Electron spectroscopy (UPS(HeI and II) and metastable impact electron spectroscopy (MIES)) applied to molecular surfaces: the interaction of atoms and molecules with solid water

We report studies of the interaction of atoms and molecules with solid molecular surfaces, water in particular, by combining photoelectron spectroscopy, UPS with HeI and II, and metastable impact electron spectroscopy (MIES). In MIES charge exchange processes of the Auger-type taking place between metastable He atoms and the surface under study are utilized to gain information on their electronic structure. The MIES spectra give a rather direct image of the surface DOS. We concentrate on the following processes taking place on water films produced at 80 K:(1) Interaction of Na Atoms with Amorphous Solid H₂O Films: emphasis was on the role of the 3sNa electrons in the water dissociation process. In order to make a detailed comparison with density functional theory (DFT), DOS (density of states) information is compared with the MIES spectra. Our results are consistent with the theoretical prediction that the 3s-electron is delocalized from the Na-core and trapped (solvated) between the Na-core and water molecules of the surrounding water shell.(2) Ionization and Solvation of NaCl Interacting with Amorphous Solid Water: at 90 K there is no interpenetration of H₂O and NaCl. However, ionic dissociation of NaCl takes place when H₂O and NaCl are in direct contact. At 105 K the solvation of the ionic species Cl⁻ and Na⁺ becomes significant. The desorption of H₂O from the mixed film takes place between 145 and 170 K; those species bound ionically to Na⁺ and Cl⁻ are removed last.(3) The Interaction of PBTs (persistent, bio-accumulative, and toxic substances), chlorobenzene and chlorophenyl, with amorphous solid water: the organic layers produced at 80 K were annealed up to 200 K under in situ control of MIES and UPS. The different behaviour of the interfaces for the three studied cases is traced back to the different mobilities of the molecules with respect to that of water. The interaction between H₂O and the benzene derivatives is discussed on the basis of qualitative free energy profiles.     

Pseudogap in Fermionic Density of States in the BCS-BEC Crossover of Atomic Fermi Gases

We study pseudogap behaviors of ultracold Fermi gases in the BCS-BEC crossover region. We calculate the density of states (DOS), as well as the single-particle spectral weight, above the superfluid transition temperature T _c including pairing fluctuations within a T-matrix approximation. We find that DOS exhibits a pseudogap structure in the BCS-BEC crossover region, which is most remarkable near the unitarity limit. We determine the pseudogap temperature T ^* at which the pseudogap structure in DOS disappears. We also introduce another temperature T ^** at which the BCS-like double-peak structure disappears in the spectral weight. While one finds T ^*>T ^** in the BCS regime, T ^** becomes higher than T ^* in the crossover and BEC regime. We also determine the pseudogap region in the phase diagram in terms of temperature and pairing interaction.     

Synthesis and characterization of spherical ZrO2:Eu phosphors by spray pyrolysis process

Europium doped zirconia (ZrO₂:Eu³⁺) powder phosphors consisting of spherical, dense and submicrometer size particles were successfully synthesized by a spray drying process followed by a post annealing treatment process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscope (SEM), photoluminescence (PL) spectra as well as lifetimes were utilized to characterize the prepared samples. The results of XRD indicated that the samples began to crystallize at 500 °C, and the crystallinity increased with increasing the annealing temperature. The powders with metastable tetragonal symmetry were obtained at relatively low temperature. The effects of annealing temperature, the Eu³⁺ concentration as well as the morphology on the PL intensity were investigated in this work.     

Nucleation studies and surface SHG analysis of l-arginine phosphate monohydrate (LAP) family crystals

The metastable zone width and the nucleation parameters, such as interfacial tension, radius of the critical nucleus and critical free energy change have been estimated for pure l-arginine phosphate (LAP), potassium thiocynate (KSCN) mixed LAP (LAP:KSCN) and sodium sulfite (Na₂SO₃) mixed LAP LAP:Na₂SO₃ single crystals. Pure and additive mixed LAP single crystals are grown by slow cooling technique. The surface second harmonic generation (SHG) analysis is done on (100) face of the grown crystals. The SHG intensity on (100) face of the crystals are measured.     

