Xe Atoms Research Articles

Energy-Transfer Kinetics for Xe (6p[1/2]0) Atoms in Kr, Ar, Ne, and He.

The kinetic processes for the Xe (6p[1/2]0) atoms in Kr, Ar, Ne, and He buffer gases were studied. We found that Kr, Ar, and Ne atoms can be used to switch the amplified spontaneous emission (ASE) channel from 3408 nm (6p[1/2]0-6s'[1/2]1) to 3680 nm (5d[1/2]1-6p[1/2]1), while Xe and He atoms do not show such a phenomenon. This ASE channel switch is mainly ascribed to the fast transfer of 6p[1/2]0 → 5d[1/2]1. On the basis of the rate equations for two-state coupling (energy-transfer processes between the two states are very rapid), the reason why the ASE channel switch effect normally coincides with a double exponential decay of the spontaneous emission at 828 nm (6p[1/2]0-6s[3/2]1) is explained. The actual situations in Xe, Ar, Ne, and He follow this rule. However, the strictly single exponential decay of the spontaneous emission at 828 nm and strong ASE channel switch effect simultaneously emerge in Kr. This indicates that the transfer of 6p[1/2]0 → 5d[1/2]1 in Kr does not occur via two-state coupling, but via two steps of near-resonance collision through the 5s[3/2]2 (Kr) state as the intermediate state (6p[1/2]0 → 5s[3/2]2 (Kr) → 5d[1/2]1). In addition, we found Xe (6p[1/2]0) atoms strongly tend to reach the 6p[3/2]2, 6p[3/2]1, and 6p[5/2]2 states through the 5s[3/2]2 (Kr) state as the intermediate state in Kr. The 5s[3/2]2 (Kr) state plays a very important role in the energy-transfer kinetics for the Xe (6p[1/2]0) atoms. Kr is probably an excellent buffer gas for laser systems based on Xe.

Resonance-like enhancement in high-order above threshold ionization of atoms and molecules in intense laser fields.

We investigate the high-order above-threshold ionization (HATI) of atoms (Ar and Xe) and molecules (N2 and O2) subjected to strong laser fields by numerically solving time-dependent Schrödinger equation. It is demonstrated that resonance-like enhancement of groups of adjacent peaks in photoelectron spectrum of HATI is observed for Ar, Xe, and N2, while this peculiar phenomenon is absent for O2, which is in agreement with experimental observation [ Phys. Rev. A88, 021401 (2013)]. In addition, analysis indicates that resonance-like enhancement in HATI spectra of atoms and molecules is closely related to excitation of the high-lying excited states.

Chemical shift extremum of 129Xe(aq) reveals details of hydrophobic solvation

The 129Xe chemical shift in an aqueous solution exhibits a non-monotonic temperature dependence, featuring a maximum at 311 K. This is in contrast to most liquids, where the monotonic decrease of the shift follows that of liquid density. In particular, the shift maximum in water occurs at a higher temperature than that of the maximum density. We replicate this behaviour qualitatively via a molecular dynamics simulation and computing the 129Xe chemical shift for snapshots of the simulation trajectory. We also construct a semianalytical model, in which the Xe atom occupies a cavity constituted by a spherical water shell, consisting of an even distribution of solvent molecules. The temperature dependence of the shift is seen to result from a product of the decreasing local water density and an increasing term corresponding to the energetics of the Xe-H2O collisions. The latter moves the chemical shift maximum up in temperature, as compared to the density maximum. In water, the computed temperature of the shift maximum is found to be sensitive to both the details of the binary chemical shift function and the coordination number. This work suggests that, material parameters allowing, the maximum should be exhibited by other liquids, too.

