Luminescent manifestations of tunneling and ionic processes in alkali feldspars

With the aim of determining the reasons for fading in paleodosimeters based on alkali feldspars, we have carried out a broad experimental study of various luminescent characteristics of these materials over a broad temperature range (from nitrogen temperatures to 450°C), relaxation processes at elevated temperatures, and also the effect of different heat treatments on these characteristics. Based on new experimental data on tunneling luminescence, we have critically analyzed the possibility of explaining fading by tunneling processes. In order to consistently explain all the data obtained, including the observed fading, we need to draw on radiation-induced and thermally induced ionic processes. We have carried out a series of direct experiments to measure ionic conductivity in feldspars. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 321–326, May–June, 2007.     

Tunneling Processes and Anomalous Fading in Natural Feldspars Extracted from Quaternary Deposits

Reasons for a temporal decrease (fading) in the accumulated energy in feldspars used as paleodosimeters are considered. The tunneling processes capable of leading to fading as well as temperature and time dependences of tunnel afterglow have been studied in detail. It is shown that this glow bears no direct relation to the possible tunnel depletion of the dosimetric trapping centers. With a view toward directly observing tunneling from these centers, the temperature dependences (from −200°C to +100°C) of the intensity of the luminescence stimulated in the IR band with λ ≈ 900 nm (of the optically stimulated luminescence (OSL) signal) have been studied in detail. Within the limits of sensitivity of the equipment used, tunneling from the dosimetric centers has not been detected. It has been inferred that in fading of the OSL signal, an essential role can be played by the traps that are depleted at 200°C and have a lifetime of about 2 years at room temperature.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 2, pp. 207–212, March–April, 2005.     

The problem of dating quartz 2: Synchrotron generated X-ray excited optical luminescence (XEOL) from quartz

The luminescence emission of quartz is used in optically stimulated luminescence dating (OSL), however the precise origins of the emission are unclear. A suite of quartz samples were analysed using X-ray excited optical luminescence (XEOL). Radiation dose effects were observed whereby the UV emissions (3.8 and 3.4 eV) were depleted to the benefit of the red emission (1.9–2.0 eV). Samples were excited at ∼7 keV. Understanding why some quartz emit light more brightly than others will increase the efficiency and precision of OSL analyses.     

Red luminescence from potassium feldspar for dating applications: a study of some properties relevant for dating

Aspects of the red thermoluminescence (RTL) and IR (833±5 nm) stimulated red (λ_emission=600–750 nm) luminescence (orange-red IRSL) of potassium feldspar from different origins are described. Anomalous fading of RTL (300–500°C) from a selection of potassium feldspar samples was tested. High temperature RTL (300–450°C) exhibits less anomalous fading in comparison to UV luminescence, for the samples under study. The result supports the contention of Zink and Visocekas (1997) that the red TL emission from feldspar does not fade. It was found that RTL is bleachable due to IR exposure, and the relationship between RTL lost and orange-red IRSL produced is linear. It is shown that around one third of the trapped charge responsible for the orange-red IRSL signal gives rise to an RTL signal, indicating that some traps and luminescence centres are shared for RTL and orange-red IRSL.

Specific characteristics of orange-red IRSL from feldspar were identified. It was found that the orange-red IRSL decay curve is bleachable by IR and daylight and can be described by the sum of three exponential components. Orange-red IRSL fading was tested. Short-term storage tests (up to 2 weeks) showed no fading. Longer-term (ca. months) storage of orange-red IRSL do in fact indicate fading, though at levels considerably lower than for the UV emission. The contradictory result is possibly due to the detection wavelength. As such, it is highly likely that the long-term fading experiment is strongly influenced by the feldspar emission centred at ca. 570 nm, which exhibits anomalous fading, while the short-term fading experiment is more greatly influenced by the far red emission centred at ca. 710 nm that in comparison to UV emission shows no or less fading.     

Optically stimulated luminescence from quartz measured using the linear modulation technique

The optically stimulated luminescence (OSL) from heated natural quartz has been investigated using the linear modulation technique (LMT), in which the excitation light intensity is increased linearly during stimulation. In contrast to conventional stimulation, which usually produces a monotonically decreasing signal, linearly increasing the stimulation power gives peaks in the signal as a function of time. In cases where the OSL signal contains more than one component, the linear increase in power of the stimulation light may result in a curve containing overlapping peaks, where the most easily stimulated component occurs at a shorter time. This allows the separation of the overlapping OSL components, which are assumed to originate from different traps. The LM-OSL curve from quartz shows an initial peak followed by a broad one. Deconvolution using curve fitting has shown that the composite OSL curve from quartz can be approximated well by using a linear combination of first-order peaks. In addition to the three known components, i.e. fast, medium and slow components from continuous-wave-OSL studies, an additional slow component is also identified for the first time. The dose responses and thermal stabilities of the various components are also studied.     