Optical characterization of Bi doped SrS nanophosphors

Optical properties of Bi³⁺ doped SrS nanophosphors synthesized by solid state diffusion method in the presence of sodium thiosulfate as a flux have been reported. UV-vis absorption and photoluminescence (PL) spectra of SrS phosphors doped with trivalent Bi³⁺ ions either alone or in combination with charge compensating ions, were also studied. These studies reflect that the incorporation of Bi³⁺ into host lattice is facilitated by the charge compensating Na⁺ ions. PL emission for SrS:Bi shows a peak at 481 nm at an excitation wavelength of 430 nm, which is attributed to the transition from the ³P₁ to ¹S₀ states of Bi³⁺. We have also investigated the effect of different dopant concentrations on PL emission intensity.     

KxNa1 − xNbO3 powder synthesized by molten-salt process

K_xNa_1 − xNbO₃ ceramic powders have been successfully synthesized in different salts (NaCl, KCl, NaCl-KCl). Our results reveal that K_xNa_1 − xNbO₃ powders with single-phase perovskite structure can be formed at a low temperature such as 750 °C. The type of salts has significant effects on the morphology and chemical composition of the powders. As Na⁺ has a higher diffusing rate and occupies the A-site in the perovskite structure more easily as compared to K⁺, the powder contains only a small amount of K⁺ (x ∼ 0.10) when it is synthesized according to formula K_0.5Na_0.5NbO₃ and in a flux containing the same molar content of Na⁺ and K⁺. By using a NaCl or KCl salt, the K⁺ concentration x can be adjusted to almost 0 and 0.77, respectively.     

Hysteresis at First-Order Wetting Transitions of 4He on Weak-Binding Substrates

⁴ He shows a first-order wetting transition on the weak-binding substrates Cs and Rb, as predicted theoretically and observed in experiment by several research groups. In these systems, the lifetime of metastable wetting states appears to be unusually long. The origin of this effect continues to be subject of a debate focussing on the relevance of thermal vs. substrate-disorder induced barriers between metastable states. It is shown here that wetting hysteresis strongly depends on the chosen initial state and its boundary conditions: depending on them, different hysteretic effects are probed. Quantitative estimates of the lifetime of two representative metastable states allow to assess the relevance of the two physically different hysteresis mechanisms for experiment.     

Mass-Spectrometric Study of the Kinetics of Ionic and Molecular Sublimation of Sodium Chloride Single Crystals

A mass-spectrometric method is used to study the kinetics of ionic and molecular sublimation of sodium chloride single crystals. The positive ions Na⁺, Na₂ ⁺, and Na₂Cl⁺ were identified in the mass spectrum in the temperature range 820–970 K. Up to 970 K no emission of negative ions was observed. The temperature dependence of thermal ion currents, lnI _i – (1/T), exhibits a maximum and a minimum successively with increasing temperature. These features are identified with a change in the electrical properties of the surface. It is inferred that in the intrinsic temperature range the sign of the surface charge in NaCl is negative and thus the Gibbs free energy of formation of cation vacancies is greater than that of anion vacancies. In the electron impact ionization mass spectrum of molecular fluxes vaporized from an open crystal surface, Na⁺, NaCl⁺, and Na₂Cl⁺ ions originating from NaCl and Na₂Cl₂ precursors are detected in the temperature range 779–982 K. The dimer-to-monomer ratio is found to be of the same order of magnitude in the cases of equilibrium and free-surface vaporization. However, the dependence on temperature of this ratio is different in these two cases. Mechanisms which account for this observation are discussed in light of the terrace–ledge–kink and surface charge models.     

Precipitation in LiF crystals doped with MgF2

X-ray, and optical and electron microscopy studies have been made of precipitation in single crystals of LiF doped with MgF₂. MgF₂ was found to nucleate only with difficulty, which probably accounts for the large MgF₂ precipitates that were observed. However, a metastable phase was discovered which readily nucleated, and, at low concentrations (e.g. 0.045 mol % MgF₂), it formed even on slow cooling. Its structure appears to be similar to that proposed by Suzuki for the NaCl/CdCl₂ system. The morphology of the metastable phase was seen by electron microscopy to be intersecting {100} platelets.On holding crystals containing the metastable phase at temperatures of about 300° C, particles of MgF₂ nucleated at, and grew at, the expense of the metastable phase. The observation of precipitation of the metastable phase at low Mg⁺⁺ ion concentrations is notable, for precipitation has almost certainly occurred during other studies of ionic conductivity, dielectric loss, and colour centres, etc., where crystals have been held at temperatures below 250° C.Solid-solubility measurements have been made to establish the MgF₂ solvus line at concentrations up to 6.5 mol % MgF₂ and the metastable phase solvus line up to 0.225 mol % MgF₂.Kinetic measurements indicate that the growth of the metastable phase is controlled by bulk diffusion of Mg⁺⁺ ions, whereas the growth of MgF₂ appears to be controlled by the motion of vacancy pairs.Part of a thesis presented for the degree of Doctor of Philosophy at Cornell University.     

