Up-conversion emissions of Er<Superscript>3+</Superscript>-Yb<Superscript>3+</Superscript> codoped Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles by the arc discharge synthesis method

The Er³⁺-Yb³⁺ codoped Al₂O₃ nanoparticles with an average particle size of about 50 nm have been synthesized by an arc discharge synthesis method. The green and red up-conversion emissions centered at about 526, 547 and 677 nm, corresponding respectively to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, were detected by a 978-nm semiconductor laser diode excitation. The Annealing has evident effect on the up-conversion emissions of the samples: The red up-conversion emission is noticeable before annealing; however, the green up-conversion emission becomes predominant after annealing. The mixture of (Er,Yb)₃Al₅O₁₂ and α-(Al,Er,Yb)₂O₃ phases is more favorable for green up-conversion emissions due to an enhancement of the ESA (I) of ⁴I_11/2+a photon→⁴F_7/2 and ET (III) of ²F_5/2(Yb³⁺)+⁴I_11/2(Er³⁺)→²F_7/2(Yb³⁺)+⁴F_7/2(Er³⁺) processes. The two-photon absorption up-conversion process is involved in the green and red up-conversion emissions. The results have proved that arc discharge synthesis is a new promising preparation technology for optical materials. Supported by National Natural Science Foundation of China (Grant No. 10804015), the Scientific Research Foundation for Doctor of Liaoning Province (Grant No. 20071095), and the Educational Committee Foundation of Liaoning Province (Grant No. 2008123)     

Study of spectroscopic characteristics of holmium-doped double sodium-yttrium fluoride crystals Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Ho<Superscript>3+</Superscript>

We have grown crystals Na_0.4Y_0.6F_2.2:Ho³⁺ (NYF:Ho³⁺) by the Bridgman-Stockbarger method. The optical spectra and luminescence kinetics of NYF:Ho³⁺ crystals have been studied. Based on the analysis of low-temperature absorption spectra, we determine the structure of the Stark splitting of holmium levels in NYF:Ho³⁺ crystals. From absorption spectra examined at T = 300 K, we calculate absorption cross-section spectra and oscillator strengths of transitions from the ground state of holmium to excited multiplets. We show that the absorption spectra of NYF:Ho³⁺ crystals consist of broad bands that lie in the UV, visible, and near-IR ranges. The most intense bands are observed in the visible range, they correspond to transitions ⁵ I ₈ → (⁵ F ₁, ⁵ G ₆) and ⁵ I ₈ → (⁵ F ₄, ⁵ S ₂), and their maximal absorption cross sections are σ_abs^max (λ = 450.3 nm) = 1.16 × 10⁻²⁰ cm² and σ_abs^max (λ = 535.1 nm) = 0.9 × 10⁻²⁰ cm². The intensity parameters Ω _t have been calculated by the Judd-Ofelt method taking into account 10, 12, and 20 transitions from the ⁵ I ₈ ground state to excited multiplets. We show that, with an increasing number of transitions taken into account in the calculation, the parameters Ω _t somewhat increase. For 20 transitions, we have obtained the following intensity parameters: Ω₂ = 0.97 × 10⁻²⁰, Ω₄ = 1.74 × 10⁻²⁰, and Ω₆ = 1.15 × 10⁻²⁰ cm². With these parameters, we have calculated the probabilities of radiative transitions, the radiative lifetimes, and the branching ratios. The rates of multiphoton nonradiative transitions have been estimated. The luminescence decay kinetics from excited holmium levels ⁵ F ₃ (⁵ F ₄, ⁵ S ₂) and ⁵ F ₅ have been studied upon selective excitation in the range of 490 nm, and the lifetimes of these levels have been experimentally determined. We find that the calculated and experimental rates of radiative and nonradiative relaxation from excited holmium levels agree well with each other. We show that, upon pumping in the range of 490 nm, the multiplet (⁵ F ₄, ⁵ S ₂) is populated as a result of the radiative and nonradiative excitation relaxation from the ⁵ F ₃ level, while the lower-lying ⁵ F ₅ level is populated due to direct radiative transitions ⁵ F _{3, 2} → ⁵ F ₅, obviating the cascade scheme ⁵ F ₃ → (⁵ F ₄, ⁵ S ₂) ↝ ⁵ F ₅. We conclude that NYF:Ho³⁺ crystals are processable; admit doping by holmium in high concentrations (up to 100%); and, with respect to all their radiative characteristics, can be considered as potential active media for solid-state continuously tunable lasers in the IR and visible ranges.     

