Kr Atoms Research Articles

Relativistic coupled-cluster calculations ofN20e,A40r,K84r, andX129e: Correlation energies and dipole polarizabilities

We have carried out a detailed and systematic study of the correlation energies of inert gas atoms Ne, Ar, Kr, and Xe using relativistic many-body perturbation theory and relativistic coupled-cluster theory. In the relativistic coupled-cluster calculations, we implement perturbative triples and include these in the correlation energy calculations. We then calculate the dipole polarizability of the ground states using perturbed coupled-cluster theory.

Angular correlations in the sequential two-photon double ionisation of noble gases

An expression for the angular correlation function of the two photoelectrons in the sequential two-photon double ionization (TPDI) of atoms is derived, which can be used for analyzing angle-resolved coincidence experiments. Making use of this expression, we have calculated in particular the cross sections, angular distributions and angular correlation functions for the TPDI of Ne, Ar and Kr atoms. The results are compared with recent experiments from the free-electron laser (FLASH) facility as far as available.

Absolute differential bremsstrahlung cross sections for 0.4–2-keV electrons scattered by Ar, Kr, and Xe atoms

The dependences of absolute differential bremsstrahlung cross sections on 0.4--2-keV electron scattering by supersonic gas jets of Ar, Kr, and Xe versus electron energy as well as the atom number $Z$ were studied experimentally. The bremsstrahlung photons have been detected at an angle $97\ifmmode^\circ\else\textdegree\fi{}$ to the incident electron beam in the ultrasoft x-ray spectral region 70--200 eV. It is found that as the electron energy is increased from 0.4 to 2 keV, the absolute bremsstrahlung cross section for Xe increases throughout the whole electron energy range and the energy dependencies of cross sections for Ar and Kr displayed maxima at electron energies of $\ensuremath{\sim}0.7\text{ }\text{keV}$ (Ar) and $\ensuremath{\sim}1\text{ }\text{keV}$ (Kr). These experimental results cannot be explained in the framework of Bethe and Heitler and Sauter theory, based on the first Born approximation. The experimental absolute differential bremsstrahlung cross sections for Ar, Kr, and Xe are considerably larger then the tabulated values of ordinary bremsstrahlung. The obtained $Z$ dependence of bremsstrahlung cross section differs with calculated one, particularly for large atomic number $Z$.

Detecting Weak Interactions between Au− and Gas Molecules: A Photoelectron Spectroscopic and Ab Initio Study

We show that anion photoelectron spectroscopy can be a very sensitive probe for weak intermolecular interactions between gold anion and a noble-gas atom or other nonreactive molecule. High-level ab initio calculations support the measured trend of relatively weak intermolecular interactions among various gold anion-atom complexes. The interaction between Au(-) and H(2)O is much stronger, comparable to a strong hydrogen bond. The interaction between Au(-) and O(2) is weaker than that between Au(-) and a noble-gas atom (Ar, Kr, or Xe).

Sequential two-photon double ionization of Kr atoms

Sequential two-photon double ionization of Kr atoms is theoretically considered. The angular distribution of emitted electrons is calculated with the dipole amplitudes evaluated within the multiconfigurational Hartree–Fock and Dirac–Fock approaches. The difference in angular distributions for two-photon and single-photon ionization is discussed.

Ab initio study of Kr in hcp Ti: Diffusion, formation and stability of small Kr–vacancy clusters

Ab initio electronic structure calculations have been performed to study the formation and migration of Kr impurities, and the stability of small Kr–vacancy clusters for clusters with up to four vacancies and four Kr atoms, in hcp Ti. Both the substitutional and the interstitial configurations of Kr are found to be stable. The octahedral configuration is however found to be more stable than the tetrahedral. Interstitial Kr atoms are shown to have attractive interactions and a low migration barrier, suggesting that, at low temperature, Kr bubble formation is possible, even in the absence of vacancies. We also find vacancy clusters to be stable. The binding energies of an interstitial Kr atom and a vacancy to a Kr–vacancy cluster are obtained from the calculated formation energies of the clusters. The stability of small-vacancy clusters is found to be dependent on Kr–vacancy ratio. The trends of the calculated binding energies are discussed in terms of providing further insights on the behaviour of Kr in implanted Ti.

