First-principles study of electronic structure and insulating properties of uranium and plutonium dioxides

First-principles density functional theory calculations were carried out to investigate the electronic structure and the degree of 5f states localization of the Mott-Hubbard type insulators UO₂ and PuO₂. We used the fully relativistic cluster discrete variational method (RDV) with the local exchange-correlation potential. The energies of one-electron transition between occupied and vacant 5f^5/2 states of neighboring actinide atoms were evaluated on the base of the ground state and the excited state calculations. It is found that in UO₂ and PuO₂ the energy difference between 5f^5/2 levels of nearest metal sites in the lattice are close to 1.0 eV and 0.9 eV, despite the results of conventional band structure approach predicting that both oxides are good conductors.     

Optical excitations in electron-induced desorption of sodium from NaCl surface

Optical excitation processes stimulated by electron bombardment of alkali halides have been investigated under various experimental conditions. As an example a 500 eV e⁻ beam was used to bombard a (100) NaCl crystal. The work included measurements of both excited and ground state sodium atom yields as a function of the target temperature and the beam intensity, a cross-beam experiment with two electron beams: parallel and perpendicular to the sample surface, and a modulated beam measurement in search for time correlations between primary electrons and excited state production.The results were compared with the H-centre migration model and the results of recent publications. In particular it was found that a direct excitation of sodium atoms took place above the surface in collisions with the flux of primary and secondary electrons.     

Excitation, ionization, and electron capture cross sections for collisions of Li with ground state and excited hydrogen atoms

Using the available experimental and theoretical data, as well as the established cross section scaling relationships, a comprehensive cross section database for excitation, ionization and electron capture in collisions of Li³⁺ ions with ground state and excited hydrogen atoms has been generated. The critically assessed cross sections are represented by analytic fit functions that have the correct asymptotic behavior both at low and high collision energies. The derived cross sections are also presented in graphical form.     

Photon emission produced by particle-surface collisions

Visible, ultraviolet and infrared optical emission results from low-energy (20 eV–10 keV) particle-surface collisions. Several distinct kinds of collision induced optical radiation are discussed which provide fundamental information on particle-solid collision processes. Line radiation arises from excited states of sputtered surface constituents and backscattered beam particles. This radiation uniquely identifies the quantum state of sputtered or reflected particles, provides a method for identifying neutral atoms sputtered from the surface and serves as the basis for a sensitive surface analysis technique. Broadband radiation from the bulk of the solid is attributed to the transfer of projectile energy to the electrons in the solid. Continuum emission observed well in front of transition metal targets is believed to arise from excited atom clusters (diatomic, triatomic etc.) ejected from the solid in the sputtering process. Application of sputtered atom optical radiation for surface and depth profile analysis is demonstrated for the case of submonolayer quantities of chromium on silicon and aluminium implanted in SiO₂.     

Electron-impact excitation of argon: Optical emission cross sections in the range of 300-2500 nm

We present measurements of optical emission cross sections for excitation from the ground state of the Ar atom into over 185 excited atomic and ionic levels. Measurements were made at electron energies of 25, 50, and 100 eV, at a gas pressure of 5 mTorr. Due to radiation trapping of resonance levels, many of the cross sections depend on the target pressure. Detailed pressure dependence for over 50 levels is also provided. The energy dependence of the excitation cross sections for over 175 levels in the energy range of 0-250 eV are provided as fitted parameters for a standard analytical function.     

Database for inelastic collisions of sodium atoms with electrons, protons, and multiply charged ions

The available experimental and theoretical cross section data for inelastic collision processes of ground (3s) and excited (3p, 4s, 3d, 4p, 5s, 4d, and 4f) state Na atoms with electrons, protons, and multiply charged ions have been collected and critically assessed. In addition to existing data, electron-impact cross sections, for both excitation and ionization, have been calculated using the convergent close-coupling approach. In the case of proton-impact cross section, the database was enlarged by new atomic-orbital close-coupling calculations. Both electron-impact and proton-impact processes include excitation from the ground state and between excited states (n = 3-5). For electron-impact, ionization from all states is also considered. In the case of proton-impact electron loss, cross sections (the sum of ionization and single-electron charge transfer) are given. Well-established analytical formulae used to fit cross sections, published by Wutte et al. and Schweinzer et al. for collisions with lithium atoms, were adapted to sodium. The “recommended cross sections” for the processes considered have been critically evaluated and fitted using the adapted analytical formulae. For each inelastic process the fit parameters determined are tabulated. We also present the assessed data in graphical form. The criteria for comprehensively evaluating the accuracy of the experimental data, theoretical calculations, and procedures used in determining the recommended cross sections are discussed.     

