Luminescence properties of oxide coatings on aluminum alloys

This is a study of the luminescence properties of coatings formed on aluminum alloys by anodizing in electrolytic solutions based on oxalic, sulfuric, and tartaric-sulfonic acids. At least two emission centers, with band maxima in the ranges of 390–410 and 470–510 nm, can be reliably identified in the photoluminescence spectra. The first type of center is characterized by single-band photoluminescence excitation spectra and the second, by two-band spectra. An analysis of the two-band photoluminescence excitation (PLE) spectra in the range of 470–510 nm shows that the position of the narrow short-wavelength PLE spectrum near 272 nm is independent of the type of acid used in the anodization process. The position and shape of the other PLE spectral bands depend both on the type of acid used and on the processing of the alloy or alumina surfaces. It is assumed that defect-free alumina centers are responsible for the 272 nm PLE band, while the other photoluminescence bands are caused primarily by different divacancies of oxygen (F₂⁺ {F_2^+} , F ₂, and F₂⁺² {F_2^{+2}} centers) whose origin is governed by the type of electrolyte.     

Ultrafast delocalization of hydrogen atoms in allene in intense laser fields

Ultrafast delocalization of hydrogen atoms in allene (CH₂=C=CH₂) induced by intense laser fields was investigated by the Coulomb explosion coincidence momentum imaging method. On the basis of the kinetic energy distributions of the fragment ions produced through the two three-body Coulomb explosion pathways, C₃H₄³⁺ ? H⁺ + CH⁺ + C₂H₂⁺\mathrm{C}_{3}\mathrm{H}_{4}^{3+} \rightarrow \mathrm{H}^{+} + \mathrm{CH}^{+} + \mathrm{C}_{2}\mathrm{H}_{2}^{+} and C₃H₄³⁺ ? H⁺ + C₂H⁺ +CH₂⁺\mathrm{C}_{3}\mathrm{H}_{4}^{3+} \rightarrow \mathrm{H}^{+} + \mathrm{C}_{2}\mathrm{H}^{+} +\mathrm{CH}_{2}^{+}, and the proton maps for both pathways, it was shown that the decomposition proceeds in a stepwise manner as well as in a concerted manner. The time scale of the hydrogen migration within an allene molecule was estimated to be ∼20 fs.     

The ultimate sensitivity of a high-temperature microwave SQUID magnetometer operating in the hysteresis regime

A method for the determination of the noise spectral density in a high-temperature microwave SQUID operating in the hysteresis regime is developed. Under these conditions, the reflection coefficient serves as an output signal. It is shown that if a directional coupler used for extracting the reflected wave is placed as close to the SQUID loop as possible, the magnetometer can be designed as a microwave integrated circuit with a noise flux spectral density S_F ^1/2 < 10 ^{- 5} F₀ /\textHz^{\text0\text.5} ,\textwhere F_\text0 S_\Phi ^{1/2} < 10^{ - 5} \Phi _0 /{\text{Hz}}^{{\text{0}}{\text{.5}}} ,{\text{where }}\Phi _{\text{0}} , is the magnetic flux quantum.     

Laser-diode-excited Blue Upconversion in Tm3+/Yb3+-codoped TeO2-Ga2O3-R2O (R=Li, Na, K) Glasses

This paper reports on intense blue upconversion in Tm³⁺/Yb³⁺ codoped TeO₂-Ga₂O₃-R₂O(R=Li, Na, K) glasses upon excitation with commercial available laser diode (LD). Effects of alkali ions on the Raman spectra, thermal stability and spectroscopic properties of the tellurite-gallium glasses have also been investigated. Energy transfer and the involved upconversion mechanisms have been discussed. Intense blue upconversion emission centered at 476 nm along with a weak red emission at 650 nm has been observed upon excitation of 977 nm LD, assigned to the transitions of , and and/or of Tm³⁺, respectively. The blue upconversion intensity has a cubelike dependence on incident pump laser power, indicating a three-photon process. However, a quadratic dependence of the 476 nm upconversion intensity on the incident pump laser power has been observed when samples under excitation of 808 nm LD due to a two-photon absorption process. Enhanced upconversion luminescence have been observed with replacing K⁺ for Na⁺ and Li⁺.     

