Formation of Color Centers and Molecular Complexes with a Weak Hydrogen Bond in Lithium Fluoride Crystals Depending on the Form of OH<Superscript>¯</Superscript> Inclusion

The form of oxygen-containing inclusions in the LiF crystal was shown to be determined by the laws of isomorphic substitution and impurity accumulation in a melt during crystal growth in open air. Color centers and hydrogen-bonded complexes of different types appear in the LiF crystal after irradiation depending on the form of OH^– and Mg²⁺ inclusions. The radiative decay of OH^– ions and the properties of molecular complexes (MCs) with a weak hydrogen bond are responsible for the decreased efficiency of formation of the F- and F₂-color centers and increased concentration of the positively charged F ₂ ⁺ - and F ₃ ⁺ -centers. Radiochemical reactions involving hydrogen and fluorine atoms and ions that compete with the formation of color centers were presented.     

Cross-section and rate coefficient calculation for electron impact excitation, ionisation and dissociation of H2 and OH molecules

The weighted total cross-section (WTCS) theory is used to calculate electron impact excitation, ionisation and dissociation cross-sections and rate coefficients of OH, H₂, OH⁺, H₂ ⁺, OH^- and H₂ ^- diatomic molecules in the temperature range 1500–15000 K. Calculations are performed for H₂(X, B, C), OH(X, A, B), H₂ ⁺(X), OH⁺(X, a, A, b, c), H₂ ^-(X) and OH^-(X) electronic states for which Dunham coefficients are available. Rate coefficients are calculated from WTCS assuming Maxwellian energy distribution functions for electrons and heavy particles. One and two temperature (θ_e and θ_g respectively for electron and heavy particles kinetic temperatures) results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θ^b)exp (-c/θ).     

X形色心激光谐振腔参数的确定及类F2+型色心激光运转

文中,针对X形谐振腔的特点及结合纵向泵浦理论,分析了X形谐振腔几何参数的确定条件和模式匹配条件;根据有关NaCl(OH^-):(F₂⁺)_H和KCl(Na⁺,OH^-):(F₂⁺)AH色心激光的主要研究结果,分析了提高色心激光输出效率的可能途径.     

Electronic structure of F,-center in MgO

The embedded-cluster numerical variational method has been developed to calculate the electronic structure of perfect MgO, F and F⁺-centers in MgO. The energy band, bulk density of states has been calculated by cluster Mg₁₄O₁₃, Mg₁₄O₁₂F⁺ and Mg₁₄O₁₂F. The calculated absorption energy for F⁺ and F centers is in good agreement with experimental data. In our calculated defect energy levels, that the first excited state of F⁺-center is at CB-3.46 eV, indicates the necessity of a large photoelectron yielding energy. We also calculate the radius of color center electron, and plot the map of charge-density distribution of valence electrons in which the structure of the color center is shown directly. Received 22 May 1998     

Ionization potentials of some N-heterocyclic compounds

Static electric polarizabilities were calculated for the 10-electron isoelectronic series OH^-, F^-, NeH⁺ and Mg²⁺ in the LCAO-SCF-MO approximation. All the independent components of all polarizability tensors up to the fourth rank were included in the study. The isotropic components of the fourth order polarizability tensors C, B and γ are, in atomic units, ? = 69.10, 35.19, 1.79 and 0.24, ? = -2286, -552, -6 and -0.3 and = 94600, 12900, 24 and 0.6 for OH^-, F^- NeH⁺ and Mg²⁺, respectively.     

LiF-LiCl-B2O3系统非晶态快离子导体结构的研究

本文通过对¹¹B核磁共振(¹¹B-NMR)、红外光谱等实验方法,研究了LiF-LiCl-B₂O₃三元系统玻璃的结构和离子导电性,着重于F^-离子在玻璃网络中所起的作用,以及F^-,Cl^-和Li⁺离子对导电率的影响。LiF-LiCl-B₂O₃三元系统玻璃,随LiF含量的增加,B由三角体向四面体变化,从而F^-离子进入网络,使玻璃结构由[B₂O₃]三角体层状结构向三维空间延展,形成了含有[BO₃F]基团的三维空间网络,Cl^-离子以游离的离子存在于网络中,起着松散网络的作用,对提高电导率有利,而Li⁺离子作为传导离子,对电导率的贡献是主要的。本系统玻璃的电导率是随LiF,LiCl含量的增加而增大,在300℃时测得电导率σ=6.12×10^-4Ω^-1·cm^-1。 关键词：     

KCl:OH-中O2-色心实验研究

通过调制吸收光谱和电子自旋共振(ESR)谱的测量得知,在被高能射线辐照前后,KCl:OH^-中氧的成分主要以O₂^-的形式存在。O₂^-在样品中的浓度随辐照时间增加而增加,直至(OH)^-完全被辐照分解。O₂^-有相当强的稳定性。其存在与产生过程对增加F₂⁺激光色心的稳定性无直接作用。本文还根据ES 关键词：     

Tuneable laser operation of F+2 and (F+2)A centers in OH- and SH- doped alkali halides

Stabilization and tuneable laser operation of F⁺₂ and (F⁺₂)_A centers using OH^- and SH^- doped alkali halide crystals are reported. The new stabilization technique improves previously described laser systems and produced a new F⁺₂(KBr) laser (1.72–2 μm), covering an important gap around 2 μm. All crystals can easily be reactivated for laser operation after extended periods of storage at room temperature.     

