Solvolytic reactivity of pyridinium ions

The leaving group abilities of pyridine, 4‐methylpyridine, and 4‐chloropyridine in S_N1 solvolytic reactions have been determined by analyzing the rate constants of X,Y‐substituted benzhydrylpyridinium salts obtained in various solvents. By applying the linear free energy relationship equation, log k = s_f (E_f + N_f), the nucleofuge specific parameters of 4‐substituted pyridine have been extracted. Because of solvation in the reactant ground state, the reactivity (nucleofugality, N_f) of a given pyridine decreases as the polarity of the solvent increases. High slope parameters (s_f > 1) may be due to the spread of the energy levels of the benzhydrylium ion/pyridine pair intermediates in comparison to benzhydrylium ion/chloride pairs (s_f ≈ 1). Because of slow heterolysis step of pyridinium salts in various solvents, some are stable under normal conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

Stability of subatomic carbon films on metal surfaces against low-energy gas ion bombardment

The earlier developed original experimental technique for measuring and analyzing the parameters of low-frequency fluctuations of the field-emission current in metal film systems is used to measure the sputtering yield Y _f of carbon films (with a coverage Θ ranging from 1 to 4) applied on Fe, Nb, Ta, and U substrates. The value of Y _f is calculated by an expression derived within a theoretical model developed. The sputtering ratios were measured for the case when the carbon films are sputtered by H⁺ and He⁺ ions with an energy E _i between 2 and 10 keV. With Θ fixed, the energy dependences of Y _f are obtained for each of the ions. In addition, for each of the ions, the Θ dependences of Y _f are found for several values of E _i. In all the cases, the measured values of Y _f far exceed those for pure carbon. With another original technique that combines field-ion microscopy (FIM) and precise measurement of current and/or luminous properties of local regions in FIM images, the energy thresholds E _th of sputtering carbon films applied on the metal surfaces are found. The energy distributions of Y _f in the near-threshold energy range for various Θ are obtained.     

“Forbidden” reflections in resonant diffraction of synchrotron radiation in yttrium aluminum garnet Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>

The purely resonant Bragg reflections (13, 13, 0) and (14, 0, 0) in yttrium aluminum garnet Y₃Al₅O₁₂ at energies near the K absorption edge of yttrium have been studied experimentally and theoretically. The anisotropic tensor atomic factor of yttrium corresponding to dipole-dipole resonance transitions depends on three independent parameters changing with energy. The intensities of the reflections (14, 0, 0) and (13, 13, 0) are shown to depend on the parameter difference f ₁(E) − f ₂(E) and the parameter f ₃(E), respectively, which are attributable to distortions of the wave functions of the excited atoms and change greatly with photon energy E. Studying various reflections has allowed one to determine the various components of the tensor atomic factor and to compare them with the results of numerical calculations.     

Optical spectra of triply-charged rare-earth ions in polycrystalline corundum

Solid samples of polycrystalline corundum α-Al₂O₃ activated by triply-charged rare-earth ions RE³⁺ (R=Eu³⁺, Er³⁺, Pr³⁺) were synthesized by the sol-gel technology. Characteristic narrow-line optical absorption and luminescence spectra produced by intraconfigurational 4f-4f transitions in RE³⁺ ions have been measured. RE³⁺ ions have been established to form one dominant type of optical centers in the corundum matrix, and the energy diagram of Eu³⁺ and Er³⁺ Stark levels in corundum has been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1442–1449 (August 1998)     

Luminescence Properties of Dual Valence Eu Doped Nano-crystalline BaF2 Embedded Glass-ceramics and Observation of Eu2+ → Eu3+ Energy Transfer

Europium doped glass-ceramics containing BaF₂ nano-crystals have been prepared by using the controlled crystallization of melt-quenched glasses. X-ray diffraction and transmission electron microscopy have confirmed the presence of cubic BaF₂ nano-crystalline phase in glass matrix in the ceramized samples. Incorporation of rare earth ions into the formed crystalline phase having low phonon energy of 346 cm⁻¹ has been demonstrated from the emission spectra of Eu³⁺ ions showing the transitions from upper excitation states ⁵D_J (J = 1, 2, and 3) to ground states for the glass-ceramics samples. The presence of divalent europium ions in glass and glass-ceramics samples is confirmed from the dominant blue emission corresponding to its 5d-4f transition under an excitation of 300 nm. Increase in the reduction of trivalent europium (Eu³⁺) ions to divalent (Eu²⁺) with the extent of ceramization is explained by charge compensation model based on substitution defect mechanisms. Further, the phenomenon of energy transfer from Eu²⁺ to Eu³⁺ ion by radiative trapping or re-absorption is evidenced which increases with the degree of ceramization. For the first time, the reduction of Eu³⁺ to Eu²⁺ under normal air atmospheric condition has been observed in a BaF₂ containing oxyfluoride glass-ceramics system.     

