Heavier Noble Gas Atoms Research Articles

Coordination of noble gases by alkali-metal-like superatom cation Be3 B.

Quantum chemical calculations were carried out to investigate the noble gas binding ability of Be3 B+ cluster. Calculations reveal that heavier noble gas atoms (ArXe) form stable complexes with this cluster. Detailed bonding analyses reveal that the noble gas atoms act as donor fragment in the formation of Ng → Be donor-acceptor bonds. Three noble gas atoms can consecutively form bonds with the Be atom of the Be3 B+ cluster.

Noble-gas atoms characterized by hyperfine frequency shift of lithium atom.

We report an experimental and theoretical study on the shift of the hyperfine splitting frequency of ground-state Li atoms in noble gases, He, Ne, Ar, and Xe. The frequency shift is due to the change in the electron-spin density at the Li nuclei induced by collisions to the noble-gas atoms. The electron density is calculated along the interatomic distance in a pseudopotential and a dispersion potential. Based on the measured and the calculated frequency shifts, we find the importance of attractive force in collisions to helium as well as heavy noble-gas atoms. Taking advantage of the simple energy structure of the Li atom, we obtain the s wave scattering length for free electrons on noble-gas atoms by using the hyperfine splitting frequency as a precise measure.

Chemi-Ionization Reactions and Basic Stereodynamical Effects in Collisions of Atom-Molecule Reagents.

A new theoretical method, developed by our laboratory to describe the microscopic dynamics of gas-phase elementary chemi-ionization reactions, has been applied recently to study prototype atom–atom processes involving reactions between electronically excited metastable Ne*(3P2,0) and heavier noble gas atoms. Important aspects of electronic rearrangement selectivity have been emphasized that suggested the existence of two fundamental microscopic reaction mechanisms. The distinct mechanisms, which are controlled by intermolecular forces of chemical and noncovalent nature respectively, emerge under different conditions, and their balance depends on the collision energy regime investigated. The present paper provides the first step for the extension of the method to cases involving molecules of increasing complexity, whose chemi-ionization reactions are of relevance in several fields of basic and applied researches. The focus is here on the reactions of Ne* with simple inorganic molecules as Cl2 and NH3, and the application of the method discloses relevant features of the reaction microscopic evolution. In particular, this study shows that the balance of two fundamental reaction mechanisms depends not only on the collision energy and on the relative orientation of reagents but also on the orbital angular momentum of each collision complex. The additional insights so emphasized are of general relevance to assess in detail the stereodynamics of many other elementary processes.

Positronium collisions with atoms and molecules

We review recent theoretical efforts to explain observed similarities between electron-atom and positronium(Ps)-atom scattering which also extends to molecular targets. In the range of the projectile velocities above the threshold for Ps ionization (break-up) this similarity can be explained in terms of quasi-free electron scattering and impulse approximation. However, for lower Ps velocities more sophisticated methods should be developed. Our calculations of Ps scattering by heavy noble-gas atoms agree well with experiments at Ps velocities above the Ps ionization threshold. However, in contrast to electron scattering cross sections, at lower velocities they exhibit maxima whereas the experimental cross sections tend to decrease toward lower velocities indicating the same similarity with electron scattering cross section observed above the threshold. Our preliminary results for Ps-N2 scattering confirm experimental observation of a resonance similar to the ∏g resonance in electron-N2 scattering.

Influence of radiation damage on the thermal properties of silicon carbide implanted with heavy noble gas ions

Diffusion of heavy noble gas atoms in irradiation damaged single crystalline silicon carbide and the thermal etching of it is investigated at temperatures of 1300°C and 1400°C. For this purpose 360keV krypton and xenon ions were implanted in commercial 6H-SiC wafers at 600°C, which is far above the critical amorphization temperature of the target material. Width broadening of the implantation profiles and the retention of krypton and xenon during isothermal annealing was determined by RBS-analysis, whilst damage profiles were simultaneously obtained by α-particle channelling. No diffusion and no loss of the implanted species is detected in the implanted samples after isothermal annealing for 40h at 1400°C. However, thermal etching of the target material is observed at both annealing temperatures and leads at 1400°C to a significant shift of the implantation profile towards the surface due to sublimation. RBS analysis shows that this occurs mainly during the initial stage of isothermal annealing, while surface loss during prolonged annealing is minimal. The resulting topographical modification of the surface during annealing was studied by scanning electron and atomic force microscopy. It indicates that the observed phenomenon is due to a relatively strong dependence of thermal etching on the defect density in the surface region, while the evolving surface roughness seems not to play a decisive role.