luminescence of BaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals activated by Eu<Superscript>2+</Superscript> and Ce<Superscript>3+</Superscript> ions

We have studied the effect of doping with Eu²⁺ and Ce³⁺ ions on the photoluminescence (PL) of BaGa₂Se₄ crystals in the temperature range 77–300 K. We have established that the broad bands with maxima at wavelengths 456 nm and 506 nm observed in the photoluminescence spectra of BaGa₂Se₄:Ce³⁺ crystals are due to intracenter transitions 5d → ²F_7/2 and 5d →²F_5/2 of the Ce³⁺ ions, while the broad photoluminescence band with maximum at 521 nm in the spectrum of BaGa₂Se₄:Eu²⁺ is associated with 4f⁶ 5d → 4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ion. We show that in BaGa₂Se₄:Eu²⁺,Ce³⁺ crystals, excitation energy is transferred from the Ce³⁺ ions to the Eu²⁺ ions.     

Optical spectra and excited state relaxation dynamics of Sm<Superscript>3+</Superscript> in Gd<Subscript>2</Subscript>SiO<Subscript>5</Subscript> single crystal

Single crystals of gadolinium orthosilicate Gd₂SiO₅ containing 0.5 at% and 5 at% of Sm³⁺ were grown by the Czochralski method. Optical absorption spectra, luminescence spectra and luminescence decay curves were recorded for these systems at 10 K and at room temperature. Comparison of optical spectra recorded in polarized light revealed that the anisotropy of this optically biaxial host affects the intensity distribution within absorption and emission bands related to transitions between multiplets rather than the overall band intensity. It has been found that among four bands of luminescence related to the ⁴G_5/2→⁶H_J (J=5/2–11/2) transitions of Sm³⁺ in the visible and near infrared region the ⁴G_5/2 →⁶H_7/2 one has the highest intensity with a peak emission cross section of 3.54×10⁻²¹ cm² at 601 nm for light polarized parallel to the crystallographic axis c of the crystal. The luminescence decay curve recorded for Gd₂SiO₅:0.5 at% Sm³⁺ follows a single exponential time dependence with a lifetime 1.74 ms, in good agreement with the ⁴G_5/2 radiative lifetime τ _rad=1.78 ms calculated in the framework of Judd-Ofelt theory. Considerably faster and non-exponential luminescence decay recorded for Gd₂SiO₅:5 at% Sm³⁺ sample was fitted to that predicted by the Inokuti-Hirayama theory yielding the microparameter of Sm³⁺–Sm³⁺ energy transfer C _da=1.264×10⁻⁵² cm⁶×s⁻¹.     

Optical properties of pyridine funtionalized TbF<Subscript>3</Subscript> nanoparticles

The preparation of pyridine functionalized TbF₃ nanoparticles are described in this report. Synthesized nanoparticles were characterized using the TEM, UV/Vis, FTIR and photoluminescence spectroscopy. TEM micrograph reveals the nanorod shaped, uniform in size with a particles size in the range of 20–30 nm. FTIR spectrum shown characteristic absorption bands of pyridine and a small intensity band at 411 cm⁻¹ corresponding metal nitrogen ν(Tb–N) bonding. Uv-vis spectrum shown the characteristic absorption transitions of Tb³⁺ ion. A strong emission transition at 540 nm (⁵D₄ → ⁷F₅) was observed on excite by visible light at 414 nm.     

Phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>NbOF<Subscript>5</Subscript> oxyfluoride

The thermal and dielectric properties of the (NH₄)₂NbOF₅ oxyfluoride have been investigated. It has been established that the structural phase transitions Cmc2₁ → C2 → Ia observed at the temperatures T ₁ = 258.0 K and T ₂ = 218.9 K exhibit a nonferroelectric nature. The hydrostatic pressure, which stabilizes the initial phase and destabilizes the low-temperature phase, hardly affects the temperature range of stability of the intermediate phase. The model of sequential ordering of the structural elements due to phase transitions has been analyzed using experimental data on the entropies of the phase transitions.     