Upconversion blue emission dependence on the pump mechanism for Tm<Superscript>3+</Superscript>-heavy-doped NaY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

Upconversion blue emissions of Tm³⁺-ion-heavy-doped NaY(WO₄)₂ crystals are investigated with three different near-infrared pump mechanisms. The dependence of upconversion efficiency on the pump mechanism is analyzed from the scope of the concentration quenching effect. Three cross-relaxation processes, , , and , which influence the upconversion dominantly in the Tm³⁺-heavy-doped system, are demonstrated theoretically and experimentally. The results indicate that Yb³⁺ ions can weaken the concentration quenching effect of Tm³⁺ ions significantly so that the blue emission efficiency can be enhanced by one order of magnitude. At the same time, the wavelength of the pump source also has considerable influence on both the population of some crucial energy levels and the upconversion mechanism. Experiments show that the upconversion blue emission in Tm³⁺/Yb³⁺ co-doped NaY(WO₄)₂ crystal under 980-nm laser diode excitation is the most intensive of these three different near-infrared pump mechanisms. The conclusions are confirmed by spectra measurements and calculations of Judd–Ofelt theory and Miyakawa–Dexter theory. PACS 42.70.Hj; 78.55.-m     

Tripolyphosphate as Precursor for REPO4:Eu3+ (RE = Y, La, Gd) by a Polymeric Method

A modification of the Pechini method was applied to obtain luminescent rare earth orthophosphates. The developed synthetic route is based on the ability of the tripolyphosphate anion () to act both as a complexing agent and as an orthophosphate precursor. Heating of aqueous solutions containing RE³⁺, Eu³⁺, , citric acid, and ethylene glycol led to polymeric resins. The ignition of these resins at different temperatures yielded luminescent orthophosphates. The produced nanosized phosphors (YPO₄:Eu³⁺, (Y,Gd)PO₄:Eu³⁺, and LaPO₄:Eu³⁺) were analyzed by infrared and luminescence spectroscopies, X-ray diffractometry, and scanning electron microscopy.     

Electrical behavior of an octahedral layered OL-1-type manganese oxide material

OL-1-type material (birnessite) is synthesized by an oxidoreduction process. Different physicochemical techniques were used to characterize the obtained material. AC impedance spectroscopy results show processes associated to the electrical conduction in bulk and grain boundary at high frequency, and an ionic conduction at low frequency. Here σ′(ω) shows a universal Jonscher’s law behavior associated to the electron hopping and charge polarization, and the value of 8.39?×?10^?6 Ω^?1?cm^?1 found in the high frequency region at room temperature suggests its semiconductor nature. The combined results of AOS, TGA, and AA suggest the following chemical formula $ {\text{N}}{{\text{a}}^{ + }}_{{0.28}}\left( {{\text{M}}{{\text{g}}^{{2 + }}}_{{0.16}}{\text{M}}{{\text{n}}^{{4 + }}}_{{0.46}}{\text{M}}{{\text{n}}^{{3 + }}}_{{0.54}}} \right){{\text{O}}_{{2.03}}} \cdot 0.6{{\text{H}}_2}{\text{O}} $ . Finally, the XRD pattern is characteristic of OL-1-type materials, the BET area was 56,25 m² g^?1, and the behavior of N₂ isotherms suggests the presence of microporous and mesoporous structures. With the purpose of obtaining a better understanding of the ionic conductivity in these types of materials, magnesium exchange material was prepared and electrical properties at room temperature were analyzed. These results indicate that there is interplay among the structural, morphological, and textural properties with the electrical performance of these materials.     