Modifying the Fullerene Surface Using Endohedral Noble Gas Atoms: Density Functional Theory Based Molecular Dynamics Study of C70O3

We have performed a series of ab initio molecular orbital and molecular dynamics calculations to ascertain the influence of an endohedral noble gas atom on the reactivity of the surface of the model system C(70)O(3). Our simulations show that the minimum energy pathways for the ozone ring-opening reaction are influenced by the presence of the endohedral atom. The effect is isomer dependent, with the enthalpy of the reaction increasing for a,b-C(70)O(3) and decreasing for e,e-C(70)O(3) when doped with the heavy noble gas atoms Xe and Rn.

Applicability of the lowest-order two-electron Breit–Pauli relativistic correction in many-electron heavy and super-heavy elements

The applicability of the lowest-order two-electron Breit–Pauli (2e-BP) relativistic correction incorporated with the infinite-order Foldy–Wouthuysen (IOFW) transformation was examined in heavy and super-heavy elements. Comparing with the non-relativistic Coulomb repulsion term, in cases of moderately heavy noble-gas atoms (He to Ar), the 2e-BP correction gave the SCF energies practically equivalent to those obtained with the Dirac–Fock/Coulomb method. The spin-orbit splittings were also corrected reasonably. On the other hand, in heavy noble-gas atoms (Kr to Rn), the 2e-BP correction tended to overshoot energy lowering, and in a super-heavy atom (E118), evidences of collapse of the 2e-BP correction were observed.

The importance of spin-orbit coupling and electron correlation in the rationalization of the ground state of the CUO molecule.

We present calculations at the relativistic coupled cluster theory that predict the (1)Sigma(0) (+) ground state of CUO to lie 58.2 kJ/mol below the first excited state, (3)Phi(2). This can be contrasted with the outcome of earlier density functional theory and complete active space second order perturbation theory (CASPT2) calculations that both predicted a (3)Phi(2) ground state upon inclusion of spin-orbit coupling in the calculations. Our result gives further justification to the interpretation of the measured frequency shifts of this species in various noble gas matrices as being caused by significant interaction between the uranium and the heavier noble gas atoms.

Noble gas–uranium coordination and intersystem crossing for the CUO(Ne)x(Ng)n(Ng = Ar, Kr, Xe) complexes in solid neon

Atomic uranium excited by laser ablation reacts with CO in excess neon to produce the novel CUO molecule, which forms weak complexes CUO(Ne)m with neon and stronger complexes CUO(Ne)x(Ng)n (Ng = Ar, Kr, Xe) when the heavier noble gas atoms are present. The heavier CUO(Ne)m − 1(Ng) complexes are identified through the effects of CO isotopic and Ng substitution on the neon matrix infrared spectra and by comparison to DFT frequency calculations on the model complexes CUO(Ng) (Ng = Ne, Ar, Kr, Xe). The U–C and U–O stretching frequencies of CUO(Ne)m − 1(Ng) complexes are slightly red shifted from the 1047 and 872 cm−1 frequencies for the 1Σ+ CUO ground state neon complex, which identifies singlet ground state CUO(Ne)m − 1(Ng) complexes in solid neon. The next singlet CUO(Ne)x(Ng)2 complexes in excess neon follow in like manner. However, stretching modes and the isotopic shifts of the higher CUO(Ne)x(Ng)n complexes approach those of the pure argon matrix CUO(Ar)n complex, which characterizes triplet ground state complexes by comparison to DFT frequency calculations.