Imaging Agent for Redox‐Active Molecules

Redox‐active and cofactor molecules (such as glutathione, NAD+/NADH, and NADP+/NADPH) are involved in a wide array of important biological reactions. Their cellular concentrations are maintained within a range of values, and an imbalance is often linked to decreased cellular defense capability against oxidative stress. Currently, the most common techniques for quantifying these molecules require cell lysis. The goal of this project is to design sensor molecules that can be used in live cells. Our approach relies on the use of oligodeoxynucleotide strands (called aptamers) as the recognition moiety for the redox compound of interest. Aptamers, when selected for a specific target molecule, can display selectivity and affinity rivalling those of antibodies. Conjugation of the aptamer to a xenon‐containing organic molecule will yield an imaging agent that will report the concentration of the target as a measurable frequency value in the xenon nuclear magnetic resonance.Isolation of aptamers for a target molecule is achieved via an iterative in vitro selection technique called Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Starting from a large pool of combinatorial library of aptamers (70‐base long strands containing 30 randomized bases), each iteration consists of: 1) incubation with the target molecule, 2) partitioning of the target‐binding and non‐binding aptamers, 3) amplification of the binding aptamers through polymerase chain reaction (PCR), and 4) separation of the amplicons into single‐stranded aptamers. This newly generated pool is then used as the starting library for the consecutive SELEX cycle. In the last round, the aptamer pool is cloned in E.coli and the sequence of each target binding strand is elucidated.The magnetic resonance frequency reporter is composed of 129Xe atom encapsulated in an organic ‘cage’ molecule. Starting from commercially available precursors, the synthesis of the Xe‐based component follows an established multi‐step organic synthesis procedure. In the final stage, conjugation of the aptamer to the organic ‘cage’ bearing the Xe atom is achieved via Huisgen 1,3‐dipolar cycloaddition or carboxylic acid‐amine condensation reaction.Toward this end, we have completed several rounds of SELEX and closely monitored the progress of each cycle. We started by incubating the aptamer pool with a high concentration of glutathione (10 mM), and observed aptamer binding to the target after several rounds. We have subsequently increased the selection pressure by gradually reducing the glutathione solution to 0.5 mM. At the same time, we have successfully completed several synthetic steps toward the Xe organic ‘cage’ molecule. The product of each step was fully characterized using standard organic chemistry methods.Upon completion of our project, we will have developed a magnetic resonance imaging (MRI) agent for glutathione which will be extended to other redox‐active and cofactor molecules.Support or Funding InformationFunding for the project came from the California State University East Bay (CSUEB) Faculty Support Grant. D.S. was a recipient of the CSUEB Center for Student Research Scholarship.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Collective resonances of atomic xenon from the linear to the nonlinear regime

XUV nonlinear spectroscopy has recently discovered that there is more than one collective dipole resonance state in the energy range of the giant dipole resonance (GDR) of atomic Xe. This resonance-state substructure, hidden in the linear regime, raises imminent questions regarding our understanding of the collective electronic behavior of Xe, which has been largely founded on linear spectroscopic studies. Here, we approach the collective response of Xe from a new perspective: we study directly the resonance eigenstates, and then analyze their spectroscopic manifestations. We find that linear spectroscopy captures only partial information on the resonance substructure as a result of quantum interferences. Moreover, we show that the resonance state dominating the GDR in linear spectroscopy has no adiabatic connection to the resonance state governing the corresponding cross section when multielectron interactions are neglected. Going beyond the dipole-allowed correlated electronic structure, we predict the existence of collective multipole resonances of Xe. Unlike any known collective feature in atoms, these resonances live exceptionally long (more than 100 attoseconds), thus providing a new playground for studying the collective nonlinear response of Xe using advanced light sources.

Polarized electron beams elastically scattered by atoms as a tool for testing fundamental predictions of quantum mechanics

Quantum information theory deals with quantum noise in order to protect physical quantum bits (qubits) from its effects. A single electron is an emblematic example of a qubit, and today it is possible to experimentally produce polarized ensembles of electrons. In this paper, the theory of the polarization of electron beams elastically scattered by atoms is briefly summarized. Then the POLARe program suite, a set of computer programs aimed at the calculation of the spin-polarization parameters of electron beams elastically interacting with atomic targets, is described. Selected results of the program concerning Ar, Kr, and Xe atoms are presented together with the comparison with experimental data about the Sherman function for low kinetic energy of the incident electrons (1.5eV–350eV). It is demonstrated that the quantum-relativistic theory of the polarization of electron beams elastically scattered by atoms is in good agreement with experimental data down to energies smaller than a few eV.