OH impurities in co-doped LiNbO3:Cr :ZnO congruent crystals

Infrared optical absorption has been used to study OH⁻impurities into congruent co-doped LiNbO₃:Cr³⁺:ZnO crystals doped with different Zn²⁺ concentration. The OH⁻ IR absorption spectra present three bands that can be associated with different OH⁻ complex centres available in the lattice. For crystals with lower Zn²⁺ concentrations (<4.7%) only one IR absorption band centred at 2867 nm (3490 cm⁻¹) is reported which is associated with the OH⁻ unperturbed vibration. For crystals with higher Zn²⁺ concentrations (>4.7%), two new bands associated with OH⁻vibration in distortion environment are reported. These bands are centred at 2827 nm (3537 cm⁻¹) and 2847 nm (3512 cm⁻¹) and can be associated with OH⁻-Zn²⁺ and Cr³⁺(Li⁺)-OH⁻-Zn²⁺(Int.) complex centres, respectively. Electron paramagnetic resonance (EPR) has been used to identify the Cr³⁺ centres in the lattice of the doped LiNbO₃:ZnO crystals.     

The roles of Li and F ions in Li-F-codoped TiO2 system

An overall comparative study was carried out on Li-doped, F-doped, and Li-F-codoped TiO₂ powders in order to elucidate the roles of Li⁺ and F⁻ ions in photocatalyst. The characteristic data were based on the analysis of XRD, XPS, and PL spectra. The effects of atomic ratio of Li/Ti and F/Ti on the photocatalytic activity were also investigated. As the results, Li doping accelerated the phase formation of rutile in lower temperature while F doping prevented the phase transition from anatase to rutile. Li doping inducted a large amount of O_OH on the surface of TiO₂, while F doping consumed much of O_OH. Li⁺ ions acted as the roles of recombination center of electron-hole pairs while F doping could restrain the recombination of electron-hole pairs on the center of Li⁺ ions. The roles of Li⁺ and F⁻ ions were also confirmed in the experimental section, where the photocatalytic activity of TiO₂ was improved greatly by synergistic reaction of Li⁺ and F⁻ ions.     

(OH)-离子对F2+心的稳定作用

基于我们的实验结果,本文讨论了(OH)^-离子在LiF晶体中对F₂⁺心的稳定作用,给出了样品在室温下放置九个月时,其中的F₂⁺心吸收峰,此时F₂⁺心仍达到实现激光振荡所要求的浓度。 关键词：     

Preparation and optical properties of Ag enwrapped Y2O3:Eu nanoparticles in solution and in powders

Ag enwrapped Y₂O₃:Eu³⁺ nanoparticles were prepared by a wet chemistry method, which was dispersed in liquid (glycol) or dried to powders. Their luminescence properties were studied in comparison to those in the un-enwrapped ones. The results demonstrated that in glycol the ⁵D₀-⁷F₂ transitions for Ag enwrapped Y₂O₃:Eu³⁺ nanoparticles became stronger than that for un-enwrapped ones, while the excitation charge transfer band shifted blue. On the contrary, the ⁵D₀-⁷F₂ transitions in Ag enwrapped Y₂O₃:Eu³⁺ powders became weaker than those in the un-enwrapped ones. It was suggested that in liquid the Ag shells thinly deposited in the surface of Y₂O₃:Eu³⁺ and insulated the Y₂O₃:Eu³⁺ from the liquid, which contained large organic vibration modes. As a result, the surface nonradiative energy transfer from Eu³⁺ to the organic modes decreased, and emission intensity of ⁵D₀-⁷F₂ increased. In the Y₂O₃:Eu³⁺ powders, the Ag shells absorbed the excitation light, leading to the decrease in excitation density and the intensity of ⁵D₀-⁷F₂.     

