Transformation of radiation-induced defects responsible for theF-band of absorption in NaF crystals subjected to irradiation with light and annealing

It has been found that the F-band of optical absorption which is observed in ψ-irradiated NaF crystals consists of three overlapping bands. The band at 345 nm of width 40 nm is independent of the impurity composition of the crystals; it disappears upon exposure to radiation at 345 nm and appears again upon annealing at 460±15 K for 15 min. This band is correlatable in intensity with the superfine structure of an EPR spectrum. The width (65–110 nm) and the spectral position (355–375 nm) of the second band depend on the impurity composition of the crystal. The band of width 90–110 nm at 320–325 nm disappears upon annealing and appears after exposure to light simultaneously with the disappearance of an EPR signal. It is established that the band at 345 nm is caused by quasimolecules based on fluorine atoms, that at 355–375 nm is triggered by F-centers with a different impurity composition near the haloid vacancy, and the band at 320–325 nm owes its appearance to F-centers in a negatively charged state (F′). Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 849–853, November–December, 1999.     

Molecular probe FET: Long-wavelength fluorescence excitation effect and proton transfer reaction

We studied the steady-state fluorescence spectra of solutions of FET (4′-(diethylamino)-3-hydroxyflavone) in acetonitrile that were excited at different temperatures by quanta with different energies located in the range of the main absorption band and in its long-wavelength wing. We found that, at room temperature, the emission intensity ratio of the bands of the normal and tautomeric forms, which are located at 505 and 570 nm, respectively, depends on the excitation wavelength. In the range of the main absorption band 300–360 nm, this ratio remains nearly the same, i.e., 1.45, while, upon excitation in the range of the long-wavelength wing 360–380 nm of the main band, it decreases to 1.33 at a wavelength of 460 nm. In this same range, a long-wavelength excitation effect that is unusual for liquid inviscid solvents at room temperature, i.e., a bathochromic shift of the entire short-wavelength emission band by 11 nm, manifests itself. We propose to explain these dependences using energy diagrams, which take into account the dependence of free energy on the orientational polarization of the polar solvent. The observed effect of the long-wavelength shift of the fluorescence spectrum with increasing excitation wavelength is explained in terms of the inhomogeneous broadening of electronic spectra of polar solutions, and it should be described using the scheme of energy states that takes into account sublevels of orientational broadening due to orientational dipole-dipole interactions of the fluorophore with nearest molecules of the polar solvent, as well as the relation between the fluorophore lifetime in the excited state and the dielectric relaxation time of solvent molecules in the field of the fluorophore dipole.     

Luminescence of oxygen molecular ion in neodymium-doped yttria

Pulsed cathodoluminescence spectra of Nd³⁺:Y₂O₃ compacts registered after their annealing in air at a temperature above 950°C exhibit a structured band in the range 610–660 nm with four well-resolved components located at wavelengths of 620.6, 630.6, 645.3, and 655.6 nm. At the same time, the lattice parameter of the cubic yttria increases, and the color of samples changes from light blue to yellowish. In addition, the nearly complete absorption of the intrinsic luminescence band of yttria is observed in the range of 380–600 nm with a center at λ = 485 nm. We assume that these effects are caused by oxygen molecular ions O₂⁻, which are formed in cubic yttria due to the penetration of oxygen into natural anion vacancies. The frequencies of vibrations of the ground and excited states of the oxygen molecular ion has been determined.     

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.     

Time-Resolved Vacuum Ultraviolet Spectroscopy of Er<Superscript>3+</Superscript> Ions in the SrF<Subscript>2</Subscript> Crystal

The photoluminescence and photoexcitation spectra as well as the luminescence decay kinetics of Er³⁺ ions in the visible ultraviolet and vacuum ultraviolet (VUV) regions have been studied by the method of low-temperature, time-resolved VUV-spectroscopy on excitation by synchrotron radiation. In the VUV spectral region of the luminescence of SrF₂:1% Er³⁺, the 146.5-nm band with a time of decay of less than 0.6 nsec was revealed together with the well-known emission band at 164.3 nm (decay constant in the microsecond range). Its possible nature is discussed. The specific features of the formation of photoexcitation spectra of the f-f and f-d transitions in the Er³⁺ ion are considered. Competition between the processes of excitation of f-f and d-f luminescence has been revealed. It manifests itself in the inverse relationship of their photoexcitation spectra in a range of energies of incident photons that are close to the position of the 4fⁿ⁻¹5d configuration levels. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 519–523, July–August, 2005.     