Charge-transfer reactions in clusters excited with synchrotron radiation

Charge-transfer reactions are observed in a photoluminescence study of NF₃\rm NF_3-doped free krypton clusters. They show up in emissions from Kr⁺F^-\rm Kr^{+}F^{-}free excimers ejected from the clusters, and from excited Kr₂⁺F^-\rm Kr_2^{+}F^{-}and Kr₂⁺(NF₃)_m^-{\rm Kr}_2^{+}({\rm NF}_3)_m^{-} (m 3(m\geq 1) solvated in the clusters. The results show that reaction dynamics in clusters differs considerably from that in the gas and solid phases.     

Collisional damping of transverse waves in a plasma

The effect of collisions on transverse waves in a homogeneous, field free plasma is investigated by means of Gross-Krook collision model. The dispersion relation is calculated by assuming the collision frequency to be small andKλ _D ?1. The damping rate ω _I is obtained as $$\omega _I = \frac{{\nu _{ei} }}{2}\frac{{\omega _p^2 }}{{\omega _0^2 }}\left[ {1 + \frac{{3K^2 \lambda _D^2 \omega _p^2 }}{{\omega _0^2 }} - \frac{{K^2 \lambda _D^2 \omega _p^4 }}{{\omega _0^4 }}} \right] + \frac{{\nu _{ee} }}{2}\frac{{\omega _p^2 }}{{\omega _0^2 }}\left( {\frac{{K^2 \lambda _D^2 \omega _p^2 }}{{\omega _0^2 }}} \right)$$ where ω ₀ ² =c ² K ²+ω ₂ ^p , andv _ei andv _ee are electron-ion and electron-electron collision frequency respectively.     

Aggregate color center formation processes in lithium fluoride crystals after irradiation

Lithium fluoride crystals were irradiated by different doses of gamma photons at a temperature of 77 K. We measured the aggregation kinetics for the color centers with different annealing temperatures above the temperature of anion vacancy mobility. We show that the lifetimes of the vacancies decrease while the lifetimes of the F₂⁺ F_2^{+} centers increase as the irradiation dose increases. We explain these types of dependences based on the aggregation processes for color centers in the post-radiation period. We determine the time constants and energies (analogous to activation energies in the Arrhenius equation) for the various processes involving rise and fall in the concentration of aggregate color centers. Based on the experimental data obtained, we have established the processes forming F ₂ and F₃⁺ F_3^{+} centers in the post-radiation period. The F ₂ centers are formed when vacancies ν_a add to F₁^- F_1^{-} centers. Vacancies arising during irradiation of the crystal participate in their creation in the first fast stage. In the long final stage, vacancies are used which appear in the post-radiation period on occurrence of the reaction F₂⁺ F_2^{+} + H → ν_a + fluoride ion at the lattice site, where H is an interstitial fluorine atom. The F₃⁺ F_3^{+} centers are formed both by merging F₂⁺ F_2^{+} and F ₁ centers and as a result of addition of vacancies to F ₂ centers. In this case, vacancies are used that are generated not only during irradiation of the crystal but also in the post-radiation period. The rise in the concentration of F₃⁺ F_3^{+} centers occurs faster than the rise in the concentration of F ₂ centers.     

