Synchrotron and laser excitation of luminescence in PbWO<Subscript>4</Subscript>:Tb crystals at different temperatures

The effect of temperature on the spectral luminescence characteristics of PbWO₄:Tb³⁺ crystals with synchrotron and laser excitation is studied. If PbWO₄:Tb³⁺ is excited by synchrotron radiation with λ = 88 nm at 300 K, a faint recombination luminescence of the impurity terbium is observed against the matrix luminescence. When the temperature is reduced to 8 K, the luminescence intensity of PbWO₄:Tb³⁺ increases by roughly an order of magnitude and the characteristic luminescence of the unactivated crystal is observed. Excitation of PbWO₄:Tb³⁺ by a nitrogen laser at 300 K leads to the appearance of emission from Tb³⁺ ions. At 90 K, a faint matrix luminescence is observed in addition to the activator emission. The formation of the luminescence excitation spectra for wavelengths of 60–320 nm is analyzed and the nature of the emission bands is discussed.     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.     

Luminescence excitation spectra and characteristics of luminescence centers with overlapping absorption bands

For an ensemble of different types of luminescence centers with overlapping absorption bands, with no restrictions on the optical densities, we have obtained relations describing the luminescence excitation spectra for each type of center. We consider transformations of the relations in some limiting cases. We suggest a procedure for using the equations obtained to determine the characteristics of the luminescence centers. Some of these procedures have been experimentally implemented in study of intrinsic radiation color centers in lithium fluoride crystals. We have determined the ratios of the luminescence quantum yields for F₂ and F₃⁺ color centers, and we have observed that a major role is played by nonradiative transitions in deactivation of the first excited singlet state of F₃⁺ centers. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 365–371, May–June, 2008.     

Time-resolved spectroscopy of natural and synthetic BeO crystals

The results of comparative luminescence investigation of natural and synthetic BeO crystals are presented. Time-resolved luminescence (2.5–8 eV) and luminescence excitation spectra, and the kinetics of glow decay were measured using ultraviolet-vacuum-ultraviolet (VUV) synchrotron radiation (5–22 eV) or x-radiation (50–620 eV or 3–62 keV) ranges. X-ray and thermostimulated luminescence of natural BeO crystals were compared to the glow of additively colored synthetic crystals. The characteristic luminescence of F and F ⁺ centers was found in natural crystals. In synthetic crystals similar luminescence is observed only after additive or radiation coloration by virtue of the creation of F ⁻ and F ⁺ centers on anion vacancies. The defects found in the crystal lattice of a natural BeO crystal testify to the degree of mineral metamictization of the given deposit.     

Sensitized anti-Stokes luminescence centers in microcrystals of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed dye molecules and few-atomic silver clusters

The sensitized anti-Stokes luminescence excited by radiation with wavelengths from 610 to 750 nm and flux densities of 10¹⁴–10¹⁵ quanta/(cm²·s) is detected for microcrystals of Zn _0.6 Cd _0.4 S solid solutions with adsorbed organic malachite green and methylene blue dye molecules. The position of its excitation spectra coincides with that of the absorption spectra of adsorbed dye molecules, which suggests the cooperative mechanism of its occurrence. The possibility of amplification of the anti-Stokes luminescence by means of adsorption of silver atoms and few-atomic silver clusters, in addition to the dye molecules, on the Zn _0.6 Cd _0.4 S surface is investigated. It is assumed that in the latter case, the anti-Stokes luminescence is excited as a result of two-quantum optical transitions with electron or electron excitation energy transfer from the dye molecules adsorbed on the Zn _0.6 Cd _0.4 S surface to silver atoms and few-atomic silver clusters creating deep local states with photoionization energies of 1.8–2.0 eV in the gap. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 21–26, March, 2008.     

Explosive luminescence of silver azide

Spectroscopic-kinetic investigations of the luminescence accompanying the explosive decomposition of silver azide are performed. A new phenomenon is observed: predetonation luminescence. A comparison of the predetonation luminescence spectrum with the band structure is in agreement with a model in which the exothermic reaction 2N ₃ ⁰ →N₆ provides the energy for the explosion. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 101–103 (25 July 1997)     

Persistent luminescence — Quo vadis?