Photoluminescence and Raman scattering in spatially inhomogeneous heteroepitaxial InGaN layers

Spatial inhomogeneities of the indium distribution in In _x Ga_1–x N epitaxial layers grown on sapphire substrate with a GaN buffer layer were investigated using photoluminescence (PL) in addition to confocal scanning Raman spectroscopy (RS) and PL. Broad emission bands from In-enriched InGaN nanoclusters (700–900 nm) and from the volume outside the clusters (about 460 nm) were observed in PL spectra of an epitaxial InGaN layer with an average In content of 25.7%. It was established that larger micro-PL intensities corresponded to energetically shallower clusters. The observed broadly asymmetric A₁(LO) RS band of InGaN confirmed that the In concentration in the layer was highly variable. Modeling the LO phonon band by two Lorentzian curves gave an average In concentration of 21% in the volume outside the clusters and 37% in the nanoclusters, which was considerably higher than the average concentration in the layer and agreed well with their PL band positions.     

Blue emission in Ti-sapphire laser crystals

The time resolved emission spectrum of the blue band of Ti:sapphire laser crystal has been investigated as a function of temperature (range 10–290 K) and UV (266 nm) laser excitation intensity. Two blue emission bands, centred at 420 nm and 460 nm, have been detected. The 420 nm band is attributed to Ti⁴⁺ centres whereas the 460 nm one is proposed to be due to Ti³⁺ ions. The evolution of the emission spectrum vs the UV excitation intensity has shown that the concentration of Ti⁴⁺ centres is increased under UV irradiation at the cost of the centres responsible for the 460 nm band.     

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.     

Laser initiation of a mixture of PETN with NiC nanoparticles at elevated temperatures

The temperature dependence of the probability of the explosion of pentaerythritol tetranitrate (PETN) with an admixture of NiC particles (0.3 wt %) initiated by laser pulses (1064 nm, 20 ns) was studied over the temperature range 295–450 K. At 295–350 K, a weak temperature dependence was observed. The determining contribution to explosion initiation was made by the absorption of laser radiation by nanoparticles. The threshold of explosive decomposition at 295 K decreased by ∼40 times compared with samples free of NiC nanoparticles. Over the temperature range 400–450 K, the threshold of the explosive decomposition of samples containing NiC nanoparticles decreased with the activation energy ∼0.4 eV. A decrease in the threshold of explosive decomposition with a ∼0.4 eV activation energy over the temperature range 340–440 K was also observed for laser action on PETN samples not containing NiC. A hypothesis was suggested according to which the absorption of a light quantum caused the transfer of an electron from the valence band of the crystal to a level in the forbidden band with subsequent thermal positive ion dissociation to the carbocation and NO₃ radical.     

Emission of Ionic Molecules (KrXe)<Superscript>+</Superscript> on Excitation by a Hard Ionizer

The molecular luminescence band appearing in the 445–510-nm region on excitation of a Kr-Xe mixture by the α particles of polonium 210 as well as by the products of nuclear reaction ³He(n,p)T has been investigated. Radiation at the transition of the heteronuclear ionic molecule (KrXe)⁺ has a high excitation efficiency and is observed in a temperature range from −100 to +600°C. The kinetics of the processes in the Kr-Xe mixture and the possibility of using radiation with λ ∼ 490 nm in radioactive light sources are considered. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 301–304, May–June, 2005.     

Influences of capping molecules on optical properties of nanocrystalline ZnS:Cu

ZnS:Cu nanocrystals capped with different capping molecules have been successfully synthesized by a simple aqueous method. The prepared nanocrystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive analysis by X-rays (EDAX). The surface characterization of the nanocrystals was done by FTIR spectroscopy. The effect of capping agents on absorption and photoluminescence (PL) spectra of the ZnS:Cu nanocrystals was studied. A blue shift of the absorption peaks was observed and attributed to a quantum confinement effect, which increases the band gap energy. The photoluminescence spectra of the capped ZnS:Cu nanocrystals showed a broad peak in the range of 460–480 nm. The intensity of the PL spectra strongly depended on the capping agents.     