Coordination of Ti4+ and Ge4+ ions in Na2O-TiO2-GeO2 glasses—an approach based on crystallization behaviour,X-ray absorption and IR spectroscopy

The coordination state of Ti⁴⁺ and Ge⁴⁺ ions in Na₂O-TiO₂-GeO₂ glasses has been investigated. Upon crystallization of the glasses, the thermodynamically metastable hexagonal GeO₂ crystals containing 4-fold coordinated Ti⁴⁺ ions together with stable Na₄Ge₉O₂₀ crystals containing 6-fold coordinated Ti⁴⁺ ions were precipitated. The distribution of Ti⁴⁺ ions between these two crystalline phases is a reflection of the coordination state of Ti⁴⁺ ions in the starting glass. The majority of Ti⁴⁺ ions are in 6-fold coordination at a small TiO₂/Na₂O ratio and they are contained in Na₂Ge₉O₂₀, while the majority of Ti⁴⁺ ions are in 4-fold coordination at a large TiO₂/Na₂O ratio and they are contained in GeO₂ crystals. Concentrations of 4- and 6-fold coordinated Ti⁴⁺ ions have been confirmed to be similar to that in Na₂O-TiO₂-SiO₂ glasses as measured by the X-ray absorption spectrometry of Ti⁴⁺ ions. The coordination state of Ge⁴⁺ ions in the glasses have been examined by infrared (IR) spectrometry. It is found that in Na₂O-TiO₂-GeO₂ glasses Ge⁴⁺ ions have 6-fold coordination preferentially, while Ti⁴⁺ ions have 4-fold coordination except at small contents of TiO₂, indicating that the addition of TiO₂ to Na₂O-GeO₂ glasses leads to the replacement of 4-fold coordinated Ge⁴⁺ ions by 4-fold coordinated Ti⁴⁺ ions.     

Luminescence and energy transfer in Er3+/Nd3+ ion-codoped Ge–In–S–CsBr chalcohalide glasses

This study investigates the emission properties of the Er³⁺/Nd³⁺ ions codoped 70GeS₂–10In₂S₃–20CsBr chalcohalide glasses. The vacuumed melt-quenching technique is employed to synthesize the glasses. The absorption spectra, upconversion and near-IR emission spectra as well as fluorescence decay curves are collected. With the increasing concentration of Er³⁺ ions, the lifetimes at 1073 nm for Nd³⁺ ions decrease from 538 to 420 μs under 808 nm excitation. Meanwhile, the lifetimes at 1540 nm for Er³⁺ ions decrease from 245 to 214 μs with the increasing concentration of Nd³⁺ ions. The emission spectra and lifetimes show that energy transfer exists between the Nd³⁺ and Er³⁺ ions. The luminescence and detailed energy transfer mechanisms are schematically proposed.     

Bombardment Induced Electron-Capture Processes at Sodium Halide Surfaces

Discrete features observed in the energy distribution of electrons emitted from ion-bombarded sodium halide surfaces can be attributed to a new type of collisional deexcitation mechanism. Such a mechanism involves sodium atoms in bombardment-excited autoionizing states that are the result of cascade collisions within the crystal lattice. This deexcitation process, in contrast to that for a metal, is not simply a consequence of the inner-shell lifetime of the initial collisionally excited sodium Na⁺* ion. Rather, the deexcitation consists of a sequence of lattice collisions during which the excited Na⁺* ion captures an electron to form the inner-shell-excited Na⁰* states responsible for the observed transitions. The formation of such autoionizing Na⁰* states is described within the framework of a new model in which excitation processes and localized collisional electron-transfer mechanisms are taken into account. These localized electron-transfer processes make possible new channels for electronic deexcitation, chemical dissociation, and defect production; they are critical for understanding inelastic ion-surface collisions in solids.     