Strong-field ionization of xenon dimers: The effect of two-equivalent-center interference and of driving ionic transitions

Strong-field ionization (SFI) of the homonuclear noble gas dimer ${\mathrm{Xe}}_{2}$ is investigated and compared with SFI of the Xe atom and of the ArXe heteronuclear dimer by using ultrashort Ti:sapphire laser pulses and photoelectron momentum spectroscopy. The large separation of the two nuclei of the dimer allows the study of two-equivalent-center interference effects on the photoelectron momentum distribution. Comparing the experimental results with a new model calculation, which is based on the strong-field approximation, actually reveals the influence of interference. Moreover, the comparison indicates that the presence of closely spaced gerade and ungerade electronic state pairs of the ${{\mathrm{Xe}}_{2}}^{+}$ ion at the ${\mathrm{Xe}}_{2}$ ionization threshold, which are strongly dipole coupled, affects the photoelectron momentum distribution.

Theoretical study of the NMR chemical shift of Xe in supercritical condition.

In this work we investigate the level of theory necessary for reproducing the non-linear variation of the 129Xe nuclear magnetic resonance (NMR) chemical shift with the density of Xe in supercritical conditions. In detail we study how the 129Xe chemical shift depends under supercritical conditions on electron correlation, relativistic and many-body effects. The latter are included using a sequential-QM/MM methodology, in which a classical MD simulation is performed first and the chemical shift is then obtained as an average of quantum calculations of 250 MD snapshots conformations carried out for Xe n clusters (n = 2 - 8 depending on the density). The analysis of the relativistic effects is made at the level of 4-component Hartree-Fock calculations (4c-HF) and electron correlation effects are considered using second order Møller-Plesset perturbation theory (MP2). To simplify the calculations of the relativistic and electron correlation effects we adopted an additive scheme, where the calculations on the Xe n clusters are carried out at the non-relativistic Hartree-Fock (HF) level, while electron correlation and relativistic corrections are added for all the pairs of Xe atoms in the clusters. Using this approach we obtain very good agreement with the experimental data, showing that the chemical shift of 129Xe in supercritical conditions is very well described by cluster calculations at the HF level, with small contributions from relativistic and electron correlation effects.

Kinematics of the Doped Quantum Vortices in Superfluid Helium Droplets

Recent observation of quantum vortices in superfluid $$^{4}$$ He droplets measuring a few hundreds of nanometers in diameter involved decoration of vortex cores by clusters containing large numbers of Xe atoms, which served as X-ray contrast agents. Here, we report on the study of the kinematics of the combined vortex–cluster system in a cylinder and in a sphere. Equilibrium states, characterized by total angular momentum, L, were found by minimizing the total energy, E, which sums from the kinetic energy of the liquid due to the vortex and due to orbiting Xe clusters, as well as solvation energy of the cluster in the droplet. Calculations show that, at small mass of the cluster, the equilibrium displacement of the system from the rotation axis is close to that for the bare vortex. However, upon decrease in L beyond certain critical value, which is larger for heavier clusters, the displacement bifurcates toward the surface region, where the motion of the system is governed by the clusters. In addition, at even smaller L, bare orbiting clusters become energetically favorable, opening the possibility for the vortex to detach from the cluster and to annihilate at the droplet’s surface.