Anomalous absorption in H<Subscript>2</Subscript>CN and CH<Subscript>2</Subscript>CN molecules

Structures of H₂CN and CH₂CN molecules are similar to that of H₂CO molecule. The H₂CO has shown anomalous absorption for its transition 1₁₁–1₁₀ at 4.8 GHz in a number of cool molecular clouds. Though the molecules H₂CN and CH₂CN have been identified in TMC-1 and Sgr B2 through some transitions in ortho as well as in para species, here we have investigated the condition under which transitions 1₁₁–1₁₀ and 2₁₂–2₁₁ of these molecules may show anomalous absorption. For the present investigation, we have calculated energy levels and radiative transition probabilities. However, we have used scaled values for collisional rate coefficients. We found that relative values of collisional rate coefficients can produce the required anom-alous absorption in 1₁₁–1₁₀ and 2₁₂–2₁₁ transitions in the molecules.      

Anisotropy of absorption spectra of the rare-earth orthoaluminate TbAlO<Subscript>3</Subscript> in polarized light

Polarized spectra of the optical absorption of the 4f → 4f transition ⁷ F ₆ → ⁵ D ₄ in rare-earth orthoaluminate TbAlO₃ were experimentally studied at temperature T = 78 K. It was shown that the nontrivial character of the absorption anisotropy of TbAlO₃ at low temperatures could be related either to the symmetry of wave functions of the Stark sublevels of ⁷ F ₆ and ⁵ D ₄ multiplets combining in the optical transitions under study or to optical experiment geometry that takes into account the orientation of incident light polarization relative to the crystal axes and the axes of local coordinate systems of a rare-earth ion in orthoaluminate.     

Optical absorption spectra and magnetic phase transitions in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Optical absorption spectra of the trigonal crystal of TbFe₃(BO₃)₄ in the vicinity of the ⁷F₆ → ⁵D₄ transition in a Tb³⁺ ion were studied as a function of temperature (2–70 K) and magnetic field strength (0–60 kOe) at 2 K. The splitting of the excited states of Tb³⁺ due to both the magnetic ordering of iron and an external magnetic field was determined. Abrupt splitting of the absorption lines of Tb³⁺ at temperature T_N of the magnetic ordering of the subsystem of iron was revealed, suggesting that the nature of such splitting is not entirely magnetic.     

On the Luminescence of Ce<Superscript>3+</Superscript>, Eu<Superscript>3+</Superscript>, and Tb<Superscript>3+</Superscript> in Novel Borate LiSr<Subscript>4</Subscript>(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>

This paper reports on the photoluminescence (PL) and time-resolved properties of Ce³⁺, Eu³⁺, and Tb³⁺ in novel LiSr₄(BO₃)₃ powder phosphors. Ce³⁺ shows an emission band peaking at 420 nm under 350-nm UV excitation. Energy transfer from Ce³⁺ to Mn²⁺ takes place in the co-doped samples. Eu³⁺ shows red emission under near UV excitation. LiSr₄(BO₃)₃:Eu³⁺ phosphor could be a suitable candidate for phosphor-converted solid state lighting. The luminescence lifetime is 2.13 ms for Eu³⁺ in LiSr₄(BO₃)₃:0.001Eu³⁺. As Eu³⁺ concentration increasing, the decay curves deviate from exponential behavior. Tb³⁺ shows the strongest ⁵D₄→⁷ F₅ emission line at 540 nm. Decay curves of ⁵D₄→⁷ F₅ and ⁵D₃→⁷ F₅ emission with different Tb³⁺ concentrations were also measured. Cross-relaxation process is discussed based on the decay curves.     

Spectral and kinetic characteristics of CdI<Subscript>2</Subscript> and CdI<Subscript>2</Subscript>:Pb scintillators

We have studied the effect of lead dopant on the optical absorption, photoluminescence, and x-ray luminescence spectra, and the scintillation characteristics of CdI₂ at room temperature. The crystals for the study were grown by the Stockbarger-Bridgman method. Activation of CdI₂ from the melt by the compound PbI₂ leads to the appearance in the absorption spectra in the near-edge region of an activator band at 395–405 nm, which is interpreted as an A band connected with electronic transitions from the ¹S₀ state to the ³P₁ levels in the Pb²⁺ ion. For x-ray excitation, CdI₂:Pb²⁺ crystals with optimal dopant concentration (∼1.0 mol%) are characterized by a light yield with maximum in the 570–580 nm region that is an order of magnitude higher than for CdI₂ crystals in the 490–500 nm band. For α excitation, the radioluminescence kinetics for cadmium iodide is characterized by a very short (∼0.3 nsec) rise time and fast decay of luminescence, with τ₁ ≈ 4 nsec and τ₂ = 10–76 nsec. Depending on the conditions under which the crystals were obtained, the fast component fraction is 95%–99%. The crystal is characterized by a similar scintillation pulse in the case of excitation by x-ray pulses. The radioluminescence pulse shape for CdI₂:Pb in the decay stage is predominantly exponential, with luminescence decay time constants τ₁ ≈ 10 nsec and τ₂ = 200–250 nsec. This system is characterized by low afterglow, at the level for the Bi₄G₃O₁₂ scintillator. We have demonstrated the feasibility of using CdI₂:Pb as a scintillator for detecting α particles. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 825–830, November–December, 2008.     