The present status of the persistent luminescence mechanisms is reviewed and the remaining unsolved details are discussed. These details include the identification and role of defects in the Eu²⁺-doped and R³⁺ co-doped alkaline earth aluminates (MAl₂O₄) and disilicates (M₂MgSi₂O₇; M:Ca, Sr, Ba) which can be partly resolved by the thermoluminescence (TL) measurements. The use of the synchrotron radiation - presently only sparsely used in the studies of persistent luminescence - is introduced: the oxidation state of the presumed R²⁺/R³⁺/R^IV species occurring in the persistent luminescence materials during the luminescence processes were examined with synchrotron radiation XANES (and EXAFS) methods. The band gap energies (E_g), the defect-related luminescence as well as the 4f⁷→4f⁶5d¹ transition energies were derived from the synchrotron radiation excitation spectra of the materials. Subsequently, the early steps of the density functional theory (DFT) calculations involving the solution of the persistent luminescence mechanisms (band gap energies, position of the Eu²⁺ levels) are discussed. Some remaining challenges are eventually highlighted.     

Zeeman effect in the luminescence spectra of terbium-yttrium-aluminum garnet

The Zeeman effect is investigated in the green luminescence band corresponding to the 4f→4f radiative transition ⁵D₄ → ⁷F₅ of Tb ³⁺ ions in Tb _0.2 Y _2.8 A _l5 O ₁₂ terbium-yttrium-aluminum garnet at the temperature T = 85 K. It is demonstrated that an external field causes not only the Zeeman shifts of the resonant frequencies of radiative transitions in the luminescence spectra but also changes in the emission line intensities caused by modification of the optical transition probabilities in the σ₋ and σ₊ opposite circular polarization states. Special features of field dependences of the Zeeman effect for the emission lines are qualitatively explained based on the results of numerical energy spectrum calculations for the ⁵D₄ and ⁷F₅ multiplets of the Tb ³⁺ ion in yttrium-aluminum garnet in the geometry of the longitudinal and transverse Zeeman effects. The possibility of nonequilibrium population of sublevels of the (Γ₁, Γ₄) quasi-doublet state of the ⁵D₄ multiplet with energy of ∼20585 cm⁻¹ is demonstrated in an external magnetic field. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 38–45, June, 2008.     

Analysis of the latent structure of luminescence and absorption spectra of thallium complexes

Comprehensive investigations of luminescence, excitation, and absorption spectra as well as of the luminescence kinetics of a frozen LiCl-Tl⁺ solution are carried out. It is established that the luminescence spectrum consists of four components. One component is caused by luminescence of the matrix and the remainder by luminescence of one luminescence center, namely, by the saturated complex of thallium TlCl₂(H₂O)Cl₄. The absorption spectrum consists of three components. Their parameters have been evaluated. Each component of the luminescence spectrum is excited in several components of the absorption spectrum. It is found that the luminescence spectrum components and their intrinsic absorption bands are located differently on the frequency axis. These data are similar to those obtained for other activated solutions of electrolytes. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 75–78, January–February, 2000.     

Spectral luminescence properties of Eu(III) complexes with tetracycline antibiotics and hydrogen peroxide

We have studied the spectral luminescence properties of mixed-ligand complexes of Eu(III) ions with tetracyclines and hydrogen peroxide. We have established that incorporation of hydrogen peroxide into the inner sphere of these complexes leads to a significant increase (by a factor of 10–25) in the intensity of luminescence of the Eu(III) ions, redistribution of the intensities of the ⁵D₀ → ⁷F_j transitions, in particular to an anomalous increase in the intensity of the band for the forbidden transition ⁵D₀ → ⁷F₀. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 310–314, May–June, 2007.     

Recombination luminescence of CaSO4:Tb3+ and CaSO4:Gd3+phosphors

A comparative study of the excitation of luminescence by VUV radiation as well as of thermally and photostimulated luminescence has been carried out for CaSO₄:Tb³⁺ and CaSO₄:Gd³⁺ phosphors, where Na⁺ or F⁻ ions are used for charge compensation. The distinction in hole processes for the phosphors with Na⁺ or F⁻ compensators is determined by the differing thermal stability of the holes localized at/near Tb³⁺Na⁺ and Gd³⁺Na⁺ (up to 100–160 K) or at/near Tb³⁺F⁻ V _Ca and Gd³⁺F⁻ V _Ca centers involving also a cation vacancy (up to 400–550 K). Tunnel luminescence in the pairs of localized electrons and holes nearby Tb³⁺ or Gd³⁺ has been detected. The mechanisms of electron-hole, hole-electron and tunnel recombination luminescence as well as a subsequent released energy transfer to RE³⁺ ions are considered.     