Ultrafast band-structure variations induced by fast Au ions in BeO

Auger-electron spectra associated with Be atoms in the pure metal lattice and in an oxide have been investigated for 1.8 MeV/u ¹²⁹Au⁴¹⁺ ions and 2.7 keV primary electrons. The excitation and local energy transfer by such fast primary particles in solids is dominated by electronic processes. The electron-induced spectrum is compared to calculated band-structure results and it is relatively well understood. For the heavy-ion case, however, we observe a significant variation of the Auger electron spectrum, related to a variation of the electronic band structure. This spectrum points to a formation of a metal-like meta-stable electronic density of states.     

Angular distributions of Ni and Ti atoms sputtered from a NiTi alloy under He and Ar ion bombardment

The angular distributions of sputtered components were measured for NiTi polycrystalline alloy under 9 keV Ar⁺ and He⁺ ions bombardments with various fluences in ultrahigh vacuum. Combination of Rutherford Backscattering Spectrometry (RBS) and Auger Electron Spectrometry (AES) techniques allowed us to observe enhanced concentration of Ni over a layer with thickness comparable to a primary He⁺ ions penetration depth due to selective sputtering of Ti atoms and radiation-induced diffusion processes. A preferential emission of Ni atoms towards the surface normal was observed during bombardment by both He⁺ and Ar⁺ ions. More forward-peaked “over-cosine” angular distributions of sputtered Ni in comparison with those for Ti atoms have been measured. Nonstoichiometric sputtering of NiTi alloy dependent on emission angle was observed for bombardment fluence of He⁺ well below that needed for the steady-state altered layer formation. To explain the peculiarities of NiTi sputtering, an interpretation is discussed in terms of sputtering due to backscattered He⁺ ions.     

Atomic line spectra from electron-bombarded KCl:Tl and KBr:Tl crystals

Optical emission spectra obtained from electron-bombarded KCl:Tl and KBR:Tl crystals are reported. The spectra consist of sharp lines and broad bands. The broad bands are due to the characteristic Tl⁺ emission. The sharp lines coincide in energy with emission lines from electronic transitions of isolated neutral potassium atoms. The production of the excited state potassium atoms was found to depend on the incident beam current. A linear dependence was found in the low current region, a third power dependence for intermediate currents, and greater than third power dependence for high currents.     

The fragmentation of sputtered cluster ions and their contribution to secondary ion mass-spectra

The emission of cluster ions Ta⁺_n Nb⁺_n, Cu⁺_n and Ag⁺_n (n ⩽ 15) from samples of Ta, Nb, Cu, Ag due to projectiles has been investigated. These clusters have been found to be emitted from the surface in an excited state. On the whole way from the target to the ion collector of the mass-spectrometer these ions can undergo monomolecular fragmentation as a result of excitation energy redistribution inside the ion with the formation of stable (energetically advantageous) fragmented ions with a smaller number of atoms. The quantity of these fragmented ions is so large that the ion composition usually recorded by secondary ion mass spectrometry (SIMS) is determined to a great extent by monomolecular fragmentations of “parent” ions and does not reflect the original composition of the latter ones as emitted from the target.     

Hartree-Fock-Roothaan wavefunctions for diatomic molecules: III first-row heteronuclear systems,AB, AB±, AB1

Tables of electronic wavefunctions are presented for almost all heteronuclear diatomic systems which arise from combinations of the first-row atoms Li through F. These wavefunctions are obtained for the experimental (or theoretical) internuclear separation and are expansion solutions to the Restricted Hartree-Fock equations with extensive sets of Slater-type functions. The repertory includes the ground states of the systems indicated, several low-lying excited states, and a few positive and negative ions. The results are arranged into sequences for fluorides, AF, oxides, AO, nitrides, AN, etc. The calculations are, in general, restricted to states involving either closed-shell electronic configurations or open-shell configurations with only one incompletely filled open shell. Several of these systems are of doubtful existence, that is, they have not been experimentally characterized, and some should be regarded as arising only from the repulsive interaction of the two atoms involved.     