Absorption spectra of actinide compounds

From the energy levels of the gaseous ions Ac⁺⁺, Th⁺⁺⁺, and Th⁺⁺, it can safely be extrapolated that the ground electron configuration of all actinide ions with ionic charges at least + 3 contains only 5f-electrons outside the emanation shells. This is further supported by the 5fⁿ → 5f^n-16d transitions observed in Pa(IV), U(III), Np(III) (though their wave numbers are too high to be measured in U(IV) and Np(IV)). The configuration f² has a very characteristic distribution of energy levels between ³H₄ and ³P₂ with ¹S₀ as much higher, isolated level. The absorption spectra of U(IV) complexes and of PuF₆ suggest interelectronic repulsion parameters ～60 per cent of the values in the corresponding lanthanides, while the Landé factors are about twice as large. This would also explain the recent measurements of Cm(III) complexes, compared to Gd(III). The larger average radius of the 5f-shell causes larger perturbation effects from the ligands than in the lanthanides. The J-values will therefore be distributed between several adjacent, observed energy levels without making the general treatment of the fⁿ-configurations impossible. The variation with oxidation numbers in isoelectronic series are discussed, e.g. U(III) and Np(IV); Pu(III) and AmF₄; and Am(III) and CmF₄. Band positions are predicted in Bk(III) and BkF₄. Finally, the chemical properties of the actinide elements are discussed, and it is emphasized that no necessary connections exist between the presence of f-electrons and constant trivalency.     

Theoretical studies of the spin-Hamiltonian parameters for the orthorhombic Pr4+ centers in Sr2CeO4 crystals

Theoretical studies of spin-Hamiltonian (SH) parameters associated with Pr⁴⁺ in Sr₂CeO₄ single crystals have been made by using the complete diagonalizing energy matrix method (CDM) for the 4f ¹ electronic configuration. The calculated results are in excellent agreement with the experimental data. The negative signs of the anisotropic g _i -factors and hyperfine structure constants A _i (where i = ∥ or ⊥) for the orthorhombic Pr⁴⁺ ion in Sr₂CeO₄ are suggested from the calculations. By comparing the results obtained by the CDM with the experimental data, one finds it is valid to interpret the SH parameters for 4f ¹ ions in crystals. The results are discussed.      

Collective bands of the positive parity states inf 7/2 shell nuclei

Collective bands of the positive parity states in odd-A f _7/2 shell nuclei are described in the framework of deformed Hartree-Fock theory by projecting states from lowest energy intrinsic states with (d ₃ ^2/−1 f ₇ ^2/n+1 ) one hole configurations. In the calculation empirical (d _3/2 −f _7/2)² effective matrix elements have been used to test the tacit assumptions of the Bansal and French model.     

Weak electric and magnetic form factors for semileptonic baryon decays in a relativistic quark model

Weak electric and magnetic form factors for semileptonic baryon decays are calculated in a relativistic quark model based on the Dirac equation with the independent-quark confining potential of the formV _q(r)=1/2(1+γ ⁰)(a ² r+V ₀). The values obtained for (g ₂/g ₁) are not very much different from the nonrelativistic results of Donoghue and Holstein. The values of (g ₁/f ₁) extracted from our model calculations of (f ₂/f ₁) in the Cabibbo limit compare well with the experimental values. The values of (f ₂/f ₁) for various semileptonic transitions are also estimated incorporating phenomenologically the effect of nonzerog ₂ in the ratio (g ₁/f ₁). It is found that the SU(3)-symmetry breaking does not generate significant departures in (f ₂/f ₁) values from the corresponding Cabibbo predictions.     

Energy transfer between europium and lead close pairs in monocrystalline NaCl

The optical properties of lead and europium doubly doped NaCl single crystals have been analyzed. It is found that energy transfer not only occurs from the Pb²⁺P₁ level to the 4f⁶ 5d (t_2g) state of the Eu²⁺ ions, but also from the 4f⁶ 5d (e_g) state of Eu²⁺ to the ³P₁ level of the lead ions. This transfer is observed in samples which have been quenched from high temperatures, as well as for low impurity concentration. The results obtained suggested that the energy transfer occurs between Pb-Eu close pairs. In order to get a better insight into this point, electron paramagnetic resonance measurements were also performed in parallel with the optical ones. The analysis of the Eu-EPR spectra shows that the site symmetry normally occupied by the Eu²⁺ ions in NaCl is strongly modified by the presence of perturbing neighboring lead ions.     