Intraband luminescence spectra of insulators and semiconductors excited by pulsed electron beams or electric fields

The features of the intraband luminescence spectrum of wide-gap insulators (KI, KBr, CsCl, etc.) and semiconductors (GaP, CdS, α-SiC, and ZnS) are studied in the temperature interval 80–760 K. The spectra of the intraband luminescence are compared with the spectra of the pre-breakdown electroluminescence of GaP and α-SiC surface-barrier diode structures and of a ZnS thin-film electroluminescence indicator. In alkali halide crystals the short-wavelength boundary hν _m of the intraband luminescence is less than the band gap E _g and is governed by complex excitonic processes. In semiconductors with indirect transitions hν _m>E _g. The differences in the spectra of the intraband luminescence and the intraband pre-breakdown electroluminescence can be explained by differences in the distributions of the hot charge carriers over levels of the allowed bands and in the maximum energies of the carriers involved in the formation of the spectra. Fiz. Tverd. Tela (St. Petersburg) 39, 613–617 (April 1997)     

Formation of a plane layer of plasma under irradiation by a soft X-ray source

We investigate theoretically the formation of a plasma in a plane layer of polymer foam (density ρ = 0.002 g/cm³ and thickness 800 μm) under the action of an external source of soft X-ray radiation under the conditions of PHELIX experiments. The incident flux is assumed to have a Planck’s distribution over the spectrum with T _rad = 20–40 eV. In numerical calculations, the flux of incident X-ray radiation and the spectral constants of the target substance are varied. The action of an external X-ray radiation source on a low-density foam substance with a density of 2 mg/cm³ causes a plasma to be formed with relatively homogeneous profiles of density and temperature T = 15–35 eV. Absorption of externalradiation energy is distributed in the volume. The plasma temperature increases with increase in the external energy, and the energy passed through the plasma also increases. The results prove to be sensitive to the values of optical constants used in numeral simulation. The spectral flux of external radiation passed through the plasma is chosen as a criterion of correctness of the optical constants used in the calculations. In future experiments using the PHELIX facility, we plan to investigate the slowing-down of an ion beam in a plasma formed as a result of indirect heating of low-density polymer triacetate cellulose (TAC) foam with densities ρ = 0.001–0.01 g/cm³ under the action of a pulse of X-ray radiation, into which the laser radiation is preliminarily transformed.     

Formation mechanisms and structure of the luminescence spectra of a dense resonant medium

The purely thermal visible and infrared radiation emitted by a dense resonant medium (sodium vapor) heated nonuniformly to temperatures of 600–1200 K was investigated experimentally for the first time under conditions where the photon mean free path is comparable with the emission wavelength. The profile of the recorded spectra and the absolute luminescence intensities in the different spectral ranges show good agreement with the results of a numerical simulation using a previously developed theory of resonance radiation transport which assumes a Boltzmann spectral distribution of the resonant level population proportional to exp(−ℏω/T). The self-reversed resonant sodium line exhibited strong asymmetry and it was shown that under certain conditions, the luminescence spectrum of the medium may exhibit an additional broad peak on the far “red” limb of the resonance line. Calculations and measurements demonstrated that the intensity of the thermal emission of sodium vapor at this red peak is several orders of magnitude higher than that obtained from the standard theory of resonance radiation transport. This effect is arbitrarily termed an infrared “ catastrophe.” It is noted that in a solar corona plasma and in gas-discharge lamps, the far red limbs of the resonant lines may make a substantial contribution to the total luminescence intensity and in some cases, considerably exceed the intensity of the photorecombination and bremsstrahlung continuum. Zh. éksp. Teor. Fiz. 114, 135–154 (July 1998)     

Anti-stokes luminescence of solid AgCl<Subscript>0.95</Subscript>I<Subscript>0.05</Subscript> solutions

An anti-Stokes luminescence band with λ_max = 515 nm of microcrystals of solid AgCl_0.95I_0.05 solutions excited by a radiation flux of density 10¹³–10¹⁵ quanta/cm²·sec in the range 600–800 nm at 77 K was detected. It is shown that the intensity of this luminescence and the frequency of its excitation depend on the prior UV-irradiation of samples. Analysis of the stimulated-photoluminescence spectra and the anti-Stokes luminescence excitation spectra of the indicated microcrystals has shown that to the centers of anti-Stokes luminescence excitation correspond local levels in the forbidden band of the crystals. These states are apparently due to the atomic and molecular disperse silver particles that can be inherent in character or formed as a result of a low-temperature photochemical process. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 738–742, November–December, 2005.     