g-factors of rotational states in176Hf,177Hf,and180Hf

g-factors of rotational states in¹⁷⁶Hf and¹⁸⁰Hf were measured with the twelve detector IPAC-apparatus of our laboratory [1]. The natural radioactivity 3.78·10¹⁰y¹⁷⁶Lu and the 5.5 h isomer^180mHf were used which populate the ground-state rotational bands of¹⁷⁶Hf and¹⁸⁰Hf. The integral rotations ofγ-γ directional correlations in strong external magnetic fields and in static hyperfine fields of (Lu→Hf)Fe₂ and HfFe₂ were observed. The following results were obtained: $$\begin{array}{l} ^{176} Hf: g\left( {4_1^ + } \right) = + 0.334\left( {38} \right) \\ ^{180} Hf: g\left( {2_1^ + } \right) = + 0.305\left( {14} \right) \\ g\left( {4_1^ + } \right) = + 0.358\left( {43} \right) \\ {{ g\left( {6_1^ + } \right)} \mathord{\left/ {\vphantom {{ g\left( {6_1^ + } \right)} {g\left( {4_1^ + } \right)}}} \right. \kern-\nulldelimiterspace} {g\left( {4_1^ + } \right)}} = + 0.95\left( {12} \right) \\ \end{array}$$ . The hyperfine field in (Lu→Hf)Fe₂ was calibrated by observing the integral rotation of the 9/2^? first excited state of¹⁷⁷Hf populated in the decay of 6.7d¹⁷⁷Lu. Theg-factor of this state was redetermined in an external magnetic field as $$^{177} Hf: g\left( {{9 \mathord{\left/ {\vphantom {9 {2^ - }}} \right. \kern-\nulldelimiterspace} {2^ - }}} \right) = + 0.228\left( 7 \right)$$ . Finally theg-factor of the 2 ₁ ⁺ state of¹⁷⁶Hf was derived from the measuredg(2 ₁ ⁺ ) of¹⁸⁰Hf by use of the precisely known ratiog(2 ₁ ⁺ ,¹⁷⁶Hf)/g(2 ₁ ⁺ ,¹⁸⁰Hf) [2] as $$^{176} Hf: g\left( {2_1^ + } \right) = + 0.315\left( {30} \right)$$ .     

Photoluminescence and Optical Absorption of Cs2NaScF6:Cr3+

The main objective of this paper is the characterization of the spectroscopic properties of new materials that are prospective laser media. This approach allows for the comparison of the properties of the Cr³⁺ in different environments. Here, we have studied the photoluminescence and optical absorption of Cs₂NaScF₆:Cr³⁺ single crystals. On the basis of near-infrared luminescence measurements at 2, 77, and 300 K the observed lines originated from the Cr³⁺-centres were associated with the transition and the lifetimes were obtained. In spite of the quenching observed as a function of temperature at least 10% of the 2 K emission intensity for Cs₂NaScF₆ doped with 1% of Cr³⁺ remains at room temperature. Besides, the 2 K emission broad band could be well described in terms of normal modes of the octahedral complex [CrF₆]³⁻, and the Racah and crystal-field parameters calculated.     

The band spectrum of SbO: A new doublet system

The band spectrum of SbO was excited in a heavy current discharge from a 2000 volt D. C. generator. A new doublet system of bands occurring in the region λ 2800 toλ 3600 arising from a transition of the type²Δ_r?² Π _r was identified. The lower² Π _r state is found to be common to those of the three band systems reported earlier, which is in all probability the ground state of the SbO molecule. The band heads of the high frequency and low frequency components could be represented by the following quantum formulae:

$$\begin{gathered} ^2 \Delta _{\tfrac{3}{2}} - ^2 \Pi _{\tfrac{1}{2}} : \hfill \\ v = 29754 \cdot 6 + 570 \cdot 6 (v' + \tfrac{1}{2}) - 3 \cdot 52 (v' + \tfrac{1}{2})^2 - 820 \cdot 5 (v'' + \tfrac{1}{2}) + 4 \cdot 62 (v'' + \tfrac{1}{2})^2 \hfill \\ ^2 \Delta _{\tfrac{5}{2}} - ^2 \Pi _{\tfrac{3}{2}} : \hfill \\ v = 28044 \cdot 8 + 568 \cdot 1 (v' + \tfrac{1}{2}) - 3 \cdot 28 (v' + \tfrac{1}{2})^2 - 819 \cdot 2 (v'' + \tfrac{1}{2}) + 4 \cdot 62 (v'' + \tfrac{1}{2})^2 . \hfill \\ \end{gathered} $$     

Luminescence and radiation-induced color centers in anion-defective alumina crystals after high-dose irradiation

A method of UV spectroscopy was used to measure photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra in anion-defective alumina crystals exposed to high doses of gamma-radiation. An additional emission band in the range of 1.6–2.75 eV appears in the exposed crystals. Aggregate F₂-type centers in different charge states are responsible for this band. It was found that growing intensity of PL aggregate centers occurs at doses corresponding to saturation of dose response and is accompanied by a sharp drop in the intensity of F⁺-band in the PL spectrum resulting from combination of F⁺-centers into aggregates. Uncharged F₂-centers are formed when electrons are trapped by F₂⁺ and F₂²⁺-centers. The main role of F⁺-centers in radiation-induced transformations of color centers under high-dose irradiation of anion-defective alumina crystals was indicated.     

