Single Helium Atom Research Articles

Behavior of Foreign Interstitial Hydrogen and Helium Atoms at Tungsten/Beryllium Interface from First-Principles Calculations

The behavior of foreign interstitial hydrogen and helium atoms and its effect on the physical properties of the tungsten/beryllium interface structure were computationally studied by first-principles calculations. Briefly, as part of our study of helium irradiation damage and hydrogen detention, the following properties were calculated: (1) the electronic properties of the tungsten/beryllium interface structure with a single interstitial hydrogen or helium atom and Hen vacancy or Hn vacancy complexes, and (2) the isotropy (polycrystalline) elastic modulus (bulk, torsion, Young’s modulus), anisotropy factor and minimum thermal conductivity of the previously described tungsten/beryllium interface systems. This study found that defect atoms are more likely to be concentrated in beryllium, but the tungsten layer is more sensitive to changes in mechanical properties caused by interstitial atoms. The ability of the beryllium vacancies to capture interstitial atoms is smaller than that of the tungsten vacancies. Based on the computational results, a preliminary assumption of the judgment of the tungsten/beryllium interface structure on the resistivity for plasma-facing materials is introduced. These computational studies provide a critical evaluation of the radiation resistivity and hydrogen retention of tungsten/beryllium interface materials. The calculated interface properties can be incorporated into radiation damage resistance property evaluation systems to develop and test tungsten-based composite materials.

Molecular dynamics exploration of helium bubble nucleation and growth mechanisms in Fe70Ni11Cr19 austenitic stainless steel.

The growth of helium bubbles impacts structural integrity of materials in nuclear applications. Understanding helium bubble nucleation and growth mechanisms is critical for improved material applications and aging predictions. Systematic molecular dynamics simulations have been performed to study helium bubble nucleation and growth mechanisms in Fe70Ni11Cr19 stainless steels. First, helium cluster diffusivities are calculated at a variety of helium cluster sizes and temperatures for systems with and without dislocations. Second, the process of diffusion of helium atoms to join existing helium bubbles is not deterministic and is hence studied using ensemble simulations for systems with and without vacancies, interstitials, and dislocations. We find that bubble nucleation depends on diffusion of not only single helium atoms, but also small helium clusters. Defects such as vacancies and dislocations can significantly impact the diffusion kinetics due to the trapping effects. Vacancies always increase the time for helium atoms to join existing bubbles due to the short-range trapping effect. This promotes bubble nucleation as opposed to bubble growth. Interestingly, dislocations can create a long-range trapping effect that reduces the time for helium atoms to join existing bubbles. This can promote bubble growth within a certain region near dislocations.

Comparative study on the radiation resistance of pure and He-doped Gd2Zr2O7 pyrochlore by DFT + U calculation

The helium accumulation in the pyrochlore ascribed to alpha decay has important effects on its performance during service. This work adopts first-principle method to investigate the structural evolution, discrepancy in mechanical properties and radiation resistance before and after He doping in Gd2Zr2O7 pyrochlore. By calculating each configuration of pure and He-doped Gd2Zr2O7, it is found that the unit cell expands and distorts due to the doping helium. Single helium atom prefers to occupy the octahedral interstitial site and Gd vacancy. The variation of elastic constants and elastic modulus implies that helium doping has no obvious damage to mechanical stability. He-Gd2Zr2O7 systems have lower antisite defects formation energy and higher vacancy defects formation energy, indicating better radiation resistance. Moreover, the electronic structure analysis shows that <Zr-O> bonds have a greater influence on radiation resistance than <Gd-O> bonds. This study demonstrates that Gd2Zr2O7 pyrochlore would exhibit excellent radiation resistance in long-term service.

Bayesian optimal control of the ultrashort circularly polarized attosecond pulse generation by two-color polarization gating.

We present ab initio simulations of optimal control of high-order-harmonic generation spectra that enable the synthesis of a circularly polarized 53-attosecond pulse in a single Helium atom response. The Bayesian optimization is used to achieve control of a two-color polarization gating laser waveform such that a series of harmonics in the plateau region are phase-matched, which can be used for attosecond pulse synthesis. To find the underlying mechanisms for generating these harmonics, we perform a wavelet analysis for the induced dipole moment in acceleration form, and compare the time-energy representation with the quantum paths extracted from the semiclassical calculation. We found that these coherent harmonics are excited along the short trajectories. The proposed method has the potential to migrate to laboratories for generation of isolated circularly polarized ultrashort attosecond pulses.