Thermally stimulated recombination processes and luminescence in Li<Subscript>6</Subscript>(Y,Gd,Eu)(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li₆(Y,Gd,Eu)(BO₃)₃ have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃: Eu, and Li₆Gd(BO₃)₃: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the ⁶ P _J → ⁸ S _7/2 transitions in the Gd³⁺ ion and the group of lines at 580–700 nm due to the ⁵ D ₀ → ⁷ F _J transitions (J = 0–4) in the Eu³⁺ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.     

Thermal and optical properties of Tm<Superscript>3+</Superscript>:NaLa(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

A Tm³⁺-doped NaLa(WO₄)₂ single crystal with a dimension of Φ20 mm×40 mm was grown by the Czochralski method. Anisotropic thermal expansion coefficients of this crystal were investigated. Polarized absorption spectra, emission spectra and decay curve were recorded at room temperature. The absorption and emission cross-section were presented. Based on the Judd–Ofelt analysis, we obtained the three intensity parameters: Ω₂=10.21×10^-20, Ω₄=2.66×10^-20, and Ω₆=1.46×10^-20 cm². The radiative probabilities, radiative lifetimes, and branch ratios of Tm³⁺:NaLa(WO₄)₂ were calculated, too. Luminescence lifetime of the ³ H ₄ level was measured to be 220 μs. The stimulated emission cross-section for the ³ F ₄→³ H ₆ transition were determined using the reciprocity method, potential laser gain for this transition were also investigated, the gain curves implied that the tunable range is up to 200 nm. PACS 42.70.Hj; 78.20.-e     

A study in depth of f<Subscript>0</Subscript>(1370)

Claims have been made that f₀(1370) does not exist. The five primary sets of data requiring its existence are refitted with suitable Breit–Wigner amplitudes. Major dispersive effects due to the opening of the 4π threshold are included for the first time; the σ→4π amplitude plays a strong role. Crystal Barrel data on p̄p→3π⁰ at rest require f₀(1370) signals of at least 32 and 33 standard deviations (σ) in ¹S₀ and ³P₁ annihilation respectively. Furthermore, they agree within 5 MeV for mass and width. Data on p̄p→ηηπ⁰ agree and require at least a 19σ contribution. This alone is sufficient to demonstrate the existence of f₀(1370). BES II data for J/Ψ→φπ⁺π^- contain a visible f₀(1370) signal >8σ. In all cases, a resonant phase variation is required. Cern–Munich data for ππ elastic scattering are fitted well with the inclusion of some mixing between σ, f₀(1370) and f₀(1500). Values of Γ_2π for f₂(1565), ρ₃(1690), ρ₃(1990) and f₄(2040) are determined. PACS 13.25.Gv; 14.40.Gx; 13.40.Hq     

Defect Structure and Up-conversion Luminescence Properties of ZrO<Subscript>2</Subscript>:Yb<Superscript>3+</Superscript>,Er<Superscript>3+</Superscript> Nanomaterials

The up-converting ZrO₂:Yb³⁺,Er³⁺ nanomaterials were prepared with the combustion and sol–gel methods. FT-IR spectroscopy was used for analyzing the impurities. The crystal structures were characterized with X-ray powder diffraction and the mean crystallite sizes were estimated with the Scherrer formula. Up-conversion luminescence measurements were made at room temperature with IR-laser excitation at 977 nm. The IR spectra revealed the conventional and OH⁻ impurities for the combustion synthesis products. The structure of the ZrO₂:Yb³⁺, Er³⁺ nanomaterials was cubic except for the minor monoclinic and tetragonal impurities obtained with the sol–gel method. The materials showed red (650–700 nm) and green (520–560 nm) up-conversion luminescence due to the ⁴F_9/2→⁴I_15/2 and (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺, respectively. The products obtained with the combustion synthesis exhibited the most intense luminescence intensity and showed considerable afterglow.     