Interaction of the Fe+ cation with heavy noble gas atoms

The diatomic iron–noble gas complexes FeAr+, FeKr+, FeXe+, and FeRn+ have been theoretically investigated by means of quantum-chemical calculations including an extensive treatment of electron correlation. Potential energy curves and spectroscopic constants for the lowest 4Δ and 6Δ states are derived from an open-shell coupled-cluster approach and the relative energies of all seven low-lying electronic states are evaluated by the multireference configuration interaction method. While in FeAr+ the lowest quartet and sextet states are found to be energetically almost degenerate, the heavier Fe+–noble gas molecules are predicted to exhibit 4Φ ground states. From a qualitative point of view bonding in these species is shown to be electrostatic in origin with intrinsically higher interaction energies for the quartet as compared to the sextet states. For calibration purposes, also an accurate calculation of the 4F(4s03d7)–6D(4s13d6) energy difference in the atomic Fe+ cation is provided.

J-mixing cross sections in the intermediate nD states of a Rydberg Cs atom.

We have presented a theoretical description of J-mixing (the transfer of excitation between fine structure) cross sections of a Rydberg Cs atom induced by thermal collisions with noble-gas atoms. The theory is based on the application of impluse approximation theory and a kind of model-potential wave-function. The results agree well with available recent experimental values for He, Ne, and Ar atoms. But for the heavier noble-gas atoms Kr and Xe, the theoretical cross sections are larger than the experimental ones

Relativistic two-channel calculations of the elastic scattering of slow electrons from Ar, Kr and Xe atoms

A relativistic non-local two-channel theory is applied to the elastic scattering of 20-200 eV electrons from heavier noble-gas atoms. Total inelastic cross sections are obtained for Ar, Kr and Xe in semi-quantitative agreement with experiment. The differential cross section and spin polarisation results for Ar and Kr agree with experimental data at about the same level as results from less sophisticated (one-channel) theories. For Xe, however, the two-channel model gives a clearly improved agreement with the experimental data.

Cascade annealing of tungsten implanted with 5 keV noble gas atoms: A computer simulation

The trapping of vacancies by implanted atoms is calculated. After low energy implantation (5 keV) of tungsten with heavy noble gas atoms most of the implanted atoms are in substitutional position with one or two vacancies closer than two lattice units. Under the influence of the lattice distortion around the implanted atoms the vacancies follow a preferential migration path towards the implant during annealing. With lattice relaxation simulations migration energies close to the implanted atom are calculated. Monte Carlo theory is applied to obtain trapping probabilities as a function of implant-vacancy separation and temperature. An estimate of the initial implant-vacancy separation follows from collision cascade calculations. The results show that nearby vacancies are trapped by the implanted atoms.

Energy and momentum transfer in scattering of low-energy atoms on metal surfaces

Abstract The scattering of low-energy atoms on metal surfaces is investigated theoretically, assuming that the adatom motion is coupled to surface phonons. The trajectory approximation is used. In the strong-coupling regime, in which many phonons are excited in the course of a scattering event, the results simplify considerably and take a universal form. Essentially, only one unknown parameter remains, the average energy loss δ, which can be determined by comparison with experiments. Details of the phonon dispersion relation and of the coupling constants are lost in the superposition of energy and momentum transfers of a large number of phonons. Explicit expressions for the scattering cross section are derived in this limit. The approach is very successful in analysing the angular and energy distributions of the direct fraction of heavy noble-gas atoms (Ar, Kr, Xe) scattered on metal surfaces.

Energy and momentum transfer in scattering of low-energy atoms on metal surfaces

The scattering of low-energy atoms on metal surfaces is investigated theoretically, assuming that the adatom motion is coupled to surface phonons. The trajectory approximation is used. In the strong-coupling regime, in which many phonons are excited in the course of a scattering event, the results simplify considerably and take a universal form. Essentially, only one unknown parameter remains, the average energy loss δ, which can be determined by comparison with experiments. Details of the phonon dispersion relation and of the coupling constants are lost in the superposition of energy and momentum transfers of a large number of phonons. Explicit expressions for the scattering cross section are derived in this limit. The approach is very successful in analysing the angular and energy distributions of the direct fraction of heavy noble-gas atoms (Ar, Kr, Xe) scattered on metal surfaces.

Photoionisation cross sections of excited noble-gas atoms and dimers

Quantum-defect calculations of the photoionisation cross sections of several excited states of noble-gas atoms and two excited states of He2 are described. Results show that cross sections for some states of heavy noble-gas atoms have pronounced minima and that excited states of atomic and diatomic helium have similar sections.