Formation and behaviors of helium bubbles in Li4SiO4: A molecular dynamics simulation

Molecular dynamics simulations were conducted to investigate the formation and behaviors of helium bubbles in Li4SiO4. We found that a single helium atom easily moved to a lithium vacancy with minimal energy, and helium dimers were also observed in Li4SiO4. The temperature directly influenced the formation and diffusion of helium, and helium atoms aggregated and formed helium bubbles at a temperature of 700 K. The release of helium bubbles at several temperatures was simulated to analyze the effects of helium bubble release on the Li4SiO4 surface. The temperature was also important factor influencing the recovery of defects. These results will be useful for establishing a physical model of irradiation-induced microstructure evolution inside tritium breeding material and provide a good reference for understanding the initial stage of helium bubble formation and growth in Li4SiO4.

Migration behaviors of helium atoms near tungsten surfaces: A molecular dynamics study

Under the fusion environment, the behavior of helium atoms near tungsten (W) surfaces plays a crucial role in surface morphological evolution and near-surface structural evolution of tungsten. In this paper, the migration behaviors of single helium atoms near W (2 1 1) and W (3 1 1) surfaces were investigated using molecular dynamics simulations. The results show that these single atoms can be well-described by the theory of continuous diffusion of particles in a semi-infinite medium, and different types of trap mutations occur for both surfaces. Although the temperature has an impotent impact on the occurrence probabilities of different types of trap mutations and the probabilities of helium atoms escaping from trap sites, there is no a strict relationship between the depth and temperature for trap mutations occurring. For the W (2 1 1) surface, an approximately one-dimensional diffusion may take place along the 〈1 1 1〉 directions when the helium atoms are trapped near 2 layers below the W surface. For the W (3 1 1) surface, the behaviors of trap mutations seem more significant than (2 1 1) and (1 1 1) surfaces at T = 1000 K. To investigate the diffusion and trap mutation processes, the nudged elastic band (NEB) method was also applied. These results can be helpful for understanding of helium release, helium retention, subsurface helium clustering, as well as surface morphological evolution.

Behaviors of helium in Cr2AlC from first principles

AbstractThe properties of helium incorporation and transport in Cr2AlC have been investigated using first principles method. The results of calculation show that a single helium atom is preferred to reside at an interstitial position near the Al plane in perfect Cr2AlC crystal, attributing to the low predicted formation energy. Helium atoms are expected to aggregate in Al layers. The interaction between helium and lattice atoms is primarily elastic due to the closed‐shell electronic structure of helium. The Doping of helium leads to a weakening of the Cr–Al bonds. Furthermore, an interstitial helium atom is likely to migrate along an indirect migration pathway from the hexahedral interstitial position on Al plane (IAlC), passing by the nearest tetrahedral interstitial position near Al plane (IAlCr), and finally reach to another hexahedral interstitial position (IAlC) with an activation energy of 1.21 eV. The high activation energy suggests a relatively low migration for helium atoms in Cr2AlC as well as a slow growth rate of helium bubbles for the early stage of helium irradiation.

A molecular dynamics study of helium bombardments on tungsten nanoparticles

Abstract Molecular dynamics simulations were conducted to study the bombardment process of a single helium atom on a tungsten nanoparticle. Helium atoms ranging from 50 eV to 50 keV were injected into tungsten nanoparticles with a diameter in the range of 2–12 nm. The retention and reflection of projectiles and sputtering of nanoparticles were calculated at various times. The results were found to be relative to the nanoparticle size and projectile energy. The projectile energy of 100 eV contributes to the largest retention of helium atoms in tungsten nanoparticles. The most obvious difference in reflection exists in the range of 3–10 keV. Around 66% of sputtering atoms is in forward direction for projectiles with incident energy higher than 10 keV. Moreover, the axial direction of the nanoparticles was demonstrated to influence the bombardment to some degree.

First-Principles Study on Various Point Defects Formed by Hydrogen and Helium Atoms in Tungsten

The different point defects formed by two hydrogen atoms or two helium atoms in tungsten were investigated through first-principles calculation. The energetically favorable site for a hydrogen atom is tetrahedral interstitial site while substitutional site is the most preferred site for a helium atom. The formation energies of two hydrogen or helium atoms are determined by their positions, and they are not simply 2 times the formation energy of a single hydrogen or helium atom’s defect. After relaxation, two adjacent hydrogen atoms are away from each other while helium atoms are close to each other. The reasons for the interaction between two hydrogen or helium atoms are also discussed.

Simulation of He(+) induced afterpulses in PMTs.

