electron function rij is replaced with its square sij=rij2 that is integrable, we need only one- and two-electron integrals for the G and rG functions. The one-center one- and two-electron integrals of the rG functions are always available in a closed form. To calculate the integrals of the multi-centered rG functions, we proposed the rG-NG expansion method, in which an rG function is expanded by a linear combination of the G functions. The optimal exponents and coefficients of this expansion were given for N = 2, 3, 4, 5, 6, and 9. To show the accuracy and the usefulness of the rG-NG method, we applied the FC-sij theory to the hydrogen molecule.

Effect of pulsed hydrogen and helium plasma irradiation on the formation of solid solutions in the thin-film CdTe/CdS heterosystem

The interphase interaction of cadmium sulfide and cadmium telluride thin film layers under irradiation with pulses of hydrogen and helium plasma was investigated by the X-ray diffraction method. CdS/CdTe heterosystem samples were obtained by the hot wall method on glass substrates with a transparent FTO electrode. During deposition of CdS layers 0.3–0.32 µm thick, the temperature of the evaporator zone was 590 °C, and the substrate temperature was 395 °C. CdTe layers 3.8–4.0 µm thick were deposited at an evaporation zone temperature of 520 °C and a substrate temperature of 497 °C. After deposition, the “chloride” treatment of the heterosystem was carried out. The “chloride” treatment consisted in the deposition of a cadmium chloride film 0.7 μm thick on the surface of a CdTe by thermal vacuum evaporation without heating the substrate and by annealing the entire structure in air at a temperature of 410-415 °C for 20 min. The composition of the solid solutions was determined from the lattice period according to Vegard's rule. In the initial state, along with the cubic phase of cadmium telluride, the presence of CdTe1-xSx solid solutions with a sulfur concentration of 3 % and 8.2 % is observed. After irradiation of the CdS/CdTe heterosystems with hydrogen plasma pulses, the entire base layer turned into a solid solution with a sulfur concentration of 3 %. At the same time, two more phases of CdTe1-xSx solid solutions with sulfur concentrations of 6 % and 11.5 % were observed. The relative concentration of the cadmium telluride phase in the initial state was 84 %, after irradiation with hydrogen plasma pulses it was 82 %. In the initial state, the phases of solid solutions with sulfur content of 3 % and 8.2 % had relative concentrations of 7 % and 9 %, respectively; after irradiation, their relative concentrations became 15 % and 3 %, respectively. After irradiation of the CdS/CdTe heterosystem with helium plasma pulses, the entire base layer turned into a solid solution with a sulfur concentration of 1.5 %. In this case, two more phases with a sulfur concentration of 3.7 % and 7.9 % were observed. The relative concentration of cadmium phase telluride after irradiation with hydrogen plasma pulses decreases to 79 %. After irradiation, the relative concentration of the phase of solid solutions with sulfur contents of 3 % and 8.2 % increases to 17 % and 12 %, respectively. The experimentally observed difference in the evolution of the phase composition of the CdS/CdTe film heterosystem under irradiation with hydrogen and helium plasma pulses may be due to the fact that irradiation with more massive helium atoms leads to a greater thermal effect, which causes melting.

The density and pressure of helium nano-bubbles encapsulated in silicon

The 1 s 2 → 1 s 2 p ( 1 P ) excitation in compressed helium atoms in either the bulk material or encapsulated in a bubble is shifted to energies higher than in the free atom. For bulk helium, energy shifts predicted from non-empirical electronic structure computations are in excellent agreement with experimentally determined values. However, there are significant discrepancies both between the results of experiments on different bubbles and between these and the well established descriptions of the bulk. A critique is presented of previous analyses of earlier scanning transmission electron microscope measurements that are untrustworthy because the density dependence of the electron impact excitation cross section was neglected. Experimental electron energy loss data from an earlier study of helium encapsulated in silicon are reanalysed and it is shown that the properties of the helium in these bubbles do not differ significantly from those in the bulk, thereby enabling the densities in the bubbles to be determined. These enable bubble pressures to be deduced from a well established experimentally derived equation of state. It is shown that the errors of up to 80% in the incorrectly determined densities are greatly magnified in the predicted pressures which can be too large by factors of over seven.