The influence of projectile charge state on ionization probabilities of sputtered atoms

The ionization probability of atoms sputtered from a clean polycrystalline metal surface was measured for different charge states of the projectile used to bombard the sample. More specifically, a polycrystalline indium surface was irradiated with Ar⁺ and Ar⁰ beams of energies between 5 and 15 keV, and In⁺ secondary ions and neutral In atoms emitted from the surface were detected under identical experimental conditions regarding the sampled emission angle and energy. The resulting energy integrated ionization probability of sputtered In atoms is consistently found to be smaller for neutral projectiles, the difference decreasing with decreasing impact energy. The observed trends agree with those measured for kinetic electron emission, indicating that secondary ion formation is at least partly governed by kinetic substrate excitation.     

Plasmon excitation and electron promotion in the interaction of slow Na ions with Al surfaces

We studied the energy distributions of electrons emitted in the interaction of slow Na⁺ ions with polycrystalline Al surfaces. To study sub-threshold plasmon excitation we performed measurements as a function of emission angle which showed the excitation of bulk plasmons, confirming the kinetic nature of the excitation process.

Electron spectra show narrow transition lines due to Auger decay in vacuum of sodium projectiles excited in the 2p-shell by electron promotion in collisions with Al target atoms. Several previously unidentified transition lines could be attributed to the autoionization of doubly excited projectiles.     

Temperature Dependence of Energy Degradation in Organic Liquid Scintillators

Under high energy excitation, organic molecules are generally excited to electronic states above the first excited state. Since, at not too high solute concentrations, energy transfer in organic liquid scintillators occurs via the first excited electronic state, the efficiency of degradation to this state affects the overall light output. Experimental results for a number of systems, at temperatures from 20°C to 24°C are reported. It was found that the efficiency of degradation to the first excited state generally decreases with increasing temperature, with some variation among the solvents tested. It was found that; the greater the efficiency of degradation to the first excited state at 20°C, the less this efficiency varies with temperature.     

Implantation profiles and sputtering studied by detecting the optical radiation from sputtered particles during ion bombardment

A method of sputter-etching and simultaneous registration of the emitted light is used to obtain depth profiles of implanted Li atoms. Measured mean penetration depths (R_p) and straggling values (ΔR_p) for implanted Li into Ag, V, Si and Al are reported. We find e.g. for 10 keV Li⁺ into $\text{V: R}{}_{\mathrm{n}}\text{= 400 ± 30}\text{A}\text{?}$ and $\text{ΔR}{}_{\mathrm{p}}\text{= 250 ± 30}\text{A}\text{?}$. Known implanted profiles are used as a scale of depth to determine sputtering yields of keV He⁺ ions. With this new method the sputtering yield of 40 keV He⁺ ions bombarding Ag was found to be 0.095 ± 0.020. Sputtering through a thin film is also used to determine sputtering yields. A remarkable increase in the light intensity from excited Ag I atoms is observed during sputtering through the interface between an Ag film and the underlying Al substrate. This is found to be due to a change in the excitation mechanism. Continuous features in the observed emission spectrum have in a few cases been identified as originating from deexcitation radiation of molecules formed in the sputtering process.     

利用15N(d,p)16N反应研究16F低能级质子宽度

¹⁶F是质子滴线附近的奇特原子核,它的所有态均不稳定,会发生质子衰变。目前¹⁶F前4个态的自旋宇称及激发能已通过实验精确测定,但能级宽度仍存在较大分歧。本工作通过¹⁵N(d,p)¹⁶N反应角分布的高精度测量,确定了¹⁶N基态和前3个激发态的谱因子;进而根据镜像核的电荷对称性,用¹⁶N的中子谱因子导出了¹⁶F基态和前3个激发态的质子宽度分别为（29.9±4.1） keV、（108±13） keV、（5.04±0.48） keV和（14.5±1.4） keV。本工作通过一个独立的实验方法为¹⁶F的质子宽度提供了一个重要的交叉检验。     