We report on a Monte Carlo simulation study of afterpulses due to trace gases in EMI 9235QA photomultipliers that are widely used in many luminescence detection systems operating in single photon counting mode. The numerical simulation takes into account the nonuniform electric field distribution and processes including elastic scattering: e + He → e + He, excitation: e + He → e + He(∗), ionization: e + He → 2e + He(+), elastic scattering: He(+) + He → He(+) + He, charge transfer: He(+) + He → Hef + He(+) (Hef indicates a fast neutral) and elastic scattering: Hef + He → Hef + He(f). The simulated and the measured time of flight distributions agree well. In addition, the above simulated processes demonstrate mechanisms of the observed series of pulses brought about by a single helium atom ionization.

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.

A Single Atom Antenna

SynopsisHere we demonstrate the smallest possible implementation of an antenna-receiver complex which consists of a single (helium) atom acting as the antenna and a second (neon) atom acting as a receiver.

Effects of temperature and surface orientation on migration behaviours of helium atoms near tungsten surfaces

Molecular dynamics simulations were performed to study the dependence of migration behaviours of single helium atoms near tungsten surfaces on the surface orientation and temperature. For W{100} and W{110} surfaces, He atoms can quickly escape out near the surface without accumulation even at a temperature of 400 K. The behaviours of helium atoms can be well-described by the theory of continuous diffusion of particles in a semi-infinite medium. For a W{111} surface, the situation is complex. Different types of trap mutations occur within the neighbouring region of the W{111} surface. The trap mutations hinder the escape of He atoms, resulting in their accumulation. The probability of a He atom escaping into vacuum from a trap mutation depends on the type of the trap mutation, and the occurrence probabilities of the different types of trap mutations are dependent on the temperature. This finding suggests that the escape rate of He atoms on the W{111} surface does not show a monotonic dependence on temperature. For instance, the escape rate at T = 1500 K is lower than the rate at T = 1100 K. Our results are useful for understanding the structural evolution and He release on tungsten surfaces and for designing models in other simulation methods beyond molecular dynamics.

3He NMR studies on helium-pyrrole, helium-indole, and helium-carbazole systems: a new tool for following chemistry of heterocyclic compounds.

The (3)He nuclear magnetic shieldings were calculated for free helium atom and He-pyrrole, He-indole, and He-carbazole complexes. Several levels of theory, including Hartree-Fock (HF), Second-order Møller-Plesset Perturbation Theory (MP2), and Density Functional Theory (DFT) (VSXC, M062X, APFD, BHandHLYP, and mPW1PW91), combined with polarization-consistent pcS-2 and aug-pcS-2 basis sets were employed. Gauge-including atomic orbital (GIAO) calculated (3)He nuclear magnetic shieldings reproduced accurately previously reported theoretical values for helium gas. (3)He nuclear magnetic shieldings and energy changes as result of single helium atom approaching to the five-membered ring of pyrrole, indole, and carbazole were tested. It was observed that (3)He NMR parameters of single helium atom, calculated at various levels of theory (HF, MP2, and DFT) are sensitive to the presence of heteroatomic rings. The helium atom was insensitive to the studied molecules at distances above 5 Å. Our results, obtained with BHandHLYP method, predicted fairly accurately the He-pyrrole plane separation of 3.15 Å (close to 3.24 Å, calculated by MP2) and yielded a sizable (3)He NMR chemical shift (about -1.5 ppm). The changes of calculated nucleus-independent chemical shifts (NICS) with the distance above the rings showed a very similar pattern to helium-3 NMR chemical shift. The ring currents above the five-membered rings were seen by helium magnetic probe to about 5 Å above the ring planes verified by the calculated NICS index.

Probing the structure and dynamics of molecular clusters using rotational wave packets.

Rotational wave packets of the weakly bound C(2)H(2)-He complex have been created using impulsive alignment. The coherent rotational dynamics were monitored for 600 ps enabling extraction of a frequency spectrum showing multiple rotational energy levels up to J = 4. spectrum has been combined with ab initio calculations to show that the complex has a highly delocalized structure and is bound only by ca. 7 cm(-1). The experiments demonstrate how highly featured rotational spectra can be obtained from an extremely cold environment where only the lowest rotational energy states are initially populated.

Molecular dynamics simulation of the initial stages of He bubbles formation in silicon

The initial stages of the formation of bubbles due to helium implantation in silicon remain largely unknown, for they occur at time and space scales hardly accessible to experiments. To improve the current knowledge, we have developed an interatomic potential to describe the He : Si system, which can model helium for the pressure conditions expected in bubbles, and reproduce the behaviour of a single helium atom in silicon. Then molecular dynamics calculations were performed. Starting from an initial random distribution of helium interstitials and silicon vacancies, simulations revealed a two-step formation of small HenVm clusters. First, available vacancies quickly formed vacancy clusters Vm, due to their high mobility. In a second step, all clusters included a slowly increasing number of helium atoms, which were captured by diffusion or by cluster migration. Migration coalescence coarsening mechanisms were also identified during the simulation.