Universal estimate of the gap for the Kac operator in the convex case

The aim of this paper is to prove that ifV is a strictly convex potential with quadratic behavior at ∞, then the quotient μ₂/μ₁ between the largest eigenvalue and the second eigenvalue of the Kac operator defined on L²(? ^m ) by exp ?V(x)/2 · exp Δ_x · exp ?V(x)/2 where Δ_x is the Laplacian on ? ^m satisfies the condition: $${{\mu _2 } \mathord{\left/ {\vphantom {{\mu _2 } {\mu _1 {{ \leqslant \exp - \cosh ^{ - 1} (\sigma + 1)} \mathord{\left/ {\vphantom {{ \leqslant \exp - \cosh ^{ - 1} (\sigma + 1)} {2,}}} \right. \kern-\nulldelimiterspace} {2,}}}}} \right. \kern-\nulldelimiterspace} {\mu _1 {{ \leqslant \exp - \cosh ^{ - 1} (\sigma + 1)} \mathord{\left/ {\vphantom {{ \leqslant \exp - \cosh ^{ - 1} (\sigma + 1)} {2,}}} \right. \kern-\nulldelimiterspace} {2,}}}}$$ where σ is such that HessV(x)≥σ>0.     

An expansion of Hartree-Fock and correlation energies,relativistic corrections and the dipole matrix element in the powers of of 1/Z

Feynman diagrammatic technique was used for the calculation of Hartree-Fock and correlation energies, relativistic corrections, dipole matrix element. The whole energy of atomic system was defined as a polen-electron Green function. Breit operator was used for the calculation of relativistic corrections. The Feynman diagrammatic technique was developed for 〈H_B>. Analytical expressions for the contributions from diagrams were received. The calculations were carried out for the terms of such configurations as 1s² 2sⁿ¹ 2pⁿ² (2 ≧n₁≧ 0, 6≧ n₂ ≧ 0). Numerical results are presented for the energies of the terms in the form $$E = E_0 Z^2 + \Delta {\rm E}_2 + \frac{1}{Z}\Delta {\rm E}_3 + \frac{{\alpha ^2 }}{4}(E_0^r + \Delta {\rm E}_1^r Z^3 )$$ and for fine structure of the terms in the form $$\begin{gathered} \left\langle {1s^2 2s^{n_1 } 2p^{n_2 } LSJ|H_B |1s^2 2s^{n_1 \prime } 2p^{n_2 \prime } L\prime S\prime J} \right\rangle = \hfill \\ = ( - 1)^{\alpha + S\prime + J} \left\{ {\begin{array}{*{20}c} {L S J} \\ {S\prime L\prime 1} \\ \end{array} } \right\}\frac{{\alpha ^2 }}{4}(Z - A)^3 [E^{(0)} (Z - B) + \varepsilon _{co} ] + \hfill \\ + ( - 1)^{L + S\prime + J} \left\{ {\begin{array}{*{20}c} {L S J} \\ {S\prime L\prime 2} \\ \end{array} } \right\}\frac{{\alpha ^2 }}{4}(Z - A)^3 \varepsilon _{cc} . \hfill \\ \end{gathered} $$ Dipole matrix elements are necessary for calculations of oscillator strengths and transition probabilities. For dipole matrix elements two members of expansion by 1/Z have been obtained. Numerical results were presented in the form P(a,a′) = a/Z(1+τ/Z).     

Vacuum condensates in supersymmetric gluodynamics

We analyse the restrictions imposed by supersymmetric Ward identities on the dimension-six condensates in supersymmetric gluodynamics. It is shown that the system of the Ward identities admits a nonzero value for the four-fermion condensate \(\left\langle {\left( {\lambda \sigma _\mu T^a \bar \lambda } \right)^2 } \right\rangle _0 \) while 〈f ^abc F _μν ^a F _νρ ^b F _ρμ ^c 〉₀ must vanish.     