Lattice thermal conductivity of defected tungsten evaluated by equilibrium molecular dynamics simulation

Exposed to high fluxes of helium particles and heat, tungsten divertor plates will suffer various forms of damage thus degrading its performance such as its thermal conductivity. In this paper, the equilibrium molecular dynamics method is employed to evaluate the lattice thermal conductivity (LTC) changes of the tungsten lattice containing one type of defects (dislocation loops, helium bubbles and impurity helium atoms). It is found that the LTC of tungsten is slightly reduced in the presence of dilute dislocation loops and vacuum bubbles, while a remarked reduction of the thermal conductivity is engendered by dilute helium particles. A tiny number of helium atoms as randomly-distributed tetrahedral interstitials (accounting for 0.2 % of the total number of atoms in the simulation cell), result in a drastic drop of the LTC to 15 % of that of the perfect tungsten lattice. The phonon density of states of the perfect and defected tungsten lattices are compared to identify the causes of the LTC reduction at the atomic level.

MP-CITDSE: A set of ab-initio programs for the simulation of hydrogenic and helium-like atom-laser interactions

MP-CITDSE is a package of programs which solves the time-dependent Schrödinger equation for hydrogenic and helium-like atomic systems interacting with an ultra-short laser pulse (of attosecond or femtosecond duration). The output of the computations – with some minimal processing – may be used to calculate excited populations, single and double ionisation yields, kinetic, angular and radial electron distributions, and harmonic yields. For the Helium atom, a full account of the inter-electronic correlation effects is included via a configuration interaction approach; for the time propagation of the wavefunction a spectral basis expansion on the eigenstates of the field-free Hamiltonian is used; for this reason post-processing of the expansion coefficients just after the pulse lead to simple formulas for quantities of experimental interest. Program summaryProgram Title: MP-CITDSECPC Library link to program files:https://doi.org/10.17632/gv4zxmfdx3.1Developer's repository link:https://github.com/aforembs/MP-CITDSELicensing provisions: GPLv3Programming language: C++Nature of problem: The ultra-fast high-intensity laser pulses of modern experimental sources require an ab-initio theoretical treatment, as the characteristics of such pulses are generally incompatible with the established Lowest-order Perturbation Theory (LOPT) approximation. Such a treatment can be quite cumbersome to not only implement, but also describe in a clear, intuitive manner. Its inherent complexity ensures that even in its simplest form, an ab-initio TDSE solver can easily consist of several thousand lines of code. As with any project of such scope a significant amount of effort is required to keep the code up to date, running on several platforms, compilers and operating systems. As such, while the numerical problem tackled by MP-CITDSE is that of the simulation of hydrogenic and helium-like atom-laser interactions on a single node computer/workstation; its main challenges are longevity and expandability. By doing away with unnecessary abstractions within the code, using a modern standard of a widely used and performant language (C++ 17) and hosting the source code on a publicly available GitHub repository we hope that the programs provided herein prove themselves useful to many current and future AMO Physicists.Solution method: The atomic systems are considered confined in a spherical box of finite size. In the one-electron case (hydrogen) the eigenstates are calculated in a spherical coordinate system by the direct diagonalisation of the field-free Hamiltonian matrix representation on a B-splines piecewise polynomial basis expansion of the radial part of the eigenfunctions; for the two-electron case (helium) the numerically calculated one-electron eigenstates are angularly coupled to form a zero-order (uncorrelated) two-electron numerical basis; then the latter basis is further coupled via a configuration-interaction approach to eventually calculate the helium's two-electron eigenstates.Following these steps, for both systems (hydrogen and helium) the computation proceeds by expanding the time-dependent wavefunction on the corresponding field-free eigenstate basis; thus, we end up to a system of first-order ordinary differential equations (ODEs) in time for the expansion (time-dependent) coefficients; the dynamical parameters for the ODEs time propagation, namely the eigenenergies and dipole matrix elements are calculated only once prior to the ODEs propagation [2]. The structure of the propagating matrix is block tridiagonal.Additional comments including restrictions and unusual features: This paper serves as the definitive reference for the MP-CITDSE code. In this version we solve the ODEs using a Runge-Kutta-Felnberg (RKF) algorithm. Also we have restricted the code to treat hydrogen and helium but with very little effort (non-relativistic) hydrogenic and helium-like atomic systems can also be simulated. The programs are supplied with two methods of compilation, via make and cmake to ensure greater portability.