Color center formation in silica glass induced by high energy Fe and Xe ions

Silica glass samples were implanted with 1.157 GeV ⁵⁶Fe and 1.755 GeV ¹³⁶Xe ions to fluences range from 1 × 10¹¹ to 3.8 × 10¹² ions/cm². Virgin and irradiated samples were investigated by ultraviolet (UV) absorption from 3 to 6.4 eV and photoluminescence (PL) spectroscopy. The UV absorption investigation reveals the presence of various color centers (E′ center, non-bridging oxygen hole center (NBOHC) and ODC(II)) appearing in the irradiated samples. It is found that the concentration of all color centers increase with the increase of fluence and tend to saturation at high fluence. Furthermore the concentration of E′ center and that of NBOHC is approximately equal and both scale better with the energy deposition through processes of electronic stopping, indicating that E′ center and NBOHC are mainly produced simultaneously from the scission of strained Si-O-Si bond by electronic excitation effects in heavy ion irradiated silica glass. The PL measurement shows three emissions peaked at about 4.28 eV (α band), 3.2 eV (β band) and 2.67 eV (γ band) when excited at 5 eV. The intensities of α and γ bands increase with the increase of fluence and tend to saturation at high fluence. The intensity of β band is at its maximum in virgin silica glass and it is reduced on increasing the ions fluence. It is further confirmed that nuclear energy loss processes determine the production of α and γ bands and electronic energy loss processes determine the bleaching of β band in heavy ion irradiated silica glass.     

Laser sputtering in the electronic excitation regime: Comparison with electron and ion sputtering

Sputtering induced by photons and electrons is surveyed. Three types of electronic excitation are distinguished: (a) valence-electron excitation at a low photon density, (b) core excitation, and (c) valence-electron excitation at a high photon density that can be attained by laser irradiation. The survey of existing data includes only those closely related to the mechanism, such as the absolute yield, the dependence of the yield on energy and intensity of incident particles and both energy and angular distributions of the emitted particles. In interpreting these results, emphasis is placed on the role played by the electron-lattice coupling and on two crucial factors for sputtering due to electronic excitation, namely localization and instability. For sputtering induced by valence-electron excitation, localization is suggested to arise from self-trapping. It is pointed out that the self-trapped excitons in some specific solids have adiabatic instability but in others self-trapped excitons have adiabatic instability when a valence electron associated with the self-trapped exciton is excited to form two holes.     

Influence of the projectile charge state on the ionization probability of sputtered particles

We present a computational study of the effect of the projectile charge state on secondary ion formation in sputtering. A molecular dynamics simulation of an atomic collision cascade is combined with a kinetic excitation model including electronic friction and electron promotion in close atomic collisions. The model is extended to account for potential excitation following the bombardment with a highly charged ion (HCI). The spatial spreading of the excitation generated in the cascade is treated in an diffusive approach. The excitation energy density profile obtained this way is parametrized via an effective electron temperature, which is then used to calculate the ionization probability of each sputtered atom in terms of a simple charge exchange model. The results obtained for the impact of a 5 keV Ag atom onto a solid silver surface show that the average ionization probability increases from 4.7×10^-4 for a neutral projectile to 5.4×10^-4 for a highly charged projectile ion with a total ionization energy of 576 eV.     

Thermoluminescence, photoluminescence, photo-stimulated luminescence and optical studies on X-ray irradiated KBr:Ce crystals

This paper reports thermoluminescence (TL) glow and emission, optical absorption and optically stimulated emission studies made on Ce³⁺ doped KBr single crystals irradiated at room temperature. Optical absorption and photoluminescence studies confirm the presence of cerium ions in the trivalent state. A broad and intense blue emission around at 290 and 390 nm, attributable to the transition from 5d excited state to the 4f¹ ground state of Ce³⁺ ions could be observed in the photoluminescence emission. Two TL glow peaks were observed at 374 and 422 K. Presence of characteristic emission due to Ce³⁺ ions in the optically stimulated emission at the F-band confirms the participations of the Ce³⁺ ions in the defect production and recombination processes.