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.     

Intrinsic superlinear dose dependence of thermoluminescence and optically stimulated luminescence at high excitation dose rates

Superlinear dose dependence of thermoluminescence (TL) and optically stimulated luminescence (OSL) has been reported for many materials. The theoretical explanation has been ascribed to competition of either traps or recombination centers, during the excitation stage or during the read-out phase. There has been an account in the literature on superlinearity of OSL associated with merely one trapping state and one kind of recombination center. This had to do with the process taking place during the read-out stage, namely the optical stimulation. In the present work, we report on a model of one trapping state and one kind of recombination center which results in a superlinear filling of the center. Thus, one can expect a superlinear dose dependence of the area under the resulting TL glow peak as well as the OSL signal. We follow this situation by writing the simultaneous nonlinear rate equations for the one-trap-one-recombination-center (OTOR) model and study the expected results by numerical simulation consisting of solving the equations with sets of the trapping parameters. We also present analytical results based on simplifying assumptions, and compare the analytical and numerical results. The effect is significant at relatively high dose rates. The main implication is that when one tries to evaluate by TL dosimetry a dose applied at a high rate, calibration of the TL dosimeter using much smaller dose rates may result in inaccurate results.     

Excitation spectra and structure of luminescence centers of manganese ions in single crystals of zinc sulfide

We investigate the photoluminescence excitation spectra in ZnS:Mn single crystals at room temperature and at the temperature of liquid nitrogen with a different concentration of Mn²⁺ ions. The strongest bands peaking at 557, 578, 600, and 637 nm are associated with a different position of the Mn ion in the lattice of the crystals under investigation. The difference obtained in the excitation spectra can be explained by the resonance transfer of energy between the Mn ions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 208–210, March–April, 2000.     

Spectral characteristics of luminescence of solutions of complex molecules under conditions of electronic excitation transfer to a saturable acceptor in the case of inhomogeneous broadening of electronic energy levels

Spectral characteristics of solutions of complex molecules under conditions of inhomogeneous broadening of energy levels are considered in the case when the nonlinear dependence of the population of molecular states on the excitation intensity is determined not by saturation of molecular levels but by exchange of the electronic excitation energy with the environment. Calculations have shown that the dependence of the position of the fluorescence spectrum on the excitation frequency is nonmonotonic in solutions of the type and varies substantially with the excitation intensity. Belarusian State University, 4, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 164–168, March–April, 1997.     

HVPE外延GaN膜中黄带的光致发光激发谱研究

测量了氢化物气相外延方法生长的非特意掺杂和掺碳GaN外延膜的光致发光谱，并在光致发光谱峰位2．25eV(550nm)附近分别测量了光致发光激发谱，对两者进行了比较．从光致发光谱中发现掺碳使黄带明显增强，从光致发光激发谱中看到了掺碳引起的约3．38～2．67eV(367～465nn)范围内的特征激发带．利用CC曲线模型说明了特征激发带和黄带之间的关系，分析了黄带的可能起因．     

Concentration self-quenching in solid solutions of complex organic molecules under intense excitation

A theoretical study was carried out to investigate concentration self-quenching which is nonlinear in intensity in a molecular system caused by nonradiative electronic excitation energy transfer to relatively long-lived acceptors that are formed in a solution and whose function is performed by molecules that have acquired the triplet state. It is shown that at ratios of constants of radiative and nonradiative transitions typical of complex molecules at concentrations of ∼10⁻³–10⁻² mole/liter substantial (2- to 10-fold) fluorescence takes place at intensities of excitation of ∼10⁴–10³ of the intensity of saturation for a singlet subsystem. Polarization characteristics of the system are analyzed. Belarusian State University, 4, F. Skorina Ave., 220050, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 729–733, November–December, 1997.     

Investigation of the mechanisms of luminescence of ZnS−Ag(Al) single crystals by the method of time-resolved spectroscopy

By the methods of amplitude and time-resolved spectroscopies the mechanisms of luminescence of ZnS−Ag(Al) single crystals differing in dislocation density are studied in pulse laser excitation. Experimental results are analyzed based on the theory of donor—acceptor pairs and the theory of centers surrounded with defect clouds. The data obtained are in agreement with the second theory. Ural State Technical University, 19 Mir Str., Ekaterinburg, 620002, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 857–860, November–December, 1999.     

Intrinsic ultraviolet luminescence of LiB<Subscript>3</Subscript>O<Subscript>5</Subscript> single crystals under inner-shell excitation

Ultraviolet photoluminescence (PL) of LiB₃O₅ (LBO) crystals has been studied under selective excitation by photons in the vacuum ultraviolet and ultrasoft x-ray regions, including the K-absorption edges of the Li and B cations and O anion. Radiative recombination of electron-hole pairs was established to be the main channel of the intrinsic PL excitation at 4.2 eV. Features were observed in the PL excitation spectra near the lithium and boron K-absorption edges originating from excitation of the cation 1s core excitons. Experimental evidence of the multiplication of Li 1s excitons in LBO was obtained. It is shown that excitation of the O 1s core excitons does not affect the PL yield noticeably. The differences in the appearance of the Li, B, and O 1s excitons in the excitation spectra of the LBO ultraviolet PL are discussed.     