Equation of state for He bubbles in W and model of He bubble growth and bursting near W{100} surfaces derived from molecular dynamics simulations

Molecular dynamics (MD) simulations are performed to derive an equation of state (EOS) for helium (He) bubbles in tungsten (W) and to study the growth of He bubbles under a W(100) surface until they burst. We study the growth as a function of the initial nucleation depth of the bubbles. During growth, successive loop-punching events are observed, accompanied by shifts in the depth of the bubble towards the surface. Subsequently, the MD data are used to derive models that describe the conditions that cause the loop punching and bursting events. Simulations have been performed at 500, 933, 1500, 2000, and 2500 K to fit the parameters in the models. To compute the pressure in the bubble at the loop punching and bursting events from the models, we derive an EOS for He bubbles in tungsten with an accompanying volume model to compute the bubble volume for a given number of vacancies (N_text {V}), He atoms (N_text {He}), and temperature (T). To derive the bubble EOS, we firstly derive the EOS for a free He gas. The derived free-gas EOS can accurately predict all MD data included in the analysis (which span up to 54 GPa at 2500 K). Subsequently, the bubble EOS is derived based on the free-gas EOS by correcting the gas density to account for the interaction between He and W atoms. The EOS for the bubbles is fitted to data from MD simulations of He bubbles in bulk W that span a wide range of gas density and sizes up to about 3 nm in diameter. The pressure of subsurface bubbles at the loop punching events as calculated using the bubble-EOS and the volume model agrees well with the pressure obtained directly from the MD simulations. In the loop punching model, for bubbles consisting of N_text {V} vacancies and N_text {He} helium atoms, the N_text {He}/N_text {V} ratio that causes the event, the resulting increase in N_text {V}, and the associated shift of the bubble depth are formulated as a function of N_text {V} and T. In the bursting model, a bubble must simultaneously reach a certain depth and N_text {He}/N_text {V} ratio in order to burst. The burst depth and N_text {He}/N_text {V} are also modeled as a function of N_text {V} and T. The majority of the loop punching events occur at bubble pressures between 20 and 60 GPa, depending on the bubble size and temperature. The larger the bubble and the higher the temperature, the lower the bubble pressure. Furthermore, our results indicate that at a higher temperature, a bubble can burst from a deeper region.

Impact of Soret effect on hydrogen and helium retention in PFC tungsten under ELM-like conditions

In our previous work, we have demonstrated using nonequilibrium molecular-dynamics simulations that the fluxes of helium and self-interstitial atoms in the presence of a thermal gradient in tungsten are directed opposite to the heat flux, indicating that species transport is governed by a Soret effect, namely, thermal-gradient-driven diffusion, characterized by a negative heat of transport that drives species transport uphill, i.e. from the cooler to the hot regions of the tungsten sample. In this work, the findings of our thermal and species transport analysis have been implemented in our cluster-dynamics code, Xolotl, which has been used to compute temperature and species profiles over spatiotemporal scales representative of plasma-facing component (PFC) tungsten under typical reactor operating conditions, including extreme heat loads at the plasma-facing surface characteristic of plasma instabilities that induce edge localized modes (ELMs). We demonstrate that the steady-state species profiles, when properly accounting for the Soret effect, vary significantly from those where temperature-gradient-driven transport is not accounted for and discuss the implications of such a Soret effect on the response to plasma exposure of plasma-facing tungsten. Although our cluster-dynamics simulations do not yet include self-clustering of helium or hydrogen blister formation, our simulation results show that the Soret effect substantially reduces helium and hydrogenic species retention inside PFC tungsten.

Rotational (de-)excitation of protonated tricarbon monosulfide HC3S+ by collision with helium atoms

A new two-dimensional potential energy surface of the interaction between HC3S+ molecule and helium atom is mapped using an explicitly correlated coupled cluster approach with a single, double and perturbative triple excitation-F12a approach with an augmented-correlation-consistent polarized valence triple-zeta Gaussian basis set. The deepest potential well is identified at θ = 95∘ and R = 6.0 bohr with a depth of 100.67 cm−1. Cross-sections are determined for total energy up to 300 cm−1 using a coupled states method. Thermal rate coefficients are derived from the determined cross-sections for temperatures ranging between 3 K and 50 K. A clear predominance of = 2 processes is observed.

Collisional quenching of the pionic helium 4He long-lived states

Collisions of metastable pionic helium atoms with helium atoms of the medium lead to the destruction of these states, as well as to the shifts and broadening of E1 spectral lines of transitions between the pionic helium states. In the paper, in order to obtain the interaction potential matrix (π −He+)−He, calculations of the potential energy surface (PES) in the unrestricted Hartree-Fock method are performed taking into account electron correlations within the second-order perturbation theory (MP2). With this potential, the system of equations of strong channel coupling is solved numerically. Various techniques are used in the calculations that eliminate degeneracy of the solution matrix in the vicinity of small distances between colliding subsystems, which arises due to strong coupling of channels in this region, owing to which the numerical solutions “forget” boundary conditions. Cross sections and rates of collisional transitions are calculated (Nσv). It is found that the collisional transition rate (n, l) = (17, 16) → (17, 15) (n, l — the principal quantum number and the angular momentum respectively) for the density of the medium N = 0.2 × 1023 cm−3 is lower than 103s−1, which indicates that it is possible to ignore the effect of collisional destruction of pionic helium long-lived states in precision laser spectroscopic experiments.

