Krypton Atoms Research Articles

State-of-the-art ab initio potential energy curve for the krypton atom pair and thermophysical properties of dilute krypton gas.

A new reference krypton-krypton interatomic potential energy curve was developed by means of quantum-chemical ab initio calculations for 36 interatomic separations. Highly accurate values for the interaction energies at the complete basis set limit were obtained using the coupled-cluster method with single, double, and perturbative triple excitations as well as t-aug-cc-pV5Z and t-aug-cc-pV6Z basis sets including mid-bond functions, with the 6Z basis set being newly constructed for this study. Higher orders of coupled-cluster terms were considered in a successive scheme up to full quadruple excitations. Core-core and core-valence correlation effects were included. Furthermore, relativistic effects were studied not only at a scalar relativistic level using second-order direct perturbation theory, but also utilizing full four-component and Gaunt-effect computations. An analytical pair potential function was fitted to the interaction energies, which is characterized by a depth of 200.88 K with an estimated standard uncertainty of 0.51 K. Thermophysical properties of low-density krypton were calculated for temperatures up to 5000 K. Second and third virial coefficients were obtained from statistical thermodynamics. Viscosity and thermal conductivity as well as the self-diffusion coefficient were computed using the kinetic theory of gases. The theoretical results are compared with experimental data and with results for other pair potential functions from the literature, especially with those calculated from the recently developed ab initio potential of Waldrop et al. [J. Chem. Phys. 142, 204307 (2015)]. Highly accurate experimental viscosity data indicate that both the present ab initio pair potential and the one of Waldrop et al. can be regarded as reference potentials, even though the quantum-chemical methods and basis sets differ. However, the uncertainties of the present potential and of the derived properties are estimated to be considerably lower.

Krypton Derivatization of an O2 -Tolerant Membrane-Bound [NiFe] Hydrogenase Reveals a Hydrophobic Tunnel Network for Gas Transport.

[NiFe] hydrogenases are metalloenzymes catalyzing the reversible heterolytic cleavage of hydrogen into protons and electrons. Gas tunnels make the deeply buried active site accessible to substrates and inhibitors. Understanding the architecture and function of the tunnels is pivotal to modulating the feature of O2 tolerance in a subgroup of these [NiFe] hydrogenases, as they are interesting for developments in renewable energy technologies. Here we describe the crystal structure of the O2 -tolerant membrane-bound [NiFe] hydrogenase of Ralstonia eutropha (ReMBH), using krypton-pressurized crystals. The positions of the krypton atoms allow a comprehensive description of the tunnel network within the enzyme. A detailed overview of tunnel sizes, lengths, and routes is presented from tunnel calculations. A comparison of the ReMBH tunnel characteristics with crystal structures of other O2 -tolerant and O2 -sensitive [NiFe] hydrogenases revealed considerable differences in tunnel size and quantity between the two groups, which might be related to the striking feature of O2 tolerance.

Commensurate Phases of Kr Adsorbed on Single-Walled Carbon Nanotubes

In this paper, we show that Krypton atoms form a commensurate solid (CS) phase with a fractional coverage of one krypton atom per every four carbons on zigzag carbon nanotubes. This is a unique phase, different from the $$\sqrt{3} \times \sqrt{3}$$ R30 $$^\circ $$ CS monolayer formed on graphite, which has a lower coverage of one krypton atom per every six carbons. Our prediction disagrees with experiments that observe in nanotubes the same solid structure found on graphite. In order to address this discrepancy, we simulated adsorption of Kr on zigzag and armchair single-walled carbon nanotubes with radii ranging from 4.7 to 28.83 A. Our simulations confirm that the CS of coverage 1/4 forms on medium-sized zigzag nanotubes. We also found the 1/6-coverage solid on graphene, which represents the infinite-radius limit of a nanotube. Our findings are key to experiments of adsorption on nanotubes where the interpretation and justification of the results are based on the monolayer coverage, such as mass or conductance isotherms measurements.