Effective radii of noble gas atoms in silicates from first-principles molecular simulation

An understanding of how noble gas atoms are dissolved in mantle minerals and melts is necessary to infer geological information from the constraints provided by noble gas geochemistry. For this purpose, first-principles molecular simulations are carried out on liquid and crystalline (stishovite) silica systems with dissolved noble gas atoms (He, Ne, Ar, Kr, and Xe). The first principles nature of the simulations, which do not involve empirical force field parameters, enables the determination of the effective radii and structural environments of the noble gas atoms. The noble gas atoms are shown to be highly compressible, so that their effective radii depend strongly on the molar volume of the system (which in turn depends on pressure). Due to the continuous nature of interatomic forces, the effective radii also depend on the extent to which the surrounding atoms can relax in response to the presence of the noble gas atom. In this regard, different definitions of effective radii are relevant in different situations: “equilibrium radii” that correspond to the optimal interatomic distances at the molar volume of the system, and “repulsive wall” radii that correspond to the interatomic distances where the interatomic potentials of mean force change from attractive to repulsive at that molar volume. The equilibrium radii determine the interatomic distances in a melt, and the repulsive wall radii determine the interatomic distances for interstitial sites in a crystal. Based on these effective radii, the structural environment surrounding the noble gas atoms at high pressure is shown to correspond to a close packing of O atoms around the central noble gas atom. Compression of the noble gas atoms is shown to correspond closely to the compression of the porosity within the silicate melt structure.

Resonance-enhanced photon excitation spectroscopy of the even-parity autoionizing Rydberg states of Kr

Kr atoms were produced in their metastable states 4p55s [3/2]2 and 4p55s’ [1/2]0 in a pulsed DC discharge in a beam, and subsequently excited to the even-parity autoionizing Rydberg states 4p5 np′ [3/2]1,2, [1/2]1 and 4p5 nf′ [5/2]3 using single photon excitation. The excitation spectra of the even-parity autoionizing resonance series from the metastable Kr were obtained by recording the autoionized Kr+ ions with time-of-flight ion detection in the photon energy range of 29000–40000 cm−1. A wealth of autoionizing resonances were newly observed, from which more precise and more systematic spectroscopic data of the level energy and quantum defects were derived.

A simple model of hydrogen bonding with particular application to trends in hydrogen-bonded dimers

A simple model has been proposed to explain trends in the computed interaction energy, bond length changes, frequency shifts and infrared intensities for the chlorofluoromethanes CFnClmH, FH and FArH on complexation with the isoelectronic diatomics BF, CO, N2 and the rare gas atoms Kr, Ar, Ne to form a series of linear or nearly linear hydrogen-bonded complexes. The dipole moment derivative of the proton donor (with respect to the stretching coordinate) and the chemical hardness of the hydrogen-bonded atom of the proton acceptor are identified as two useful parameters for rationalizing the changes in some of the molecular properties of the proton donor when the hydrogen bond is formed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Retrieving photorecombination cross sections of atoms from high-order harmonic spectra

We observe high-order harmonic spectra generated from a thin atomic medium, Ar, Kr, and Xe, by intense $800\text{\ensuremath{-}}\mathrm{nm}$ and $1300\text{\ensuremath{-}}\mathrm{nm}$ femtosecond pulses. A clear signature of a single-atom response is observed in the harmonic spectra. Especially in the case of Ar, a Cooper minimum, reflecting the electronic structure of the atom, is observed in the harmonic spectra. We successfully extract the photorecombination cross sections of the atoms in the field-free condition with the help of an accurate recolliding electron wave packet. The present protocol paves the way for exploring ultrafast imaging of molecular dynamics with attosecond resolution.

Coarse molecular-dynamics analysis of an order-to-disorder transformation of a krypton monolayer on graphite

The thermally induced order-to-disorder transition of a monolayer of krypton (Kr) atoms adsorbed on a graphite surface is studied based on a coarse molecular-dynamics (CMD) approach for the bracketing and location of the transition onset. A planar order parameter is identified as a coarse variable, psi, that can describe the macroscopic state of the system. Implementation of the CMD method enables the construction of the underlying effective free-energy landscapes from which the transition temperature, T(t), is predicted. The CMD prediction of T(t) is validated by comparison with predictions based on conventional molecular-dynamics (MD) techniques. The conventional MD computations include the temperature dependence of the planar order parameter, the specific heat, the Kr-Kr pair correlation function, the mean square displacement and corresponding diffusion coefficient, as well as the equilibrium probability distribution function of Kr-atom coordinates. Our findings suggest that the thermally induced order-to-disorder transition at the conditions examined in this study appears to be continuous. The CMD implementation provides substantial computational gains over conventional MD.