The hot corrosion of 310 stainless steel with pre-coated NaCl/Na2SO4 mixtures at 750 °C

The high-temperature corrosion behavior of 310 stainless steel has been studied at 750 °C in air with 2 mg cm⁻² mixtures of various NaCl/Na₂SO₄ ratios. The corrosion behavior and morphological development were investigated by weight gain kinetics, metallographs, depths of attack, metal losses, and X-ray analyses. The results show that weight gain kinetics in simple oxidation reveals a steady-state parabolic rate law after 3 h, while the kinetics with salt deposits display multi-stage growth rates. NaCl is the main corrosive specie in high-temperature corrosion involving mixtures of NaCl/Na₂SO₄ and is responsible for the formation of internal attack. The most severe corrosion takes place with the 75% NaCl mixtures. Uniform internal attack is the typical morphology of NaCl-induced hot corrosion, while the extent of intergranular attack is more pronounced as the content of Na₂SO₄ in the mixture is increased.     

S2O2-3对DP980高强钢在NaCl水溶液中的点蚀行为的影响

钢是一种冷轧双相高强度结构钢,成本低且具备良好的强度和塑性,在海工领域有巨大的潜在应用价值,但其在苛刻海水环境下的耐腐蚀性能,尤其是耐点蚀等局部腐蚀的能力有待深入研究.本文采用动电位极化法对DP980高强钢在含S₂O₃^2-离子的NaCl水溶液中的点蚀行为进行了研究,结果发现：在纯硫代硫酸钠水溶液中,DP980高强钢不发生点蚀;在不同质量分数的NaCl水溶液中,DP980高强钢发生点蚀,且随着Cl^-离子浓度的增大,DP980高强钢试样的耐点蚀性能下降明显;向NaCl溶液中加入浓度为0.001、0.01、0.1、1 mol/L的Na₂S₂O₃时,DP980高强钢试样的点蚀敏感性降低,点蚀受到明显抑制;随着Na₂S₂O₃浓度的升高,DP980高强钢的腐蚀主要以均匀腐蚀的形式发生;当试样浸泡在不同质量分数(0.1%、1%、10%)NaCl的1 mol/L Na₂S₂O₃溶液后,对试样表面腐蚀产物及腐蚀形貌进行扫描电镜观察及能谱分析,显示试样均发生均匀腐蚀,腐蚀产物为铁的硫化物,且未发现明显的点蚀.     

Sub-Nanosecond Lifetime Measurement Using the Recoil-Distance Method

The electromagnetic properties of low-lying nuclear states are a sensitive probe of both collective and single-particle degrees of freedom in nuclear structure. The recoil-distance technique provides a very reliable, direct and precise method for measuring lifetimes of nuclear states with lifetimes ranging from less than one to several hundred picoseconds. This method complements the powerful, but complicated, heavy-ion induced Coulomb excitation technique for measuring electromagnetic properties. The recoil distance technique has been combined with heavy-ion induced Coulomb excitation to study a variety of problems. Examples discussed are: study of the two-phonon triplet in ¹¹⁰Pd, coupling of the β and γ degrees of freedom in ^182,184W, highly deformed γ bands in ¹⁶⁵Ho, octupole collectivity in ⁹⁶Zr, and opposite parity states in ¹⁵³Eu. Consistency between the Coulomb excitation results and the lifetime measurements confirms the reliability of the complex analysis often encountered in heavy-ion induced Coulomb excitation work.     

Thermal behaviour of NaBH4-sodalites with alumosilicate framework: Influence of cage water content and the surrounding conditions

Sodalites Na₈[AlSiO₄]₆(BH₄)_1.8(H₃O₂)_0.2 and Na₈[AlSiO₄]₆(BH₄)(H₃O₂)_0.5(CO₃)_0.25(H₂O) were examined to characterize the influence of the mixed cage fillings and the surroundings on the thermal behaviour of enclathrated BH₄⁻ and the stability of the alumosilicate framework. The samples were investigated in a NaCl matrix during temperature dependent IR-measurements (TIR) and by thermal analysis in inert atmosphere (helium) and in synthetic air. The thermal products were further characterized by XRD and SEM/EDS.It could be shown that the extent of the BH₄⁻ decomposition strongly depends on the cage-fillings of the individual sodalite phase and on the environmental experimental conditions. At low to medium temperatures hydrolysis of BH₄⁻ by water molecules from decomposition of H₃O₂⁻ and H₂O/CO₃²⁻ templates leads to B(OH)₄⁻/BH₃OH⁻ and via subsequent dehydration to BO(OH)₂⁻ and finally BO₂⁻. At elevated temperatures direct oxidation of BH₄⁻ to BO₂⁻ occurs inside the sodalite cages.Further heating results in sodalite framework destruction and finally formation of nepheline with some boron in its structure besides some unknown polymeric borates outside the alumosilicate phase. Framework stability of the sodalites was found to decrease in the direction → heating in NaCl → heating in He-atmosphere → heating in air.     