Synthesis and ionic conduction of apatite-type materials

Apatite is a mineral with general formula M₁₀(XO₄)₆Z₂, where M are metallic elements such as Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ln³⁺ etc.; X=P, V, S, Si, Ge, Re, Cr etc; Z=F⁻, Cl⁻, I⁻, OH⁻, O²⁻, S²⁻ etc. Some materials with apatite structure (S.G. P6₃/m) exhibit quite high cationic (Li⁺, H⁺ etc.) and/or anionic (F⁻, Cl⁻ etc.) conduction. Recently, it was reported that some rare earth silicates, e.g., La₁₀(SiO₄)₆O₃, exhibit quite high oxide-ion conductivity. In this paper, we discuss chemical composition, structure, synthetic procedure and ionic conduction of apatite-type materials. Recent improvements are briefly reviewed. High ionic conductivity has been observed for both cation deficient, oxygen stoichiometric La_9.33(SiO₄)₆O₂ and cation stoichiometric, oxygen excess La₁₀(SiO₄)₆O₃ compositions. Grain boundary conductivity is usually low, which tends to dominate the impedance response. The resistance, particularly the grain boundary resistance is also found to depend on pO₂. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Investigation of the kinetics of OH∗ and CH∗ chemiluminescence in hydrocarbon oxidation behind reflected shock waves

The temporal variation of chemiluminescence emission from OH^?(A² Σ ⁺) and CH^?(A² Δ) in reacting Ar-diluted H₂/O₂/CH₄, C₂H₂/O₂ and C₂H₂/N₂O mixtures was studied in a shock tube for a wide temperature range at atmospheric pressures and various equivalence ratios. Time-resolved emission measurements were used to evaluate the relative importance of different reaction pathways. The main formation channel for OH^? in hydrocarbon combustion was studied with CH₄ as benchmark fuel. Three reaction pathways leading to CH^? were studied with C₂H₂ as fuel. Based on well-validated ground-state chemistry models from literature, sub-mechanisms for OH^? and CH^? were developed. For the main OH^?-forming reaction CH+O₂=OH^?+CO, a rate coefficient of k ₂=(8.0±2.6)×10¹⁰ cm³?mol^?1?s^?1 was determined. For CH^? formation, best agreement was achieved when incorporating reactions C₂+OH=CH^?+CO (k ₅=2.0×10¹⁴ cm³?mol^?1?s^?1) and C₂H+O=CH^?+CO (k ₆=3.6×10¹²exp(?10.9 kJ?mol^?1/RT) cm³?mol^?1?s^?1) and neglecting the C₂H+O₂=CH^?+CO₂ reaction.     

Photoexcitation of rare-gas neon and argon clusters doped with <Emphasis Type="Bold">H</Emphasis><Subscript><Emphasis Type="Bold">2</Emphasis></Subscript><Emphasis Type="Bold">O</Emphasis>

We have studied the fluorescence of electronically excited OH^*, H^* and H₂O^+* dissociation fragments after VUV excitation ( h ν≥11.6 eV) of rare-gas clusters (Rg = Ne, Ar) doped with H₂O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH ^* ( A ² Σ ⁺ ↦ X ² Π) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH^* ( A ) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H₂O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons. Received 7 March 2002 / Received in final form 27 May 2002 Published online 19 July 2002     

Structural and luminescence studies of trivalent europium complexes with pentaethylene glycol and 18-crown-6 ligands in the presence of picrate anion

Two new isostructural complexes of europium picrate (Eu-Pic) with pentaethylene glycol (EO5) and 18-crown-6 (18C6) ligands formed complexes of molecular formula [Eu(Pic)₂(18C6)]⁺(Pic)⁻I and [Eu(Pic)₂(EO5)]⁺(Pic)⁻II have been isolated and characterised. Compound I showed 10-coordination number through six oxygen atoms from the 18C6 ligand and two bidentate picrate anions. Meanwhile, compound II exhibited 9-coordination number via six oxygen atoms from EO5 ligand, two oxygen atoms from a bidentate and one oxygen atom from monodentate picrate anions. Photoluminescence (PL) spectra of the solid-state europium complexes display sharp lines which are assigned to ⁵D₀→⁷F_0-4 and ⁵D₁→⁷F_1,2,4 transitions. No emission of polyether ligands is observed, indicating that the energy transfer from the polyether ligands to the Eu³⁺ ion is quite efficient. The PL spectra of [Eu(Pic)₂(OH₂)₆]⁺(Pic)⁻·6H₂O III, [Eu(NO₃)₃(OH₂)₃]·(18C6) IV, [Eu(NO₃)₃·6H₂O] V and Eu₂O₃VI are also observed. Compounds I-IV exhibited high Ω₂ intensity parameter values, namely 16.93, 10.23, 17.10 and 12.35 (in units of 10⁻²⁰ cm²), respectively. These relatively high values reflect the hypersensitive behaviour of the ⁵D₀→⁷F₂ transition and indicate that the Eu³⁺ ion is located in a highly polarisable chemical environment.     