Infrared luminescence of erbium-doped sodium lead germanate glass

The infrared optical properties of Er³⁺ ions are reported for 60GeO₂–20PbO–20Na₂O glass for two Er₂O₃ concentrations. From the optical absorption spectra, the Judd–Ofelt (J–O) parameters have been obtained and have been used to calculate radiative lifetimes and stimulated emission cross sections. A narrow emission band peaked at 1536 nm with exponential decay for both Er concentrations is observed. The measured lifetime decreases for increasing Er concentration, its value being very close, in the case of the lowest doping level, to the radiative lifetime calculated from J–O analysis. This, together with the relatively high emission cross section makes this glass suitable for laser applications.     

Spectral and kinetic properties of fluorescence of immunoactive steroids in the gas phase

We have used steady-state fluorescence spectroscopy and pulsed kinetic fluorimetry with high time resolution to experimentally study the spectral and temporal characteristics of luminescence of immunoactive 8-aza-D-homogona-1,3,5(10), 13-tetraene-12,17a-dione (8-aza-D-homogonane) molecules in the gas phase. We have established the existence of several centers emitting long-wavelength and short-wavelength fluorescence. The fluorescence of high-temperature 8-aza-D-homogonane vapor (T = 550 K) when excited by radiation with λ_ex = 266 nm is represented by two spectrally separated emission bands with maxima λ _max ^fl = 465 nm and λ _max ^fl = 365 nm. Analysis of the distribution of fluorescence decay times for 8-aza-D-homogonane showed that in the spectral range of the emission wavelengths 360–590 nm, the fluorescence decay kinetics contains three components with average lifetimes <200 psec, 2.8 nsec and 13.5 nsec. The multicenter nature of luminescence of 8-aza-D-homogonane was confirmed by direct measurements of instantaneous fluorescence spectra in different stages of luminescence decay. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 309–314, May–June, 2006.     

Effect of neutron irradiation of colorless sapphire (leucosapphire) single crystals on their absorption and luminescence spectra

Features of the thermoradiative changes in the optical absorption and luminescence spectra of leucosapphire (colorless sapphire) crystals irradiated by neutron fluences within the range 5·10¹⁵ to 5·10¹⁹ cm⁻² have been studied in the visible region. The stepwise character of these processes was established as well as the basic steps involved in bleaching the induced color of the wafers as a result of isochronal annealing. Some anomalies have been observed in the temperature dependence of the optical density of the 460–620 nm bands, the activation energies for the color centers as well as the color center concentrations have been calculated, and the nature of the radiation-induced centers has been analyzed. An analytical expression is proposed to describe the accumulation kinetics for the centers during irradiation of the crystals by reactor neutrons. It is concluded that there is an interconnection between, for example, the 460 nm color centers and the 540 nm luminescence centers, and that there is a common mechanism for the process of radiation-induced defect formation, initially responsible for their formation in the crystal. The possible effect of reabsorption of radiation on the behavior of the Y(Φ) curve in the irradiated material is discussed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 247–251, March–April, 2007.     

Light-emitting Si nanostructures formed in silica layers by irradiation with swift heavy ions

Thin SiO₂ layers were implanted with 140 keV Si ions to a dose of 10¹⁷ cm⁻². The samples were irradiated with 130 Mev Xe ions in the dose range of 3×10¹²–10¹⁴ cm⁻², either directly after implantation or after pre-annealing to form the embedded Si nanocrystals. In the as-implanted layers HREM revealed after Xe irradiations the 3–4 nm-size dark spots, whose number and size grew with increase in Xe dose. A photoluminescence band at 660–680 nm was observed in the layers with the intensity dependent on the Xe dose. It was found that passivation with hydrogen quenched that band and promoted emission at ∼780 nm, typical of Si nanocrystals. In spectra of pre-annealed layers strong ∼780 nm peak was observed initially. Under Xe bombardment its intensity fell, with subsequent appearance and growth of 660–680 nm band. The obtained results are interpreted as the emission at ∼660–680 nm belonging to the imperfect Si nanocrystals. It is concluded that electronic losses of Xe ions are mainly responsible for formation of new Si nanostructures in ion tracks, whereas elastic losses mainly introduce radiation defects, which quench the luminescence. Changes in the spectra with growth of Xe ion dose are accounted for by the difference in the diameters of Xe ion tracks and their displacement cascades.     

Characteristic features of thermoluminescence kinetics in dosimetric aluminum oxide crystals

We have studied the thermoluminescence curves in the 420 nm band (F centers) and the 330 nm band (F⁺ centers) within the temperature range of the dosimetric peak (T_max = 450 K) in anion-deficient aluminum oxide crystals. Assuming general-order kinetics, we have analyzed the thermoluminescence decay curves on the rising and falling sides of the dosimetric peak, in samples with different degrees of deep trap filling. We have established differences in the order of the kinetics within different temperature ranges of the dosimetric peak. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 187–190, March–April, 2006.     