Optical attosecond pulses and tracking the nonlinear response of bound electrons.

The time it takes a bound electron to respond to the electromagnetic force of light sets a fundamental speed limit on the dynamic control of matter and electromagnetic signal processing. Time-integrated measurements of the nonlinear refractive index of matter indicate that the nonlinear response of bound electrons to optical fields is not instantaneous; however, a complete spectral characterization of the nonlinear susceptibility tensors--which is essential to deduce the temporal response of a medium to arbitrary driving forces using spectral measurements--has not yet been achieved. With the establishment of attosecond chronoscopy, the impulsive response of positive-energy electrons to electromagnetic fields has been explored through ionization of atoms and solids by an extreme-ultraviolet attosecond pulse or by strong near-infrared fields. However, none of the attosecond studies carried out so far have provided direct access to the nonlinear response of bound electrons. Here we demonstrate that intense optical attosecond pulses synthesized in the visible and nearby spectral ranges allow sub-femtosecond control and metrology of bound-electron dynamics. Vacuum ultraviolet spectra emanating from krypton atoms, exposed to intense waveform-controlled optical attosecond pulses, reveal a finite nonlinear response time of bound electrons of up to 115 attoseconds, which is sensitive to and controllable by the super-octave optical field. Our study could enable new spectroscopies of bound electrons in atomic, molecular or lattice potentials of solids, as well as light-based electronics operating on sub-femtosecond timescales and at petahertz rates.

Ultrashort light pulses shake atoms

The response of electrons in atoms to ultrashort optical light pulses has been probed by measuring the ultraviolet light emitted by the atoms. This reveals that a finite time delay occurs before the response. See Letter p.66 A fundamental speed limit for controlling matter through the electromagnetic force of light arises from the time it takes bound electrons to respond. Experiments have shown that this response is not instantaneous, but the lack of sufficiently fast probes has prevented direct measurements. Eleftherios Goulielmakis and colleagues have now produced intense light pulses in the visible and nearby spectral ranges and with energy largely confined to a half wave cycle, and show that these so-called optical attosecond pulses can control and measure the dynamics of bound electrons in krypton atoms. Proof-of-principle measurements establish the value of optical attosecond pulses for probing and manipulating bound electrons in atoms, molecules or solids, and suggest they may also find use in light-based nonlinear photonics operating on sub-femtosecond time scales and petahertz rates.

Collision-induced light scattering spectra of krypton layer confined between graphite slabs – MD simulation

We have used the molecular dynamics (MD) simulation method to obtain the collision-induced light scattering spectra of the thin krypton layer confined between two parallel graphite slabs. The simulations have been made under constant density, pressure and temperature condition. We have investigated four thin krypton layers of the thickness ranging from 13 to 24 Å. The 2-, 3- and 4-body correlation functions of collision-induced polarizability anisotropy were calculated. The spectra of colliding krypton atoms were simulated as cosine Fourier transform of the total polarizability anisotropy correlation function. The calculated correlation functions and their spectra show substantial dependence on the distance between graphite slabs. The collision-induced light scattering spectrum of krypton bulk sample was also simulated and compared with the krypton layer between graphite walls. The striking differences between these two systems are observed. We have further extended our analysis of krypton movement between graphite slabs by calculating the mean square displacement functions and diffusion coefficients. The decrease of the diffusion of krypton atoms with the increasing distance between graphite walls has been found. The structure of krypton layer has been also investigated by calculating the density profile and pressure tension across the rift. The distance between graphite slabs, for which the highest mobility of krypton's atoms occurred, has been found.

The Laser-assisted photoelectric effect of He, Ne, Ar and Xe in intense extreme ultraviolet and infrared laser fields

In this paper, we report results on two-colour above-threshold ionisation, where extreme ultraviolet pulses of femtosecond duration were synchronised to intense infrared laser pulses of picosecond duration, in order to study the laser-assisted photoelectric effect of atomic helium, neon, krypton and xenon which leads to the appearance of characteristic sidebands in the photoelectron spectra. The observed trends are found to be well described by a simple model based on the soft-photon approximation, at least for the relatively low optical intensities of up to employed in these early experiments.