Breit corrections to individual atomic and molecular orbital energies

Several issues concerning Breit correction to electron-electron interaction in many-electron systems, which are important in precise atomic and molecular calculations, are presented. At first, perturbative versus self-consistent calculations of Breit correction were studied in selected cases. Second, the Z-dependence of Breit contribution per subshell is shown, based on values calculated for selected atoms with 30 ≤ Z ≤ 118. Third, the relations between magnetic and retardation parts of Breit interaction are analyzed. Finally, Gaunt contribution calculated for Kr, Xe, and Rn noble gas atoms and its iso-electronic HBr, HI, and HAt diatomic molecules has been compared to full-Breit atomic calculations. We found that Breit corrections should be treated by self-consistent calculations and that there is a functional dependence of those corrections for subshells as εnlBreit(Z)≃a×Zb. We also found that molecular Gaunt corrections are close to their atomic counterparts for inner electrons though they are not for outer orbitals. In any case, accurate calculations must include retardation correction in addition to Gaunt.

Energy- and Momentum-Resolved Photoelectron Spin Polarization in Multiphoton Ionization of Xe by Circularly Polarized Fields.

We perform a joint experimental and theoretical study on momentum- and energy-resolved photoelectron spin polarization in multiphoton ionization of Xe atoms by circularly polarized fields. We experimentally measure the photoelectron momentum distributions of Xe atoms in circularly polarized near-infrared (800nm) and ultraviolet (400nm) light, respectively. We analyze the momentum- and energy-resolved photoelectron spin polarization by comparing the experimental photoelectron momentum distributions with the simulations, although we cannot derive the spin polarization solely from the experiment. We show that the use of circularly polarized ultraviolet light at 400nm can create better than 90% spin polarization with focal volume effect considered, which enables the separation of the spin states by momentum gating. This paves the way to produce high-degree spin-polarized electron sources from strong-field multiphoton ionization.

Calculations of positronium-atom scattering using a spherical cavity

Positronium (Ps) scattering by noble-gas atoms (He, Ne, Ar, Kr, and Xe) is studied in the frozen-target approximation and with inclusion of the van der Waals interaction. Single-particle electron and positron states in the field of the target atom are calculated, with the system enclosed by a hard spherical wall. The two-particle Ps wave function is expanded in these states, and the Hamiltonian matrix is diagonalized, giving the Ps energy levels in the cavity. Scattering phase shifts, scattering lengths, and cross sections are extracted from these energies and compared with existing calculations and experimental data. Analysis of the effect of the van der Waals interaction shows that it cannot explain the recent experimental data of Brawley et al. for Ar and Xe [Phys. Rev. Lett. 115, 223201 (2015)].

Investigation of excited states populations density of Hall thruster plasma in three dimensions by laser-induced fluorescence spectroscopy

The article deals with investigation of the excited states populations distribution of a low-temperature xenon plasma in the thruster with closed electron drift at 300 W operating conditions were investigated by laser-induced fluorescence (LIF) over the 350–1100 nm range. Seven xenon ions (Xe II) transitions were analyzed, while for neutral atoms (Xe I) just three transitions were explored, since the majority of Xe I emission falls into the ultraviolet or infrared part of the spectrum and are difficult to measure. The necessary spontaneous emission probabilities (Einstein coefficients) were calculated. Measurements of the excited state distribution were made for points (volume of about 12 mm3) all over the plane perpendicular to thruster axis in four positions on it (5, 10, 50 and 100 mm). Measured LIF signal intensity have differences for each location of researched point (due to anisotropy of thruster plume), however the structure of states populations distribution persisted at plume and is violated at the thruster exit plane and cathode area. Measured distributions show that for describing plasma of Hall thruster one needs to use a multilevel kinetic model, classic model can be used just for far plume region or for specific electron transitions.