Luminescence of biologically active 24-epicastasterone and a model compound

Biologically active brassinosteroid 24-epicastasterone, ring B of which contains a C=O group and has the nπ*-configuration for a low-lying electronic excited state, exhibits rapid fluorescence. The wavelengths of the fluorescence maxima of the steroid dissolved in hexane and acetonitrile are equal to 332 and 394 nm, respectively. The fluorescence lifetime of the steroid dissolved in acetonitrile is τ = 9.9 nsec. Solutions of 24-epibrassinolide do not luminesce. The long-wavelength electronic absorption band λ_max^abs = 340 nm in the absorption spectrum of an ethanol solution of model compound 2, ring D of which contains a C=O group π*-conjugated with the C=C double bond of ring C, like in the spectrum of the steroid, has a low extinction coefficient. An ethanol solution of 2 does not fluoresce. 24-Epicastasterone at 77 K in ethanol solution exhibits phosphorescence with λ_max^phos = 447 nm. The phosphorescence decay is exponential with τ = 0.79 msec. Compound 2 also phosphoresces. The phosphorescence spectrum of its ethanol solution has a maximum at 490 nm. The phosphorescence decay is nonexponential in the early stage. The phosphorescence lifetime is 25 msec in the exponential decay region. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 182–186, March–April, 2008.     

Electrical conductivity and dielectric properties of SiO<Subscript>2</Subscript> nanoparticles dispersed in conducting polymer matrix

Electrical and dielectric properties of conducting polypyrrole–wide band gap silica (PPY–SiO₂) nanocomposites have been investigated as a function of temperature and frequency for different concentrations of polypyrrole. The average grain size of the nanocomposites is in the range of 40–80 nm. Impedance spectra reveal two distorted semicircles corresponding to grain and grain boundary effects. The magnitude of conductivity and its temperature variation are significantly different from polypyrrole and silica. A very large dielectric constant of about 4800 at 30 kHz and at room temperature has been observed for the highest concentration of silica. Inhomogeneous behavior of nanocomposites gives rise to high dielectric constant.     

All-fiber high-energy supercontinuum pulse generator

An all-fiber supercontinuum generator with a record-high pulse energy of 40 μJ is presented. The generator is based on a nanosecond ultralong high-energy mode-locked Yb-doped fiber laser with an additional amplification stage. The supercontinuum spectrum belongs to the wavelength range 500–1750 nm, and a relatively uniform spectral distribution of the intensity is observed in the interval 1125–1550 nm. The mean power of the supercontinuum is greater than 1.5 W. The simulation of such a generator yields the integrity of the supercontinuum pulse on the nanosecond time scale and shows that the pulse can be characterized by a certain energy in contrast to the multipulse complicated trains of supercontinuum corresponding to the femtosecond and picosecond pumping.     

A UV–Visible–NIR fluorescence lifetime imaging microscope for laser-based biological sensing with picosecond resolution

This article describes the design and characterization of a wide-field, time-domain fluorescence lifetime imaging microscopy (FLIM) system developed for picosecond time-resolved biological imaging. The system consists of a nitrogen-pumped dye laser for UV–visible–NIR excitation (337.1–960 nm), an epi-illuminated microscope with UV compatible optics, and a time-gated intensified CCD camera with an adjustable gate width (200 ps-10^-3 s) for temporally resolved, single-photon detection of fluorescence decays with 9.6-bit intensity resolution and 1.4-μm spatial resolution. Intensity measurements used for fluorescence decay calculations are reproducible to within 2%, achieved by synchronizing the ICCD gate delay to the excitation laser pulse via a constant fraction optical discriminator and picosecond delay card. A self-consistent FLIM system response model is presented, allowing for fluorescence lifetimes (0.6 ns) significantly smaller than the FLIM system response (1.14 ns) to be determined to 3% of independently determined values. The FLIM system was able to discriminate fluorescence lifetime differences of at least 50 ps. The spectral tunability and large temporal dynamic range of the system are demonstrated by imaging in living human cells: UV-excited endogenous fluorescence from metabolic cofactors (lifetime ∼1.4 ns); and 460-nm excited fluorescence from an exogenous oxygen-quenched ruthenium dye (lifetime ∼400 ns). Received: 23 February 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-734/9361-905, E-mail: mycek@umich.edu