Delayed luminescence of indole derivatives in the gas phase

Spectral and kinetic characteristics of delayed luminescence of indole derivatives in the gas phase have been studied. The mechanism for delayed luminescence has been shown to depend on the disposition of the two lowest excited singlet ππ*-states, ¹L_a and ¹L_b. For vapors of 3-methyl-and 2,3-dimethylindole having a small ¹L_a-²L_b energy gap ΔE (ΔE < 500 cm⁻¹), prolonged light emissions with maxima at 525 and 540 nm, respectively, have been observed and interpreted as luminescence of free radicals formed due to dissociation of an N-H bond. For vapors of 5-methoxyindole having ΔE ≈ 3700 cm⁻¹, annihilation of delayed luminescence has been observed. The lifetimes for triplet states of 3-methyl-, 2,3-dimethyl-, and 5-methoxyindole at T = 373 K equal to 30, 26, and 65 μsec, respectively, were calculated from the kinetics of the delayed luminescence. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 503–508, July–August, 2008.     

IR luminescence of thulium-activated crystals of double sodium yttrium fluoride Na0.4Y0.6F2.2:Tm3+: Self-quenching of luminescence from the 3H4 level

Na_0.4Y_0.6F_2.2:Tm³⁺ crystals with thulium contents of 0.5–20% have been grown by the Stockbarger-Bridgman method. The concentration dependences of the steady-state spectra and luminescence kinetics from the ³ H ₄ and ³ F ₄ thulium levels upon selective laser excitation of a series of Na_0.4Y_0.6F_2.2:Tm³⁺ crystals (0.5–20% Tm) are investigated. It is shown that the luminescence from the ³ H ₄ level becomes significantly quenched with an increase in the concentration due to the nonradiative energy transfer. No concentration quenching of luminescence from the ³ F ₄ level is observed. Possible schemes of self-quenching are considered for the ³ H ₄ thulium level, the self-quenching microparameters and macrorates are estimated using model quantum-mechanical calculation, and the most likely self-quenching mechanisms and schemes are found from comparison of the calculated and experimental self-quenching rates. Good agreement is obtained between the experimental and calculated kinetic curves. It is shown that, under pumping near ∼0.8 μm, self-quenching of the ³ H ₄ thulium level leads to effective occupation of the ³ F ₄ level and increases the luminescence intensity in the range of 1.7–1.9 μm. It is concluded that Na_0.4Y_0.6F_2.2:Tm³⁺ crystals are promising active media for tunable lasers in the range of 1.7–1.9 μm, with small thermal loss under laser-diode pumping near 0.8 μm. Original Russian Text ? S.é. Ivanova, A.M. Tkachuk, A.A. Mirzaeva, F. Pellé, 2009, published in Optika i Spektroskopiya, 2009, Vol. 106, No. 6, pp. 922–930.     

Electronic excitations in ZnWO4 and ZnxNi1?xWO4 (x = 0.1 ? 0.9) using VUV synchrotron radiation

The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO₄ as well as the Zn _x Ni_1−x WO₄ solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO₆]⁶⁻ anions, becomes modulated by the optical absorption of Ni²⁺ ions in the Zn _x Ni_1−x WO₄ solid solutions; and (ii) no significant change in the excitation spectra of Zn_0.9Ni_0.1WO₄ is observed compared to pure ZnWO₄. At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO₄.     

Use of sub-Doppler optical resonances for measurement of weak magnetic fields by means of extremely thin rubidium vapor cell

We study experimentally and theoretically D ₁ lines of ⁸⁵Rb and ⁸⁷Rb atoms and show that using atomic-velocity-selective optical resonances which are formed in the transmission spectrum of an atomic rubidium-filled submicron cell at single pass of linearly polarized laser radiation, it is possible to measure weak magnetic fields beginning with 5 G. Having in mind the results obtained earlier with use of also submicron cell with ⁸⁷Rb (D ₁ line) and circularly polarized laser radiation, the entire range of measurable magnetic fields (both homogeneous and inhomogeneous) becomes 5–5000 G.