Systematic study of the K X-ray spectra for hollow krypton and xenon atoms in collision process

The satellite and hypersatellite of K X-rays resulted from additional vacancies in L-shell and M-shell for Kr and Xe atoms have been calculated systematically by using the multiconfiguration-Dirac–Fock (MCDF) method, in which the Breit interaction, QED corrections are also considered. The present calculations for diagram lines are in good agreement with available previous results. It is found that the number of spectator vacancies in L shell varies nearly linearly with the energy shifts of both K X-ray satellites and hypersatellites for Kr and Xe atoms. While an additional M shell hole has nearly no effect on the Kα energy shifts in comparison with the small but evident energy shifts of Kβ lines. The simulated spectra based on our results show a good agreement with the current experimental spectra at Heavy-ion Research Facility (HIRFL) in Lanzhou. The results of this paper will be helpful in further interpretation of various K X-ray spectra and better understanding of the collision processes.

Phase differences in the photoemission from krypton in the fine-structure-split ionization channelsP3/22andP1/22

Using the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) technique, a nonzero phase difference is observed in the two-photon ionization transition of krypton atoms into the ionization channels $^{2}P_{1/2}$ and $^{2}P_{3/2}$ when an electron is removed from the $4p$ shell. The phase difference is studied for excitation energies ranging from $\ensuremath{\approx}18\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ up to $\ensuremath{\approx}28\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. The phase differences are found to depend on the relative directions of linear polarization of the photoionization laser pulses and the pulses inducing the subsequent continuum-continuum transition. The experimental results are compared to phase differences calculated on the basis of dipole matrix elements for photoionization and scattering phases known for krypton from earlier experiments. The results indicate that the RABBITT phase differences do not give an obvious direct access to time-delay differences in the primary photoionization step of the atom.

Investigation of cold collision in a two isotope magneto-optical trap for Krypton atoms

We report simultaneous cooling and trapping of metastable 84Kr (84Kr*) and metastable 86Kr (86Kr*) atoms by overlapping the cooling laser beams for these different isotopes in the same region of a magneto-optical trap (MOT). Approximately 2 × 105 atoms of each isotope were trapped in this two isotope-MOT (TIMOT). We have investigated the heteronuclear collision trap loss rates for each isotope due to the presence of cold atoms of other isotope using the TIMOT loading curves. The two body heteronuclear loss rate coefficient (i.e. for the loss of 84Kr* due to presence of 86Kr*) and the reciprocal loss rate coefficient (i.e. for the loss of 86Kr* due to presence of 84Kr*) are measured to be (8.7 ± 0.8) × 10−10 cm3 s−1 and (8.8 ± 0.8) × 10−10 cm3 s−1 respectively for laser beam intensity values of 21 mW cm−2 (for cooling of 84Kr*) and 64 mW cm−2 (for cooling of 86Kr*). The dependence of heteronuclear cold collision loss rate on cooling laser beams intensity has also been studied and compared with homonuclear cold collision loss rate.

Krypton tagging velocimetry of an underexpanded jet.

In this work, we present the excitation/emission strategy, experimental setup, and results of an implementation of krypton tagging velocimetry (KTV). KTV is performed as follows: (i)seed a base flow with krypton; (ii)photosynthesize metastable krypton atoms with a frequency-doubled dye laser to form the tagged tracer; (iii)record the translation of the tagged metastable krypton by imaging the laser-induced fluorescence (LIF) that is produced with an additional dye laser. The principle strength of KTV, relative to other tagging velocimetry techniques, is the use of a chemically inert tracer. KTV results are presented for an underexpanded jet of three mixtures of varying Kr/N2 concentration. It is demonstrated that KTV can be used in gas mixtures of relatively low krypton mole fraction (0.5% Kr/99.5% N2), and the KTV data from that experiment are found to be in good agreement with an empirical fit found in the literature. We find that KTV is useful to perform instantaneous velocity measurements with metastable krypton as a chemically inert, dilute, long-lifetime tracer in gas-phase flows.