5d-4f luminescence of Nd3+, Gd3+, Er3+, Tm3+, and Ho3+ ions in crystals of alkaline earth fluorides

The vacuum ultraviolet emission spectra of alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals with rare earth impurity ions (Nd, Gd, Er, Tm, Ho) have been investigated. The main luminescence bands are described well by the transitions from the lowest excited 5d state to different 4f levels of rare earth ions.     

Influence of unstable valence of cerium ions on the crystal field in ReNi compounds

To examine the effect of hybridization of 4f electrons with conduction electrons on the crystal field potential using neutron spectroscopy, we studied the effects of the crystal electric field (CEF) in intermetallic compounds of the type ReNi, in which chemical substitution is followed by a transition of the cerium ions from an intermediate valence state to the Kondo state. Measurements were performed both on cerium ions in the compounds Ce_1−x La_xNi (x=0.5, 0.8), where they have a whole-number population of the 4f shell, and on the paramagnetic impurity ion Nd in the series of compounds Re_1−x Nd_xNi (Re=Ce, La, Y), in which the cerium ions are found either in an intermediate valence state or in the Kondo state. From the neutron inelastic magnetic scattering spectra on Nd ions, we have reconstructed the crystal field parameters in ReNi compounds and calculated the CEF level diagram of Ce ions in these compounds as functions of the interion distances Re-Ni. The results of our calculations are in good agreement with the experimentally determined splitting diagram of the ground-state multiplet of the Ce ions. We have determined that as the degree of hybridization with the conduction electrons grows the CEF potential varies considerably and the effective splitting of the 4f shell of the cerium ions increases. The estimated energy scale of the splitting of the ground-state multiplet of the Ce³⁺ ions in the ReNi CEF (Δ_CEF∼15 meV) turns out to be commensurate with the Kondo temperature (T _K ;140 K for CeNi. Analysis indicates that the CEF potential has a substantial effect on the formation of the valence-unstable ground state of the f shell in this compound. Zh. éksp. Teor. Fiz. 113, 1731–1747 (May 1998)     

Fluorescence properties of divalent and trivalent europium ions in CdWO4 single crystals grown by Bridgman method

Optical absorption, excitation, and fluorescence were investigated in Eu ion-doped CdWO₄ single crystal grown by a modified Bridgman method. The results indicate that Eu²⁺ and Eu³⁺ ions coexist in CdWO₄ crystal and an energy transfer occurs between these Eu²⁺ and Eu³⁺ ions. When the crystal is excited by 266-nm light, the energy corresponding to the 4f⁶5d to ⁸S_7/2 transition of Eu²⁺ ions results in the excitation of the Eu³⁺ ions to the ⁵D_J level. The effect on fluorescence of annealing in oxygen at various temperatures was investigated. The excitation intensity of Eu²⁺ ions at 266 nm decreases as annealing temperature increases from 300 K to 1073 K, but it remains at a certain equilibrium level when the annealing temperature is further increased.     

Crystal field spectroscopy by inelastic neutron scattering

We present a review of inelastic neutron scattering results on very diluted rare earth (RE)-Laves phases. A systematic investigation to study crystal electric fields, experienced by single rare earth ions in a metallic environment, was done on (RE,R)Al₂, with RE=Pr, Nd, Tb, Dy, Ho, Er, Tm andR=Sc, Y, La. We show the influence of the 4f-ions on the crystal fields as well as the influence of the host lattices. The rare earth atoms in ScAl₂ have been studied for the first time and most of the other alloys, with LaAl₂ and YAl₂ as hosts, have been studied for the first time with magnetic atom concentrations below 1 at% and/or with good resolution for low energy excitations. Furthermore we studied the dynamics of 4f-moments, which are coupled to the conduction electrons by determining the coupling constants. We present a comprehensive set of crystal field parameters and coupling constants from these systematic studies and discuss them qualitatively within the available theoretical models. The influence of magnetic atom concentration on crystal field spectra will be shown in part II.     

A quark model study of semileptonic baryon decays in the electronic decay modes

A relativistic quark model based on Dirac equation with the independent-quark confining potential of the form (1 +γ ⁰)[a ln(r/b)] is used to compute the weak electric and magnetic form factors for semileptonic baryonic decays in the electronic decay modes. The values obtained for (g ₂/g ₁) agree with the non relativistic results and those for (f ₂/f ₁) agree with the MIT bag model values of Donoghue and Holstein. The SU(3) symmetry breaking does not generate appreciable departures in (f ₂/f ₁) values from corresponding Cabibbo values.     