3He NMR: from free gas to its encapsulation in fullerene

The (3)He nuclear magnetic shieldings were calculated for single helium atom, its dimer, simple models of fullerene cages (He@Cn), and single wall carbon nanotubes. The performances of several levels of theory (HF, MP2, DFT-VSXC, CCSD, CCSD(T), and CCSDT) were tested. Two sets of polarization-consistent basis sets were used (pcS-n and aug-pcS-n), and an estimate of (3)He nuclear magnetic shieldings in the complete basis set limit using a two-parameter fit was established. Theoretical (3)He results reproduced accurately previously reported theoretical values for helium gas, dimer, and helium probe inside several fullerene cages. Excellent agreement with experimental values was achieved. (3)He nuclear magnetic shieldings of single helium atom approaching various points of benzene ring were tested, and an impact of (3)He confinement within fullerene cages of different size on the (3)He chemical shift was determined.

Novel carbon materials: New tunneling systems (Review Article)

This review covers recent achievements in the studies of quantum properties of the novel carbon materials (fullerite C60 and bundles of single-walled nanotubes (SWNT)) saturated with such light-mass species as helium isotopes, the homonuclear molecular hydrogens, and neon. It is shown that even some heavy dopants demonstrate kinetic phenomena, in which coherent effects play an essential role. Two theoretical concepts are surveyed which have been suggested for the explanation of the anomalous phenomena in saturation kinetics and linear thermal expansion of doped C60. Most unusual effects have been also observed in the low-temperature radial expansion of bundles of single-walled carbon nanotubes saturated with the helium isotopes. First, it was shown that low-temperature radial expansion of pure SWNT is negative, i.e., a nanotube shrinks with warming. Second, saturation of SWNT bundles with the helium isotopes entails a huge increase of the negative expansion effect, when the dopant is He. So far, no detailed physical picture has been put forward. It is worth mentioning that the dynamics of a single helium atom on an isolated nanotube corresponds to that of a tight-bound quasiparticle with a band width of about 10 K.

X-ray observation of a helium atom and placing a nitrogen atom inside He@C60 and He@C70

Single crystal X-ray analysis has been used as a powerful method to determine the structure of molecules. However, crystallographic data containing helium has not been reported, owing to the difficulty in embedding helium into crystalline materials. Here we report the X-ray diffraction study of He@C60 and the clear observation of a single helium atom inside C60. In addition, the close packing of a helium atom and a nitrogen atom inside fullerenes is realized using two stepwise insertion techniques, that is, molecular surgery to synthesize the fullerenes encapsulating a helium atom, followed by nitrogen radio-frequency plasma methods to generate the fullerenes encapsulating both helium and nitrogen atoms. Electron spin resonance analysis reveals that the encapsulated helium atom has a small but detectable influence on the electronic properties of the highly reactive nitrogen atom coexisting inside the fullerene, suggesting the potential usage of helium for controlling electronic properties of reactive species.

Electron impact ionization of water-doped superfluid helium nanodroplets: Observation of He(H2O)n+ clusters

Electron impact mass spectra have been recorded for helium nanodroplets containing water clusters. In addition to identification of both H(+)(H(2)O)(n) and (H(2)O)(n)(+) ions in the gas phase, additional peaks are observed which are assigned to He(H(2)O)(n)(+) clusters for up to n=27. No clusters are detected with more than one helium atom attached. The interpretation of these findings is that quenching of (H(2)O)(n)(+) by the surrounding helium can cool the cluster to the point where not only is fragmentation to H(+)(H(2)O)(m) (where m < or = n-1) avoided, but also, in some cases, a helium atom can remain attached to the cluster ion as it escapes into the gas phase. Ab initio calculations suggest that the first step after ionization is the rapid formation of distinct H(3)O(+) and OH units within the (H(2)O)(n)(+) cluster. To explain the formation and survival of He(H(2)O)(n)(+) clusters through to detection, the H(3)O(+) is assumed to be located at the surface of the cluster with a dangling O-H bond to which a single helium atom can attach via a charge-induced dipole interaction. This study suggests that, like H(+)(H(2)O)(n) ions, the preferential location for the positive charge in large (H(2)O)(n)(+) clusters is on the surface rather than as a solvated ion in the interior of the cluster.