Effect of environment on the low cycle fatigue behaviour of cast nickel-base superalloy IN738LC

Low cycle fatigue tests were conducted on cast nickel-base superalloy IN738LC in vacuum and air and on specimens coated with a layer of NaCl and Na₂SO₄ at 900°C and two different strain rates. The fatigue life, determined in terms of the cyclic plastic strain, decreased markedly as the severity of the environment increased, ie in the order vacuum, air, NaCl + Na₂SO₄. The marked strain-rate effect found in air and saline environments disappeared when the experiments were conducted in a vacuum of 1.3 × 10^?2 Pa. Cyclic plastic deformation had the effect of producing coarsening which was associated with cyclic softening. Crack nucleation occurred in favourably oriented grains in vacuum while for air and for NaCl + Na₂SO₄ coated specimens the cracks always initiated at hot-corroded grain boundaries near the surface or generally at oxide spikes in surface-connected grain boundaries. The crack propagation from these nucleation sites was essentially transgranular for all environments studied.     

Spectroscopic studies of Sm3+-doped phosphate glasses

Optical properties of Sm³⁺ ion in phosphate glassy matrices were studied. The absorption, fluorescence, excitation spectra and decay patterns were obtained at room temperature. The oscillator strengths of the transitions betweenJ manifolds were calculated using Judd-Ofelt theory and compared with the ones obtained experimentally. Various laser intensity parameters such as J-O parameters, radiative transition probabilities, lifetimes, branching ratio, and integrated absorption cross-section were evaluated. Radiative lifetimes of the excited states were determined and used to obtain nonradiative transition rates and quantum yields. Integrated intensity and dynamics of the fluorescence originating from the excited states were also studied as a function of Sm³⁺ concentration.     

Elastic properties and lattice dynamics of alkali chalcogenide compounds Na2S,Na2Se and Na2Te

We report on first principles study of the elastic, vibrational and dielectric properties of the alkali chalcogenide compounds Na₂S, Na₂Se and Na₂Te using the pseudopotential method within the local density approximation and linear response theory. The calculated lattice constants for the studied compounds are in good agreement with available experimental data as well as with other theoretical results. The phonon dispersion curves and phonon density of states are calculated by using density functional perturbation theory. For Na₂S the experimental features of lattice dynamics data are well reproduced by our calculations. The width of the acoustic frequency range decreases with increasing chalcogen atomic number. The phonon densities of states show that Na⁺ ions are involved in the lower frequency modes in Na₂S. However, the inverse occurs in the case of Na₂Se and Na₂Te. The mean-square ionic displacements show that Na⁺ ions perform large thermal vibrations even at temperatures well below the melting point. The Born effective charge increases, while the IR oscillator strength decreases with anion atomic number.     

Structural dependence of ionic motion at interfaces between NaCl crystal surfaces and supersaturated solutions in crystallization process

The effects of surface structure and solution concentration on crystal morphology have been investigated through molecular dynamics (MD) simulation in the NaCl crystallization process. Four types of crystal surface structures were prepared for simulation. The probability of ion existence suggests that the solute ions were hardly taken into flat surfaces, while they were easily taken into roughs with some kinks and steps. This is because solute ions are more stable at a kink than at a terrace. The present simulated results suggest that the mechanism of crystal growth is dependent on the solution concentration. It was demonstrated that some ions are slotted on the crystal surface at low supersaturation after moving to the crystal surface. Under high supersaturation, the clusters are formed and adsorbed on the crystal surface. When comparing the crystal growth rates of Na⁺ and Cl⁻, the attractive force of Na⁺ from the crystal surface was found to be larger than that of Cl⁻. Moreover, the repulsive force of Na⁺ from the crystal surface was smaller than that of Cl⁻ in approaching the crystal surface. The adsorption of Na+ was also found to occur before that of Cl⁻.