Signs of phase transitions and a thermooptic memory effect in absorption spectra of (N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>)<Subscript>2</Subscript>Zn<Subscript>0.8</Subscript>Ni<Subscript>0.2</Subscript>Cl<Subscript>4</Subscript> solid solutions

We have studied the signs of phase transitions and spatial modulation of the structure in the absorption spectra of an (NCH₃)₄)₂Zn_0.8Ni_0.2Cl₄ crystal. We have observed the existence of phase transitions in the given solid solution at temperatures of 155 K, 168 K, 275 K, 280 K, and 296 K. We have established that the thermooptic memory effect observed in the absorption spectra is completely consistent with a model of defect ordering in the sample in the field of the modulated structure. According to this model, stabilization of the sample in an incommensurable phase leads to fixing of a certain symmetry in the crystal (usually a lower symmetry than the average symmetry of the incommensurable phase) and a metal-halogen complex corresponding to the defect wave. As a result, we observe an appreciable shift of the intra-ionic absorption bands and an increase in their intensity. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 717–723, September–October, 2008.     

Photochromic effect in Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystals doped with molybdenum

The steady-state and photoinduced absorption spectra of Bi₁₂SiO₂₀ crystals doped with molybdenum are investigated. It is demonstrated that the impurity absorption is associated with the Mo⁶⁺ and Mo⁵⁺ ions occupying the positions close to the tetrahedral Si⁴⁺ position. The mechanism of the photochromic effect is proposed. This mechanism involves the change in the charge state of molybdenum impurity ions according to the scheme Mo_Si⁶⁺ + e ⁻ → Mo_Si⁵⁺.     

Fundamental band edge optical absorption in Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>

Spectra of optical absorption in Bi_0.5Sb_1.5Te₃ films grown on mica and KBr substrates have been investigated for T = 145 and 300 K. The data obtained have been analyzed together with the data of investigations on the fundamental absorption edge for Bi₂Te₃ available in the scientific literature. It has been revealed that the interband absorption spectra for both Bi_0.5Sb_1.5Te₃ and Bi₂Te₃ represent a superposition of two components corresponding to direct and indirect allowed optical transitions. In this case, the least energy gap separating the valence band and the conduction band is direct for Bi_2−xSb_xTe₃ (x ≤ 1.5, T = 300 K). For Bi_0.5Sb_1.5Te₃ the temperature variation rates have been estimated for the thresholds of direct and indirect interband transitions. It has been shown that this solid solution is direct gap solution at T ≥ 145 K. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 50–52, July, 2008.     

Polarized spectral properties of Er<Superscript>3+</Superscript> ions in NaGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

Polarized spectral properties of Er³⁺:NaGd(WO₄)₂ single crystal are reported. The crystal was grown by the Czochralski method. The Judd–Ofelt theory was applied to analyze the polarized absorption spectra and then calculate the spontaneous emission probabilities, radiative lifetimes, and branching ratios. Fluorescence decay curves of the ⁴ I _13/2, ⁴ I _11/2, and ⁴ S _3/2 multiplets for the Er³⁺ ions were measured. Stimulated emission cross-sections of the ⁴ I _13/2→⁴ I _15/2 transition obtained by the Fuchtbauer–Ladenberg formula and the reciprocity method were compared. Multi-phonon relaxation rates of the crystal were estimated. Green up-conversion fluorescence around 531 and 552 nm was observed, and the possible up-conversion mechanisms were proposed. PACS 78.20.-e; 42.70.Hj     

Hypersensitive and forbidden transitions of trivalent europium ion in Tb1.8Eu0.2 (MoO4)3 single crystal

The fluorescent transitions⁵ D _0.1 →⁷ F _J (J=0−4) of the europium ion in the Tb_1.8Eu_0.2 (MoO₄)₃ single crystal were recorded at 300 and 20 K. The forbidden and the hypersensitive transitions were observed in this system. The intensity ratio between⁵ D ₀ →⁷ F ₁ and⁵ D ₀ →⁷ F ₂ which is 1:5 is discussed in the light of covalency between the Eu³⁺ ion and MoO₄ tetrahedra.