Potential energy surface and cold collision dynamics of Xe-NH(X3∑-) system

Sympathetic cooling is one of the most promising techniques for producing ultracold molecules from precooled molecules. Previous researches have shown that it is inadequate to use the ultracold alkali-metal atoms as coolant for sympathetic cooling. To explore the possibility of ultracold alkali-earth-metal atoms as coolant, in this paper a theoretical investigation is performed of the cold collision dynamics for Xe-NH(X3∑-) system in magnetic fields. The interaction potential energies of Xe-NH complex are calculated respectively by using the single and double excitation coupled-cluster theory with the noniterative treatment of triple excitations[CCSD(T)] method and complete basis set limit extrapolated method. An analytic express of potential energy surface (PES) is given for the first time. A single global minimum value occurs at R=7.14a0, θ=102.76° with an energy of-153.54 cm-1, and the PES has a weak anisotropy. Combine the ab initio PES with quantum scattering theory, then the cold collisional dynamics of Xe-NH system in a magnetic field will be studied. The elastic and inelastic transition cross sections and their ratios of NH molecules in the lowest low-field following state (n=0, mj=1) under different magnetic fields and collisional energies are calculated. The results show that the elastic cross section is independent of magnetic field, and the inelastic cross section changes with magnetic field, especially at an ultracold temperature. A common rule of thumb is that to successfully implement cooling, the ratio of elastic cross section to inelastic cross section needs to reach 100 at least. The results suggest that it is likely to be a challenging work to perform sympathetic cooling of NH molecule by ultracold Xe atom.

Predicting stability limits for pure and doped dicationic noble gas clusters undergoing coulomb explosion: A parallel tempering based study.

We have used a replica exchange Monte-Carlo procedure, popularly known as Parallel Tempering, to study the problem of Coulomb explosion in homogeneous Ar and Xe dicationic clusters as well as mixed Ar-Xe dicationic clusters of varying sizes with different degrees of relative composition. All the clusters studied have two units of positive charges. The simulations reveal that in all the cases there is a cutoff size below which the clusters fragment. It is seen that for the case of pure Ar, the value is around 95 while that for Xe it is 55. For the mixed clusters with increasing Xe content, the cutoff limit for suppression of Coulomb explosion gradually decreases from 95 for a pure Ar to 55 for a pure Xe cluster. The hallmark of this study is this smooth progression. All the clusters are simulated using the reliable potential energy surface developed by Gay and Berne (Gay and Berne, Phys. Rev. Lett. 1982, 49, 194). For the hetero clusters, we have also discussed two different ways of charge distribution, that is one in which both positive charges are on two Xe atoms and the other where the two charges are at a Xe atom and at an Ar atom. The fragmentation patterns observed by us are such that single ionic ejections are the favored dissociating pattern. © 2017 Wiley Periodicals, Inc.

Microstructural evolution and hardening of GH3535 alloy under energetic Xe ion irradiation at room temperature and 650 °C

The GH3535 alloy was irradiated with 7 MeV Xe26+ ions to a dose of 10 dpa at room temperature (RT) and 650 °C, and subsequently examined using Transmission Electron Microscopy (TEM) and nanoindentation. High numbers of nano-sized black dots, identified as dislocation loops were observed in both irradiated samples. The dislocation loops detected at the high temperature irradiated sample (size/number density: 9.5 nm/1.9 × 1021 m−3) were found to be larger in size but less in amount as compared to that of the case of RT irradiation (6.9 nm/18.7 × 1021 m−3). In addition, the large-sized Mo-Cr rich precipitates (16.4 nm/3.7 × 1021 m−3) were observed in the sample irradiated at 650 °C. Moreover, the Xe bubbles, with smaller size (2.9 nm) but higher number density (77.8 × 1021 m−3) among the irradiated induced defects, were also detected in the case of high temperature irradiated sample via the diffusion and aggregation of Xe atoms. Nanoindentaion measurements showed a hardening phenomenon for the irradiated sample, and the hardness increment is higher in the case of high temperature irradiated sample. Dispersed barrier-hardening (DBH) model was applied to predict the hardening produced from the irradiation induced defects. The yield strength increment calculated based on TEM observations and the nanohardness increment measured using nanoindentation are in excellent agreement.