A DFT + U study on diffusion and aggregation behavior of He atoms in Li2TiO3

Future fusion power plants are designed based on the deuterium-tritium fusion reactions. Yet, as an element with a half-life of 12.5 years, tritium cannot be obtained in large quantities from nature. To meet future demand, an artificial nuclear reaction is needed to use tritium breeder materials to produce tritium. At present, Li2TiO3 is the most promising tritium breeder blanket material in fusion reactor. The existing experimental results show that helium irradiation will change the crystal structure of Li2TiO3 and cause embrittlement failure. In order to study the microscopic helium embrittlement mechanism of Li2TiO3, the diffusion and aggregation behavior of helium atoms in Li2TiO3 bulk was calculated by first principles. The reasons for the formation of the most stable interstitial configuration of helium atoms and helium clusters in Li2TiO3 bulk are explained by lattice vibrational theory and electronic structure analysis. At the same time, the relatively new research results in this work are mainly as follows: In Li2TiO3 bulk, the three spatial inversion symmetries Li vacancy helium atoms tend to diffuse from all Li vacancies to Li2 vacancies. Meanwhile, the Li2 vacancy is the most likely site for helium clusters to nucleate. And helium atoms and the helium clusters tend to arrange and grow along the plane of the Li layer.

Radiation Pressure Acting on the Neutral He Atoms in the Heliosphere

Interstellar neutral helium (ISN He) is an important source of information on the physical state of the local interstellar medium. Radiation pressure acting on the neutral helium atoms in the heliosphere has always been neglected; its effect has been considered insignificant compared to gravitational force. The most advanced numerical models of ISN He take into account more and more subtle effects; therefore, it is important to check if the effect of radiation pressure is still negligible. In this paper, we use the most up-to-date version of the Warsaw Test Particle Model (WTPM) to calculate the expected helium distribution in the heliosphere and simulate the flux of the ISN He observed by the Interstellar Boundary Explorer (IBEX) and in the future by the Interstellar Mapping and Acceleration Probe (IMAP). We compare results calculated with and without radiation pressure during low and high solar activity. The results show that in the analysis of IBEX-Lo observations, the radiation pressure acting on typical helium causes flux differences at a level of 1%–4% and is comparable to the observational errors. For the more sensitive IMAP-Lo instrument, there are some regions in the considered observation configurations where radiation pressure causes potentially statistically significant changes in the calculated fluxes. The effect can be up to 9% for the indirect beam and is likely to be higher than the estimated errors. Therefore, we claim that in the future analysis of the IMAP-Lo observations, radiation pressure acting on ISN He should be considered.

Generation of collimated extreme ultraviolet radiation by single-photon process

Generating laser with short wavelength is a bottleneck problem in laser technology. The current applicable table-top extreme ultraviolet (EUV) lasers are all generated by multi-photon process with low efficiency. By utilizing metastable helium atoms excited by microwave and irradiated by a resonant laser, here we report the development of 58.4 nm collimated radiation by single-photon-excitation related anti-Stokes Raman scattering (ASRS). The conversion efficiency is much higher than that of high harmonic generation (HHG). The same divergence of 1.4 mrad as that of excitation laser indicates its stimulating character. Our results show an applicable path towards up-conversion by single-photon process to generate table-top EUV lasers.

Theoretical study of helium atom single ionization by bare carbon ions impact

Theoretical study of helium atom single ionization by bare carbon ions impact

Microstructural evolution in ODS-EUROFER steel caused by high-dose He ion implantations with systematic variation of implantation parameters