Ramsey method for Auger-electron interference induced by an attosecond twin pulse

We examine the archetype of an interference experiment for Auger electrons: two electron wave packets are launched by inner-shell ionizing a krypton atom using two attosecond light pulses with a variable time delay. This setting is an attosecond realization of the Ramsey method of separated oscillatory fields. Interference of the two ejected Auger-electron wave packets is predicted, indicating that the coherence between the two pulses is passed to the Auger electrons. For the detection of the interference pattern an accurate coincidence measurement of photo- and Auger electrons is necessary. The method allows one to control inner-shell electron dynamics on an attosecond timescale and represents a sensitive indicator for decoherence.

Semiclassical description of high-order-harmonic spectroscopy of the Cooper minimum in krypton

The Cooper minimum has recently been observed in the high-order-harmonic spectrum issued from the interaction of a short and intense laser pulse with krypton atoms [A. D. Shiner et al., J. Phys. B 45, 074010 (2012)]. Here we compare this observation to a semiclassical description using the single-active electron approximation. Particular attention is paid to the description of the interaction of the active valence electron with the ionic core in the framework of a mean-field approach. The computed harmonic spectrum presents a minimum at about 83 eV, which does not show up so clearly in the measurements probably because of second-order diffraction of the grating used to disperse the harmonic spectrum.

Quenching of Metastable 5 s′(3P0) Krypton Atoms in Collisions with Krypton and Argon Atoms in High-Pressure Laser Mixtures

We use an absorption probe method to study collisional quenching of the metastable 5 s ′ [1/2] 0 (3P0) Kr atom in electron-beam-excited high-pressure Ar–Kr mixtures with a small content of krypton. The rate constants for plasma-chemical reactions Kr* + Kr + Ar → Kr 2 * + Ar((5.3 ± 0.8) ⋅ 10− 33cm6/s), Kr* + 2 Ar → ArKr* + Ar ((4.6±1.4)·10−36 cm6/s) and Kr* + Ar → products + Ar ((4.1±0.6)·10−15 cm3/s) are measured for the first time.

Trapping of He clusters by inert-gas impurities in tungsten: First-principles predictions and experimental validation

Properties of point defects resulting from the incorporation of inert-gas atoms in bcc tungsten are investigated systematically using first-principles density functional theory (DFT) calculations. The most stable configuration for the interstitial neon, argon, krypton and xenon atoms is the tetrahedral site, similarly to what was found earlier for helium in W. The calculated formation energies for single inert-gas atoms at interstitial sites as well as at substitutional sites are much larger for Ne, Ar, Kr and Xe than for He. While the variation of the energy of insertion of inert-gas defects into interstitial configurations can be explained by a strong effect of their large atomic size, the trend exhibited by their substitutional energies is more likely related to the covalent interaction between the noble gas impurity atoms and the tungsten atoms. There is a remarkable variation exhibited by the energy of interaction between inert-gas impurities and vacancies, where a pronounced size effect is observed when going from He to Ne, Ar, Kr, Xe. The origin of this trend is explained by electronic structure calculations showing that p-orbitals play an important part in the formation of chemical bonds between a vacancy and an atom of any of the four inert-gas elements in comparison with helium, where the latter contains only 1s2 electrons in the outer shell. The binding energies of a helium atom trapped by five different defects (He-v, Ne-v, Ar-v, Kr-v, Xe-v, where v denotes a vacancy in bcc-W) are all in excellent agreement with experimental data derived from thermal desorption spectroscopy. Attachment of He clusters to inert gas impurity atom traps in tungsten is analysed as a function of the number of successive trapping helium atoms. Variation of the Young modulus due to inert-gas impurities is analysed on the basis of data derived from DFT calculations.