Determination of gaseous fission product behavior near the cerium dioxide [formula omitted] tilt grain boundary via first-principles study

Grain boundaries (GBs) are the most abundant structural defects in nanostructured nuclear fuels and play an important role in determining fission product behavior, which further affects the performance of nuclear fuels. In this work, cerium dioxide (CeO2) is used as a surrogate material for mixed oxide fuels to understand gaseous fission product behavior, specifically Xe. First-principles calculations are employed to comprehensively study the behavior of Xe and trap sites for Xe near the Σ3 (111)/[11¯0] grain boundary in CeO2, which will provide guidance on overall trends for Xe stability and diffusion at grain boundaries vs in the bulk. Significant segregation behavior of trap sites, regardless of charge states, is observed near the GB. This is mainly ascribed to the local atomic structure near the GB, which results in weaker bond strength and more negative segregation energies. For Xe, however, the segregation profile near the GB is different. Our calculations show that, as the size of trap sites increases, the segregation propensity of Xe is reduced. In addition, under hyper-stoichiometric conditions, the solubility of Xe trapped at the GB is significantly higher than that in the bulk, suggesting higher Xe concentration than that in the bulk. The results of this work demonstrate that the diffusion mechanism of Xe in CeO2 is comparable to that in UO2. The diffusion activation energies of Xe atoms in the Σ3 GB are lower than that in the bulk CeO2. These results suggest that the diffusivity of Xe atoms is higher along the GB than that in the bulk, which enhances the aggregation of Xe atoms near the GB.

Comparison Study of Strong-Field Ionization of Molecules and Atoms by Bicircular Two-Color Femtosecond Laser Pulses.

We experimentally investigate the single and double ionization of N_{2} and O_{2} molecules in bicircular two-color femtosecond laser pulses, and compare with their companion atoms of Ar and Xe with comparable ionization thresholds. Electron recollision assisted enhanced ionization is observed in N_{2} and Ar by controlling the helicity and field ratio between the two colors, whereas the enhanced ionization via the recollision is almost absent in O_{2} and Xe. Our S-matrix simulations clearly reveal the crucial role of the detailed electronic structures of N_{2} and O_{2} on the two-dimensional recollision of the electrons driven by the bicircular two-color laser fields. As compared to Ar, the resonant multiphoton excitation dominates the double ionization of Xe.

Simulation of defect formation, amorphization and cluster formation processes in nc-TiN/a-Si3N4 nanocomposite under Xe irradiation

The research of defect formation and clusterization processes by means of a molecular dynamics method both in nc-TiN nanocrystals and amorphous a-Si3N4 matrix, as the constituents of nc-TiN/a-Si3N4 nanocomposite, under exposure to Xe implantation was the aim of the present study. Dependences of the clustered Xe atoms fraction on their concentration and temperature of post-irradiation annealing were analyzed. At defect formation process in nc-TiN nanocrystals, there is a size effect consisting in intensification of the radiation point defects formation with the reduction of nc-TiN nanocrystals size and concurrent predominant formation of the dangling Si- and N-bonds in a-Si3N4 matrix. Accumulation of these defects at the irradiation leads to amorphization of nc-TiN nanocrystals with the size less than 8 nm and to formation of the nanopores in a-Si3N4 matrix. The important role of the radiation defects subsystem in transport processes of implanted Xe both in TiN close-packed lattice as well as in a-Si3N4 amorphous matrix is shown. There is a much higher extent of intensity of xenon atoms clusterization processes in the amorphous matrix. The results of the simulation are compared to existing experimental data.

Raman spectroscopy revealing noble gas adsorption on single-walled carbon nanotube bundles

The interaction of the noble atoms (Ar and Xe) with single-walled carbon nanotube (SWCNT) bundles are investigated using Raman spectroscopy in conjunction with computational modeling known as ReaxFF force field. SWCNT bundles were deposited on transmission electron microscopy (TEM) grids, and different noble gases were adsorbed onto the nanotubes at 20 K. Raman spectra acquired show significant frequency blueshifts of the radial breathing mode (RBM), G- and G′(or 2D)-bands due to gas solidification within the external groove sites (free spaces between the tubes in the bundle) and external surfaces of the bundles. This solid shell formed by the adsorbed gases contributes with a hydrostatic pressure to the system. We show from Raman measurements that the frequencies found after gas adsorption exhibit almost the same shifts indicating that the interactions between SWCNTs bundles and the gases (Ar or Xe) are nearly identical.