The third-order optical nonlinearity and upconversion luminescence of CdTe quantum dots under femtosecond laser excitation

The aqueous CdTe quantum dots (QDs) were synthesized by the electrostatic reaction method. The optical properties of CdTe QDs were investigated by femtosecond Z-scan and time-resolved luminescence technique in nonresonant spectral region. The nonlinear absorption and refraction are ascribed to two-photon absorption, and time-resolved upconversion photoluminescence produces biexponential decay pattern at infrared femtosecond laser excitation. Upconversion luminescence is composed of a band-edge excitonic state and a photoinduced trapping state. The short-lived band-edge excitonic emission is independent of the detection wavelengths, and long-lived species becomes even longer with the increase of detection wavelengths, which indicates the size dependence of surface excitonic emission. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Novel ultra-fast luminescence from incipient ion tracks of insulator crystals: electron–hole plasma formation in the track core

Measurements of fast luminescence decay and time-resolved spectra revealed novel ultra-fast luminescence with the lifetime of several tens ps in heavy-ion-irradiated single crystals of LiF, NaF, NaCl, KCl, KBr, KI, RbI, CsCl, CsBr, CsI, -alumina, and MgO. The luminescence is furthermore characterized by a super-linear increase in the efficiency with increasing excitation density, non-tailed decay curve, and temperature-insensitive decay-rate and yield. The results mean that the luminescence neither originates from localized excited states such as self-trapped excitons, free excitons, excited defects, and excited impurity centers nor their interaction. A process which does not contradict the experimental results is the formation of the e–h plasmas and the luminescence from them.     

真空紫外光激发下掺Pr3+铝酸盐的自陷激子对Pr3+发光的影响

研究了SrAl₁₂O₁₉:Pr³⁺ 和Y₃Al₅O₁₂(YAG):Pr³⁺ 在20K, 77K和300K的真空紫外(VUV)光谱，二者除了表现出Pr³⁺特有的发射性质外，还产生了一种特殊的发射现象，即自陷激子(STE)发射,并且其随着稀土离子浓度和温度的降低而增强. 还着重讨论了自陷激子对Pr³⁺发光的影响. 关键词： 自陷激子 真空紫外 3+')" href="#">Pr³⁺ 本征激发     

Mechanism of photoluminescence excitation of Mn<Superscript>2+</Superscript> ions in ZnS crystals

The mechanisms of photoluminescence excitation of Mn²⁺ ions in ZnS crystals have been investigated on the basis of complex analysis of the temperature dependences of the photoluminescence and photoluminescence-excitation spectra of ZnS:Mn crystals. The activation energy of a manganese luminescence center was estimated at E_a = 0.17 ± 0.05 eV. It is shown that E_a represents an energy band with a width ΔE_a = 0.1 eV, within which a manganese luminescence center can experience radiationless recombination. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 788–793, November–December, 2005.     

Upconverted VUV luminescence of Nd and Er doped into LiYF4 crystals under XeF-laser excitation

The upconverted VUV (185 nm) and UV (230 and 260 nm) luminescence due to 5d-4f radiative transitions in Nd³⁺ ions doped into a LiYF₄ crystal has been obtained under excitation by 351/353 nm radiation from a XeF excimer laser. The maximum upconversion efficiency, defined as the ratio of intensity for 5d-4f luminescence to overall intensity for 5d-4f and 4f-4f luminescence from the ⁴D_3/2 Nd³⁺ level, has been estimated to be about 70% under optimal focusing conditions for XeF laser radiation. A redistribution of intensity between three main components of 5d-4f Nd³⁺ luminescence is observed under changing the excitation power density, which favors the most long-wavelength band (260 nm) at higher excitation density level. The effect is interpreted as being due to excited state absorption of radiation emitted. The upconverted VUV and UV luminescence from the high-lying ²F(2)_7/2 4f level of Er³⁺ doped into a LiYF₄ crystal has also been obtained under XeF-laser excitation the most intense line being at 280 nm from the spin-allowed transition to the ²H(2)_11/2 4f level of Er³⁺, but the efficiency of upconversion for Er³⁺ emission is low, less than 5%.     

Infrared multiphoton excitation of polyatomic molecules

Results of investigations of the process of multiphoton excitation of polyatomic molecules by CO₂-laser radiation are presented. The mechanism of formation of the profiles of IR absorption bands of polyatomic molecules is discussed. New experimental methods of investigation of relaxation processes at high levels of vibrational excitation of molecules in the ground and triplet states are considered. For vapors of polyatomic molecules and their mixtures with foreign gases, the quantitative characteristics of the collisional exchange and the vibrational-energy transfer as well as the rates of intercombinational conversion ⇛ and triplet-triplet transfer are presented and their dependences on the vibrational-excitation level are discussed. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 675–693, September–October, 1998.     