SIMS analysis of xenon and krypton in uranium dioxide: A comparison of two models of gas-phase ionisation

Reasonable detection limits in secondary ion mass spectrometry (SIMS) analysis of inert gases in solids may be achieved by gas-phase ionisation. For Xe in UO 2 Desgranges and Pasquet (D&P) have recently reported that the ion intensity I Xe + can be enhanced significantly using O 2 + ion bombardment in combination with an oxygen jet directed at the sputtered area. A similar effect was reported by Portier et al. for Kr in UO 2. The enhancement was attributed to ionisation by an interaction with oxygen atoms and molecules (D&P model). The proposed mechanism is at variance with a recently outlined model of gas-phase ionisation involving charge transfer between the ejected atoms and the incoming primary ions. The purpose of this study was to clarify these contradictory views by a reanalysis of the original data. Access to all relevant original data was kindly provided by L. Desgranges. In contrast to expectation based on the D&P model, step-wise increases in the flow rate of the oxygen jet did not result in an immediate response of I Xe + . Instead, transient yield changes were observed, with characteristic rise times that were even longer for Xe + than for uranium specific signals. This observation invalidates the idea that ionisation of Xe atoms is due to interaction with oxygen molecules of the jet. Recalling well known transient phenomena in ion-bombardment induced photon emission, it is argued that the enhancement effect may be associated with the increase in the oxidation state of the sample, as a result of which the fraction of Xe (or Kr) atoms leaving the sample in an electronically excited state is presumably increasing, thus enlarging the effective cross section for charge transfer. Alternatively, the enhancement may be due to a lowering of the ejection velocity of rare gas atoms, possibly caused by the increase in near-surface sample oxidation. A second problem with the D&P study is the use of I U + as a reference signal. Literature data as well as new results reveal that UO 2 + and UO + secondary ions dominate the SIMS spectrum of oxidised uranium at energies <60 eV, the U + fraction amounting to only 1% or less. Other issues are (i) the significant variation of the sample erosion rate that occurred upon deliberate changes of the mean primary ion current density, (ii) the associated bombardment induced, progressive oxygen depletion of the sample, (iii) the presence of a background superimposed on the Xe + signals, a significant effect at low current densities and (iv) the space–charge broadening of the primary ion beam at high beam currents. Eliminating all these interfering factors, the (reduced) ionisation probabilities of Xe + were found to be in accordance with gas-phase ionisation by charge exchange, even in the pressure of excessive oxygen flooding.

Photoelectron Imaging of Helium Droplets Doped with Xe and Kr Atoms

Helium droplets doped with Xe and Kr atoms were photoionized by using VUV synchrotron radiation from the Advanced Light Source and the resulting photoelectron images were measured. A wide range of He droplet sizes, photon energies, and dopant pick-up conditions was investigated. Significant ionization of dopants was observed at 21.6 eV, the absorption maximum of 2p (1)P1 electronic excited state of He droplets, indicating an indirect ionization mechanism via excitation transfer. The photoelectron images and spectra reveal multiple photoionization mechanisms and pathways for the photoelectrons to escape the droplet. Specifically, they show sets of sharp peaks assigned to two mechanisms for Penning ionization of the dopant by He* in which the photoelectrons leave the droplet with no detectable energy loss, a broad, intense feature representing electrons that undergo significant energy loss, and a small amount of ultraslow electrons that may result from electron trapping at the droplet surface. The droplet-size dependence of the broad, intense feature suggests the development of the conduction band edge in the largest droplets seen here ((N) approximately 250,000).

Photoionization of the subvalent subshells of noble gas endohedrals: interference of three resonances

We demonstrate strong interference patterns in the photoionization cross-section of the subvalent subshells of noble gas (NG) endohedral atoms NG@F. This interference is a result of common action of three factors: the effect of neighbouring atomic subshells, reflection of photoelectron waves by the fullerene F shell and resonance modification of the incoming photon beam by the complex effect under the action of the F electrons. We have considered the outer ns-subshells for Ne, Ar, Kr and Xe noble gas atoms. The polarization of the fullerene shell is expressed via the F total photoabsorption cross-section. The photoelectron reflection from the static F potential is taken into account in the frame of the so-called bubble potential which is a spherical δ-type potential. It is assumed that the NG atom is centrally located in the fullerene. It is also assumed in accordance with the available experimental data that the fullerene radius is much bigger than the atomic radius and the thickness of the fullerene shell. These assumptions permit the NG@F photoionization cross-section to be presented as a product of the NG atomic cross-section and two calculated factors that account for polarization of the F electron shell and reflection of photoelectrons by the fullerene static potential.