Magnetic interaction in Mg, Ti, Nb doped manganites

An effect of Mn substitution with Me=Mg²⁺, Ti⁴⁺, Nb⁵⁺ in manganites has been investigated by preparing La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃ series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La_0.7Sr_0.3(Mn ^{3 +} _0.85Nb ^{5 +} _0.15)O₃ is ferromagnet with T _C = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical magnetic phase diagrams are constructed for La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃. Our results show that Mn³⁺-O-Mn³⁺ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn⁴⁺ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between antiferromagnetic Mn⁴⁺-O-Mn⁴⁺ and ferromagnetic Mn³⁺-O-Mn⁴⁺(Mn³⁺) superexchange interactions. Received 24 January 2002 Published online 9 July 2002     

Vibrational properties of the F A (Li+) center in alkali halides

Abstract

The on- or off-axis behavior and the vibrational properties of the F _A (Li⁺) center are investigated in several alkali halides by means of polarized resonant Raman scattering. The observed Raman modes are interpreted and classified according to their nature and frequency. A comparison between on- and off-axis systems and between the vibrational modes of the isolated Li⁺ and the F _A (Li⁺) center reveals a displacement of the Li⁺ equilibrium position parallel to the F _A (Li⁺) defect axis and, due to the small impurity size, away from the adjacent F center.     

Optical absorption and photoluminescence studies of Eu-doped phosphate and fluorophosphate glasses

Phosphate (P₂O₅+K₂O+BaO+Al₂O₃+Eu₂O₃) and fluorophosphate (P₂O₅+K₂O+BaO+BaF₂+Al₂O₃+Eu₂O₃) glasses with different Eu³⁺ ion concentrations have been prepared and characterized through optical absorption, photoluminescence and decay times. An intense red luminescence is observed from the ⁵D₀ emitting level of Eu³⁺ ions in these glasses. The relative luminescence intensity ratio of ⁵D₀→⁷F₂→⁵D₀→⁷F₁ transitions has been evaluated to estimate the local site symmetry around the Eu³⁺ ions. The emission spectra of these glasses show a complete removal of degeneracy for the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions. Second and fourth rank crystal-field (CF) parameters have been calculated together with the CF strength parameter by assuming the C_2v symmetry for the Eu³⁺ ions in both the phosphate and fluorophosphate glasses. Judd-Ofelt parameters have been evaluated from the luminescence intensity ratios of ⁵D₀→⁷F_J (J=2, 4 and 6) to ⁵D₀→⁷F₁ transitions. These parameters have been used to derive radiative properties such as transition probabilities, branching ratios, radiative lifetimes and peak stimulated emission cross-sections for the ⁵D₀→⁷F_J transitions. Decay curves of the ⁵D₀ level of Eu³⁺ ions in these two Eu³⁺:glass systems have been measured by monitoring the ⁵D₀→⁷F₂ transition (611 nm) at room temperature. The experimental lifetime of the ⁵D₀ level in the title glasses is found to be higher than Eu³⁺-doped niobium phosphate glasses. The analysis indicates that the lifetime of the ⁵D₀ level is found to be less sensitive to the Eu³⁺ ion concentration and addition of BaF₂ has no significant effect on the optical properties of Eu³⁺-doped phosphate glasses.     

Structures of proton solvates in the HCl-H2O-(CH3)2NCHO system as determined by IR spectroscopy and quantum-chemical calculations

The structures of proton solvates in the HCl-H₂O-(CH₃)₂NCHO (DMFA) system at H₂O: DMFA ratios ranging from 1: 1 to 21: 1 are studied by the IR spectroscopy method. It is demonstrated that H₂O?H⁺?OH₂ ions and (CH₃)₂NCHO?H⁺?OH₂ mixed solvates with a strong quasi-symmetrical hydrogen bond are formed in solutions. With an increase in the DMFA concentration, the fraction of H₅O ₂ ⁺ ions decreases. At HCl: H₂O ≥ 1: 3 and arbitrary DMFA concentrations, only mixed proton solvates are formed. The continuous absorption coefficients for the (CH₃)₂NCHO?H⁺?OH₂ ions are determined. The results obtained are compared with the results of quantum-chemical calculations of the structure and relative stability of the (DMFA) _m H⁺(H₂O) _n (m = 0–2, n = 0–3) positively charged complexes which were performed by the B3LYP/6-31++G(d,p) DFT method. We identified 19 stable configurations with chain, cyclic, and branched structures. Most of these configurations contain the (CH₃)₂NCHO?H⁺?OH₂ fragment. The parameters of the O?H⁺?O bridge show that some configurations have a strong quasi-symmetrical hydrogen bond. In some cases, the proton is located between two DMFA molecules. The H₂O?H⁺?OH₂ bridge is observed in none of the stable configurations of the (DMFA) _m H⁺(H₂O) _n (m ≠ 0) complexes.