Tunneling luminescence of Y2O3 and Sc2O3 ceramics

In the present work we investigate the tunneling luminescence (TL) of Y₂O₃, Y₂O₃:Eu, and Sc₂O₃ ceramics. From an analysis of the decay kinetics of the TL it is established that the recombinational processes are described by a model involving recombination with a complex interaction. In addition, using the method of partial light sums, we show that in the ceramics investigated the mechanisms of TL are the same and the majority of radiation-induced defects in pairs are removed to distances exceeding the radius of effective tunneling transitions with a time constant of 10–100 sec. In the Y₂O₃:Eu ceramics the distances between defects decrease. TL occurs in pairs whose electron component is F-centers that are thermally released at 176 K in the Y₂O₃ ceramics and 200 K in the Sc₂O₃ ceramics. The hole components of tunneling pairs are associated with centers that are thermally released at 110 and 122 K in the Y₂O₃ ceramics and at 121 and 146 K in the Sc₂O₃ ceramics. To whom correspondence should be addressed. I. Franko Lvov State University, 50, Dragomanov Str., Lvov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 649–651, September–October, 1999.     

The luminescence enhancement of Eu^3＋ ion and SnO2 nanocrystal co-doped sol-gel SiO2 films

SnO₂ nanocrystal and rare-earth Eu～（3+） ion co-doped SiO₂ thin films are prepared by sol-gel and spin coating methods.The formation of tetragonal rutile structure SnO₂ nanocrystals with a uniform distribution is confirmed by X-ray diffraction and transmission electron microscopy.Fourier transform infrared spectroscopy is used to investigate the densities of the hydroxyl groups,and it is found that the emission intensity from the 5 D 0 7 F 2 transitions of the Eu～（3+） ions is enhanced by two orders of magnitude due to energy transfer from the oxygen-vacancy-related defects of the SnO₂ nanocrystals to nearby Eu～（3+） ions.The influences of the amounts of Sn and the post-annealing temperatures are systematically evaluated to further understand the mechanism of energy transfer.The luminescence intensity ratio of Eu～（3+） ions from electric dipole transition and magnetic dipole transition indicate the different probable locations of Eu～（3+） ions in the sol-gel thin film,which are further discussed based on temperature-dependent photoluminescence measurements.     

The luminescence enhancement of Eu3+ ion and SnO2 nanocrystal co-doped solben gel SiO2 films

SnO₂ nanocrystal and rare-earth Eu³⁺ ion co-doped SiO₂ thin films are prepared by sol-gel and spin coating methods. The formation of tetragonal rutile structure SnO₂ nanocrystals with a uniform distribution is confirmed by X-ray diffraction and transmission electron microscopy. Fourier transform infrared spectroscopy is used to investigate the densities of the hydroxyl groups, and it is found that the emission intensity from the ⁵D₀-⁷F₂ transitions of the Eu³⁺ ions is enhanced by two orders of magnitude due to energy transfer from the oxygen-vacancy-related defects of the SnO₂ nanocrystals to nearby Eu³⁺ ions. The influences of the amounts of Sn and the post-annealing temperatures are systematically evaluated to further understand the mechanism of energy transfer. The luminescence intensity ratio of Eu³⁺ ions from electric dipole transition and magnetic dipole transition indicate the different probable locations of Eu³⁺ ions in the sol-gel thin film, which are further discussed based on temperature-dependent photoluminescence measurements.     

Optically stimulated luminescence of dosimetric anion-defective corundum crystals

The relationship between the characteristics of optically stimulated luminescence (OSL) and thermally stimulated luminescence (TSL) of dosimetric crystals based on nominally pure single-crystal, ion-defective corundum is considered. It is established that the TSL and OSL characteristics of these crystals are closely interrelated and are caused by the same active traps. It is shown that the OSL of deep traps has a form difficult to interpret, which can be due to the effect of both more shallow and deeper traps.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 140–142, January–February, 2005.     