Energy transfer of highly vibrationally excited 2-methylnaphthalene: Methylation effects

The methylation effects in the energy transfer between Kr atoms and highly vibrationally excited 2-methylnaphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at a translational collision energy of approximately 520 cm(-1). Comparison of the energy transfer between naphthalene and 2-methylnaphthalene shows that the difference in total collisional cross section and the difference in energy transfer probability density functions are small. The ratio of the total cross sections is sigma(naphthalene): sigma(methylnaphthalene)=1.08+/-0.05:1. The energy transfer probability density function shows that naphthalene has a little larger probability at small T-->VR energy transfer, DeltaE(u)<300 cm(-1), and 2-methylnaphthalene has a little larger probability at large V-->T energy transfer, -800 cm(-1)<DeltaE(d)<-100 cm(-1). However, these differences are close to our experimental uncertainty. No significant difference in the probability of very large energy transfer, such as supercollisions defined arbitrarily as DeltaE(d)<-1500 cm(-1), was observed. The possible methylation effects due to the subsequent successive collisions were discussed.

Release and efficiencies of radioactive ion beams from the JAEA-ISOL target/ion source systems

For the ISOL-based radioactive ion beam facility, TRIAC, separation efficiencies and release times of neutron-rich isotopes produced in proton-induced fission of 238U have been measured using the JAEA-ISOL target/ion source systems. With a FEBIAD-B2 type integrated target–ion source, for example, we obtained separation yields, the values were normalized to a uranium target thickness of 1g/cm2 and 1μA primary proton beam current, 1.4×106pps for 91Kr, 7×105pps for 126In, 2.6×106pps for 138Xe, respectively. Using a surface ionization type ion source, under the same normalization, the separation yields of 1.8×107pps for 93Rb, 6×107pps for 94Sr, 2.5×106pps for 143Cs and 5×106pps for 142Ba were observed. The release times of radioactive Rb, Cs, Ba, In, Kr and Xe atoms were measured using present target/ion source systems. Comparison of the measured yields of Ba, In and Xe isotopes with the calculated ones suggests that the long release time gave low separation efficiency for short-lived isotopes.

Measuring the Extreme Conditions Created During Cavitation

Extreme temperatures and pressures are produced through acoustic cavitation: the formation, growth and collapse of bubbles in a liquid irradiated with high intensity ultrasound. Single bubbles have generally been assumed to give higher temperature conditions than bubble clouds, but confirmation from the single buble sonoluminescence (SBSL) emission spectra have been problematic because SBSL typically produces featureless emission spectra that reveal little about the intra-cavity physical conditions or chemical processes. Here we present definitive evidence of the existence of a hot, highly energetic plasma core during SBSL. From a luminescing bubble in sulfuric acid, excited state to excited state emission lines are observed both from noble gas ions (Ar+, Kr+, and Xe+) and from neutral atoms (Ne, Ar, Kr, and Xe). The excited states responsible for these emission lines range from 8.3 eV (for Xe) to 37.1 eV (for Ar+) above the ground state. Observation of emission lines allows for identification of intra-cavity species responsible for light emission; the energy levels of the emitters indicate the plasma generated during cavitation is comprised of highly energetic atomic and ionic species.

Energy transfer of highly vibrationally excited naphthalene. III. Rotational effects

The rotational effects in the energy transfer between Kr atoms and highly vibrationally excited naphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity map ion imaging at various translational collision energies. As the initial rotational temperature changes from less than 10 to approximately 350 K, the ratio of vibrational to translational (V-->T) energy transfer cross section to translational to vibrational/rotational (T-->VR) energy transfer cross section increases, but the probability of forming a complex during the collisions decreases. Significant increases in the large V-->T energy transfer probabilities, termed supercollisions, at high initial rotational temperature were observed.

Relativistic quantum-chemical calculations of magnetizabilities of noble gas atoms using the Douglas–Kroll–Hess method

Quantum-chemical calculations of the magnetizability of the noble gas atoms (He, Ne, Ar, Kr, and Xe) were carried out using the second-order Douglas–Kroll–Hess (DKH2) Hamiltonian containing the vector potential A. We found that the vector potential A treated by the DKH2 method gives about a 10% correction in magnetizability of Kr and Xe. Although the basis set dependency is considerably large in the present method, we showed that it can be avoided by using well extended basis sets.

Observing molecular structures by using high-order harmonic generation in mixed gases

We demonstrate that the structure of ${\mathrm{CO}}_{2}$ molecules can be determined by measuring the phases of harmonics from them. This is evidenced by observing a phase jump of harmonics generated in a mixed gas of aligned ${\mathrm{CO}}_{2}$ molecules and their reference Kr atoms, which serve to extract the net contribution due to the structure of ${\mathrm{CO}}_{2}$ molecules. Furthermore, in the strong field approximation model, we derive the exact conditions of quantum interference during high-order harmonic generation in ${\mathrm{CO}}_{2}$ molecules analytically and we propose a method to control harmonic pulse duration.