The paper presents a detailed analysis of helium (He) bubble development in ODS-EUROFER steel caused by helium ion implantation in different regimes, with a particular attention to the role of the oxide nanoparticles in promoting the growth of He bubbles, helium accumulation and gas-driven swelling. The Transmission Electron Microscopy (TEM) characterization of steel samples implanted applying systematic variation of experimental parameters has allowed clarifying the trends of the bubble microstructure evolution depending on the implantation dose, flux, and sample temperature. It was found that in all investigated implantation regimes He bubbles formed both in the grain bulk and on various structural defects (dislocations, grain boundaries, oxide particles and carbide precipitates), but the sizes and densities of bubbles in different bubble populations were sensitive to particular irradiation conditions. In the majority of cases the main traps for implanted helium and the main contributors to the estimated swelling were bubbles associated with grain boundaries, though in some cases (high implantation dose or lower temperature) the bubbles in the grain bulk were competitive with the grain boundary bubble population. Oxide particles in ODS-EUROFER were found to be excellent nucleation sites for He bubbles and practically each observed particle hosted a single relatively large bubble, sometimes as large as the particle itself. However, the contribution of oxide-associated bubbles to the estimated swelling and He inventory was found to be minor as compared to other bubble populations because of a relatively low number density of nano-oxides. Comparison of ODS-EUROFER and EUROFER 97 samples implanted with He ions in identical regimes has demonstrated lower efficiency of ODS-EUROFER for accumulating implanted helium in bubbles and noticeably higher share of helium atoms trapped in the vacancy defects invisible by TEM.

Elastic and electron capture processes in slow He+–He collision

Aims. The elastic and electron-capture processes of He+ ions with ground helium atoms are very important for studies in astrophysics. It is essential to have reliable state-selective charge transfer, elastic, and transport cross-sections, along with the corresponding reaction rate coefficient data, especially for low collision energies. Methods. We investigated the elastic and non-radiative electron-capture processes in He+(1s)–He(1s2) collisions are investigated employing the full quantum-mechanical molecular orbital close-coupling method. The adopted ab initio adiabatic potentials and coupling matrix elements were obtained by a multi-reference single- and double-excitation configuration interaction approach. Results. We computed the elastic, charge-transfer, and transport cross-sections in the energy range of 0.01–2500eVamu−1 and the reaction rate coefficient in the temperature range of 10–109 K. Good agreement was achieved when compared to the available experimental and theoretical results. Shape resonance, Regge, and Glory oscillations were also found in the elastic and charge transfer cross-sections in the energy region considered here.

Gravitationally induced entanglement in a harmonic trap

Recent work has shown that it may be possible to detect gravitationally induced entanglement in tabletop experiments in the not-too-distant future. However, there are at present no thoroughly developed models for this type of experiment where the entangled particles are treated more fundamentally as excitations of a relativistic quantum field, and with the measurements modeled using expectation values of field observables. Here we propose a thought experiment where two particles are initially prepared in a superposition of coherent states within a common three-dimensional (3D) harmonic trap. The particles then develop entanglement through their mutual gravitational interaction, which can be probed through particle position detection probabilities. The present work gives a nonrelativistic quantum mechanical analysis of the gravitationally induced entanglement of this system, which we term the ``gravitational harmonium'' due to its similarity to the harmonium model of approximate electron interactions in a helium atom; the entanglement is operationally determined through the matter wave interference visibility. The present work serves as the basis for a subsequent investigation, which models this system using quantum field theory, providing further insights into the quantum nature of gravitationally induced entanglement through relativistic corrections, together with an operational procedure to quantify the entanglement.

Observation of Helium in Mercury's Exosphere by PHEBUS on Bepi‐Colombo

AbstractOn 1 October 2021, Bepi‐Colombo performed its first flyby of Mercury. During the maneuver, the short wavelength channel (55–155 nm) of “Probing the Hermean Exosphere by UV Spectroscopy” (PHEBUS) was activated for a total duration of 1 hr. The helium resonance line at 58.4 nm was clearly observed during the whole sequence. At large distance from the planet, the emission was due to helium atoms in the interplanetary medium (interplanetary UV glow). Just after crossing the terminator of the planet and entering the dawn side of the exosphere, PHEBUS observed a clear additional emission due to scattering of solar photons by helium atoms in the exosphere of Mercury. The first detection of the 58.4 nm line in the exosphere of Mercury was reported by Broadfoot et al. (1976, https://doi.org/10.1029/gl003i010p00577) following the Mariner 10 flybys in 1974. The PHEBUS observation of exospheric helium emissions is the first for this element since the UVS measurements. In this paper, we present the results of our analysis of the PHEBUS data at 58.4 nm. Calibration of both instruments are compared with observations of the interplanetary glow, showing that the measurements of both instruments are accurate. However, we find that the exospheric density of helium atoms deduced from the PHEBUS data is 4.5–7.5 times lower than the previous estimate from UVS on Mariner 10. Possible explanations are considered. We show that some of the helium atoms present in the exosphere of Mercury could originate from the local interstellar cloud.