Lack of the neighboring group rate effect in solvolytic reactions that proceed via participation

In order to investigate the influence of solvent polarity on the rate effect of double bonds in reactions that proceed via an extended π‐participation mechanism, the solvolysis rates (k_U) of the benzyl chloride derivative 1 and tertiary chloride 2 that have doubly unsaturated side chains were measured in absolute ethanol, 80% v/v. aq. ethanol and 97% wt. aq. trifluoroethanol. The rates of the corresponding saturated analogs 1S and 2S (k_S) were measured in 80% aq. ethanol and 97% wt. aq. trifluoroethanol, while those in pure ethanol were calculated according to LFER equation log k = s_f (E_f + N_f). In solvents with moderate ionizing power (ethanol and 80% aq. ethanol) the expected rate effects were obtained (k_U/k_S>1), while in solvent with high ionizing power (2,2,2‐trifluoroethanol) absence of the rate effect was observed (k_U/k_S≈1), indicating that in the k_S process the solvation of the transition state is very important, while in k_Δ process the breaking of the C? Cl bond is not appreciably developed in the transition state and the solvent effect is marginal. Copyright © 2007 John Wiley & Sons, Ltd.     

White-light emission from Li<Subscript>2</Subscript>Sr<Subscript>1−3<Emphasis Type="Italic">x</Emphasis>/2</Subscript>Dy<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>SiO<Subscript>4</Subscript> phosphors

Li₂Sr_1−3x/2Dy _x SiO₄ phosphors were synthesized at 900°C in the normal atmosphere by a solid-state method. The synthesized phosphors were characterized by X-ray powder diffraction (XRD) and photoluminescence (PL) spectra. The photoluminescence excitation (PLE) spectra show excitation peaks ranging from 300 to 400 nm due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid-state lighting and light-emitting diodes (LEDs). The predominant emission of Dy³⁺ ions under 349 nm excitation is observed at 479 nm (blue) due to the ⁴F_9/2→⁶H_15/2 transitions and 572 nm (yellow) due to ⁴F_9/2→⁶H_13/2 transitions, respectively. The PL results reveal that the optimal concentration of the Dy³⁺ ions in Li₂Sr_1−3x/2Dy _x SiO₄ phosphors is x=0.03 mol. The nature of the resonance energy transfer for the Dy³⁺ ions is confirmed by Huang’s rule. Simulation of the white light excited by 349 nm near-ultraviolet (n-UV) light is also performed for its potential for white LEDs.     

Solvent nucleophilicities of hexafluoroisopropanol/water mixtures

First‐order rate constants k₁ for the trapping of various donor‐ and acceptor‐substituted benzhydrylium ions in mixtures of 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP) and water ranging from 50 to 99% HFIP (w/w) were determined by laser flash photolytic generation of benzhydrylium ions from benzhydryl triarylphosphonium salts in these solvents. From these rate constants, we derived the solvent‐specific reactivity parameters N₁ and s_N for HFIP/water mixtures as defined by the linear free energy relationship lg k₁(20 °C) = s_N(N₁ + E). Copyright © 2012 John Wiley & Sons, Ltd.     

Ce(Ⅳ)和Ce(Ⅲ)化合物系列的X射线光电子能谱研究

用XPS研究了标称四价铈盐系列:硫酸铈铵、硫酸铈、二苯基羟乙酸铈、碘酸铈、过氧化铈、二氧化铈以及三价铈盐系列:草酸亚铈、硫酸亚铈、二苯基羟乙酸亚铈、碘酸亚铈和氯化亚铈.结果表明,上述标称四价铈盐都属于混合价化合物,其Ce3d电子能谱呈现Ce⁴⁺和Ce³⁺的二组谱线结构的混合.在真空中加热后CeO₂的Ols电子能谱有两个氧峰,这说明 Ce(4f⁰)O(2p^π)←→Ce(4f¹)O(2p^n-1) 类型价态波动的可能性较大.三价铈盐的3d谱线的低结合能端约3.5—4eV处则存在摔落伴峰.     

Luminescence of CaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>:Eu crystals

We have studied photoluminescence and thermoluminescence (PL and TL) in CaGa₂Se₄:Eu crystals in the temperature range 77–400 K. We have established that broadband photoluminescence with maximum at 571 nm is due to intracenter transitions 4f⁶ 5d–4f⁷ (⁸S_7/2) of the Eu²⁺ ions. From the temperature dependence of the intensity (log I–10³/T), we determined the activation energy (E _a = 0.04 eV) for thermal quenching of photoluminescence. From the thermoluminescence spectra, we determined the trap depths: 0.31, 0.44, 0.53, 0.59 eV. The lifetime of the excited state 4f6 5d of the Eu²⁺ ions in the CaGa₂Se₄ crystal found from the luminescence decay kinetics is 3.8 μsec. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 112–116, January–February, 2009.