Quenching of the resonance state of krypton atoms in collisions with krypton and helium atoms

The processes of collision quenching of the resonance state of the krypton atom are studied by the absorption probe method in electron-beam-excited high-pressure He – Kr mixtures with a low content of krypton. The rate constants of plasmochemical reactions and are measured for the first time. The rate constants of similar reactions are refined for krypton in the metastable state.

Velocity selective bipolarization spectroscopy for laser cooling of metastable krypton atoms

We report a velocity selective bipolarization spectroscopy (VS-BPS) technique to generate a background-free, dispersion-like reference signal which is tunable over a wide range of frequency. In this technique, a pair of linearly polarized weak probe beams passing through a gas cell of metastable krypton (Kr*) atoms overlaps with a pair of counterpropagating circularly polarized strong pump beams derived from an independently tunable control laser. The polarization spectroscopy signals from the two probe beams, after subtraction, result in the VS-BPS signal. The spectral shifting in the VS-BPS signal can be achieved by tuning the frequency of the control laser. The dependence of the amplitude and slope of the VS-BPS signal on the RF power used for excitation of Kr atoms in the gas cell and on the power of pump beams has been studied. The frequency stability of a diode laser locked with VS-BPS signal has been found to be better than the frequency stability of the laser locked with a saturated absorption spectroscopy signal. The VS-BPS signal is finally used for stabilization and tuning of the cooling laser frequency for a magneto-optical trap for Kr* atoms.

All-optical production and trapping of metastable noble-gas atoms down to the single-atom regime

The determination of isotope ratios of noble-gas atoms has many applications, e.g., in physics, nuclear-arms control, and Earth sciences. For several applications, the concentration of specific noble-gas isotopes (e.g., Kr and Ar) is so low that single-atom detection is highly desirable for a precise determination of the concentration. As an important step in this direction, we demonstrate the operation of a krypton atom trap trace analysis (ATTA) setup based on a magneto-optical trap (MOT) for metastable Kr atoms excited by all-optical means. Compared to other state-of-the-art techniques for preparing metastable noble-gas atoms, all-optical production is capable of overcoming limitations regarding minimal probe volume and avoiding cross-contamination of the samples. In addition, it allows for a compact and reliable setup. We identify optimal parameters of our experimental setup by employing the most abundant isotope 84Kr, and demonstrate single-atom detection within a 3D MOT.

Loading of a krypton magneto-optical trap with two hollow laser beams in a Zeeman slower

A significant enhancement in the number of cold atoms in an atomic-beam-loaded magneto-optical trap (MOT) for metastable krypton atoms is observed when hollow laser beams are used in a Zeeman slower instead of a Gaussian laser beam. In the Zeeman slower setup, a combination of two hollow laser beams, i.e., a variable-diameter hollow beam generated using a pair of axicon lenses superimposed on a fixed-diameter hollow beam, has been used to reduce the longitudinal velocity of the atoms in the atomic beam below the capture speed of the MOT. The observed enhancement in the number of atoms in the MOT is attributed to reduced destruction of the atom cloud in the MOT and increased cooling of the off-axis atoms in the atomic beam, resulting from the use of hollow beams in the Zeeman slower.

Bonding xenon and krypton on the surface of uranium dioxide single crystal

Abstract We present density functional theory (DFT) calculation results of krypton and xenon atoms interaction on the surface of uranium dioxide single crystal. A pseudo-potential approach in the generalised gradient approximation (GGA) was applied using the ABINIT program package. To compute the unit cell parameters, the 25 atom super-cell was chosen. It has been revealed that close to the surface of a potential well is formed for xenon and krypton atom due to its interaction with the atoms of oxygen and uranium. Depth and shape of the well is the subject of ab initio calculations in adiabatic approximation. The calculations were performed both for the case of oxygenic and metallic surfaces. It has been shown that the potential well for the oxygenic surface is deeper than for the metallic surface. The thermal stability of immobilising the atoms of krypton and xenon in the potential wells were evaluated. The results are shown in graphs.