Tunneling recombination luminescence under excitation of PbWO4:Mo crystals in the defect-related absorption region

Time-resolved emission and excitation spectra and luminescence decay kinetics were studied at 150-300 K for the green emission of PbWO₄:Mo crystals. It was found that the slow (μs-ms) decay component observed under excitation in the defect-related absorption region (around 3.8-3.9 eV) arises from the G(II) emission which appears at the tunneling recombination of optically created electron and hole centers. The study of the emission decay kinetics at different temperatures and excitation intensities allowed concluding that both the monomolecular and the bimolecular tunneling recombination process can be stimulated in the mentioned energy range. The monomolecular process takes place in the isolated spatially correlated pairs of electron and hole centers produced without release of electrons into the conduction band. The bimolecular process takes place in the pairs of randomly distributed centers created at the trapping of free electrons from the conduction band. The formation of electron centers under irradiation in the defect-related absorption region was investigated by the electron spin resonance (ESR) and thermally stimulated luminescence (TSL) methods. The possibility of various photo-thermally stimulated defects creation processes, which take place with and without release of free electrons into the conduction band, was confirmed.     

Up-conversion luminescence saturation under pulsed excitation of erbium-doped, SiO2–TiO2 sol–gel powders

The intensity of erbium up-conversion luminescence could be limited by a saturation effect due to increased pump power. We studied the luminescence saturation of the 550 nm emission on erbium-doped, SiO₂–TiO₂ sol–gel powders under pulsed excitation at 979 and 1532 nm. From the latter, the up-converted luminescence intensity decreased with increasing excitation power, whereas no saturation was observed at 979 nm excitation. We proposed that the saturation effect is determined by the pump power, the erbium content and the lifetime of the corresponding first excited states at different pumping schemes.     

VUV spectroscopy of pure and Tm-doped LicaAlF6 crystals

Nominally pure and Tm³⁺-doped LiCaAlF₆ crystals were grown by the Czochralski technique in a reducing atmosphere. The optical properties of transparent single crystals were studied using absorption and time-resolved luminescence spectroscopy in the VUV spectral range (330-100 nm). The strongest VUV emission peaking at 60 800 cm⁻¹ with a decay time of 5.6 μs (7 μs) at 300 K (7.4 K) was assigned to the spin-forbidden 4f¹¹5d-4f¹² transition of Tm³⁺. The fine structure observed in the VUV emission and corresponding excitation spectra indicate intermediate strength of electron-phonon coupling in this system. The efficient excitation of f-f emissions above 72 000 cm⁻¹, higher than the onset of f-d absorption at 63 000 cm⁻¹, is mainly caused by the F⁻ to Tm³⁺ charge transfer absorption. The nature of various host-related excitation processes in the energy transfer to the Tm³⁺ ions is discussed.     

Preparation of NaYF4:,Er3+/TiO2 composite and up-conversion luminescence properties under visible light excitation

The up-conversion luminescence composite NaYF 4:Er 3+ /TiO 2 is prepared using the sol-gel method.The specimen has good crystallinity and two shapes,i.e.,viereck and round,while the sizes of viereck and round particles are both micron-sized.The TiO 2 has an anatase structure,while the NaYF 4 has a hexagonal phase,which can be hardly obtained through the common sol-gel method.Due to the big particle size and the high crystallinity of pure NaYF 4:Er 3+,the composite has a small specific surface area that is less than Degussa P25 TiO 2.The NaYF 4:Er 3+ /TiO 2 composite shows several emission peaks at 211,237,and 251 nm under the excitation of 388 nm,at 395 nm and 411 nm under the excitation of 500 nm,and at 467,481,492,and 508 nm under the excitation of 570 nm.     

L‐series X‐ray emission spectra of 3d‐metal compounds with various excitation conditions

L‐series emissions of manganese, iron, and zinc oxides were studied using electron beam excitation and highly brilliant synchrotron radiation excitation. We showed that manganese and iron oxides show different Lβ/Lα intensity ratio because of their oxidation states and excitation electron voltages. On the other hand, we could not get any L‐series emissions from those bulk samples when excited by normal incident high‐energy monochromatic X‐rays, while samples of thin films and samples excited by grazing incident monochromatic X‐rays showed clear emissions. It is suggested that the difference of Lβ/Lα intensity ratio due to the oxidized states mainly concerns with the Coster–Kronig transition ratio of the samples, while self‐absorption effects should also deeply contribute the ratio, considering the experimental results. Copyright © 2016 John Wiley & Sons, Ltd.