Observation of a strong correlation betweeng(2+γvib) andg(2+rot) in strongly deformed nuclei

The E2/M1 multipole mixing parameters of cascade transitions inγ-vibrational bands of¹⁵⁴Gd,¹⁶⁶Er and¹⁶⁸Er have been determined byγ-γ directional correlation measurements as: $$\begin{array}{l} \delta \left( {^{154} Gd\left( {3_\gamma ^ + \to 2_\gamma ^ + } \right)} \right) = - 4.3_{ + 2.1}^{ - 9.4} \\ \delta \left( {^{166} Er\left( {5_\gamma ^ + \to 4_\gamma ^ + } \right)} \right) = + 1.94_{ - 0.21}^{ + 0.23} \\ \end{array}$$ and $$\delta \left( {^{168} Er\left( {3_\gamma ^ + \to 2_\gamma ^ + } \right)} \right) = + 1.42_{ - 0.04}^{ + 0.04} $$ (with conversion data [15] taken into account) These data were used to deriveg(2⁺ γvib)?g(2⁺rot). The results, together withg-factors derived from direct measurements by IPAC and Mössbuer spectroscopy [10] or by use of transient fields [9, 31] exhibit a strong correlation between bothg-factors, i.e. ifg(2⁺rot) is largeg(2⁺ γvib) is small and vice versa. The most direct and most simple interpretation is the assumption of a more or less different density distribution of protons and neutrons in the nuclei.     

Eu2+ and Eu3+ based “concentrated phosphors” as converters for UV LED light: two approaches and two new examples

The absolute majority of phosphors are composed of a host lattice and some percentage of an activator. At higher activator concentrations the concentration quenching occurs. However, there are phosphors in which only minor quenching of the emission occurs with increasing of the activator content. Based on the existence of two different valence states of the Eu ion (2+ and 3+), two approaches for the development of “concentrated phosphors”, i.e. light emitting materials in which the activator ion is a main part of the crystal lattice, are discussed. In both approaches, reduced energy migration leading to the luminescence quenching is considered as a main condition to reach a high quantum efficiency of a concentrated phosphor. Two kinds of phosphors—Eu²⁺-doped alumosilicate and Eu³⁺-doped oxyfluoride—are used as an experimental basis for this discussion. Starting from the stoichiometric Ca_1-xEu_x²⁺Al₂Si₂O₈\mathrm{Ca}_{1-x}\mathrm{Eu}_{x}^{2+}\mathrm{Al}_{2}\mathrm{Si}_{2}\mathrm{O}_{8} anorthite and Eu³⁺OF oxyfluorides, the non-stoichiometric powders with Eu²⁺_0.92Al_1.76Si_2.24O₈\mathrm{Eu}^{2+}_{0.92}\mathrm{Al}_{1.76}\mathrm{Si}_{2.24}\mathrm{O}_{8}, Eu³⁺(O, F)_2,35 and Eu³⁺(O, F)_2,16 compositions were synthesized by a solid state reaction and investigated. It was shown that—in spite of the almost 100% Eu concentration—light converters with high quantum efficiency of more than 45% can be realized. A possible application of these materials as UV LED light converters for white light emitting diodes are discussed as well.     

Small magnetic polaron effect in lithium iron phosphates

The small polarons in LiFePO₄ are associated with the presence of Fe³⁺ ions introduced by the native defects in relative concentration in the samples known to be optimized with respect to their electrochemical properties. The nearest iron neighbours around the central polaron site are spin-polarized by the indirect exchange mediated by the electronic charge in excess. These small magnetic polarons are responsible for the interplay between electronic and magnetic properties that are quantitatively and self-consistently analysed. Comparison is made with other magnetic polaron effects in other members of the family of magnetic semiconductors to which this material belongs. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 September 2007.