Red long-lasting luminescence in clinoenstatite

A long-lasting phosphor Ca_0.2Zn_0.9Mg_0.9Si₂O₆:Eu²⁺, Dy³⁺, Mn²⁺ was prepared by a sol-gel method. Nanoparticles crystallizing in a clinoenstatite structure were obtained. Long persistent phosphorescence in the red has been observed with persistence time over one hour at 680 nm and was attributed to Mn²⁺ emission. The persistent luminescence is suggested to involve Eu²⁺ as a sensitizer, Dy³⁺ or Dy³⁺-related defect as a trap center and Mn²⁺ as the luminescent center. However, the details of the mechanism are still under further investigation.     

稀土掺杂材料的上转换发光

稀土掺杂材料的上转换发光是实现光波频率转换的重要途径,也是稀土掺杂发光材料研究的重要内容。本文从介绍与上转换相关的基本概念出发,阐述了稀土离子上转换发光的发展历史;对稀土离子掺杂材料的能量传递、激发态吸收、合作敏化、合作发光、双光子吸收激发及光子吸收雪崩等上转换发光机制进行了概述,并对各机制进行了比较;对不同稀土离子掺杂体系中上转换发光的机制进行了总结;对以往研究的稀土掺杂上转换发光材料的基质,包括粉体材料、晶体材料、非晶材料进行了概括;最后对影响稀土离子上转换发光效率的因素进行了分析,提出了在上转换发光材料的设计中应重点考虑基质对泵浦光及上转换发射光的吸收、基质材料的声子能量、稀土离子的掺杂方案及泵浦途径等因素。     

Thermoluminescence and low-temperature luminescence of beryllium oxide

Beryllium oxide in the forms of either single crystals (pristine, additively-colored) or hot-pressed ceramic samples was studied in the energy range of 1.2–6.2 eV using both the thermoluminescence (TL) and steady-state X-ray induced luminescence (XRL) techniques. The XRL emission spectra were recorded at 6 and 293 K, whereas TL glow curves were studied after X-ray exposure at T₀ = 6 K upon linear heating in the temperature range from 6 to 293 K. A search for TL manifestations of shallow trapping centers was carried out using a sensitive channel for TL registration in the range of more than six decades of change in intensity. The participation of shallow trapping centers in the process of recombination luminescence excitation at 6–293 K; branching electronic excitations between different recombination channels; the dominance of the self-trapped exciton and F-center emissions in spectra of the low-temperature recombination luminescence in BeO at 6–293 K were discussed.     

Trapping and self-trapping in ytterbium-doped oxides with charge transfer luminescence

Temperature dependence of the charge transfer luminescence (CTL) of Yb-doped yttrium aluminum garnet Y₃Al₅O₁₂-Yb (YAG-Yb) and Yb-doped lutetium aluminum perovskite LuAlO₃-Yb (LuAP-Yb) crystals under X-ray excitation and their thermostimulated luminescence are investigated in the temperature range 30-350 K and compared to those of undoped crystals. Simulation using a set of kinetic equations describing the processes of creation of excitons, electron-hole pairs, their trapping and self-trapping, radiative relaxation and quenching is presented for the systems under investigation to analyze qualitatively two different types of experimentally observed temperature dependences: CTL yield decline with the temperature decrease below 110 K as in case of YAG-Yb and constant yield in the same temperature range as in case of LuAP-Yb.     

Thermal quenching of far-red Fe3+ thermoluminescence of volcanic K-feldspars

Strong thermal quenching is observed from 77 to 550 K in the far-red luminescence of K-feldspars. This far-red emission, next to the emission in the UV-to-blue spectrum recorded for thermoluminescence (TL), is reported in most alkaline feldspars with a characteristic peak centered on 710 nm with a width of 100 nm. This emission was observed by cathodoluminescence (CL) at room temperature (RT) for more than 30 K-feldspars, ranging from volcanic sanidines to granitic microclines and sediments and it is attributed to an Fe³⁺ impurity. Contrary to ‘blue’ emission in volcanic feldspars, the far-red emission displays very low anomalous fading (AF). This makes it attractive for dating purposes; however, it has weak natural TL intensity, even at saturation, which competes with the black-body emission of the heater plate. This is in contrast to an intense tunneling afterglow at liquid nitrogen temperature (LNT). Further observations show that the disadvantage of weak TL can be overcome. Photoluminescence (PL) under UV shows a very strong thermal quenching effect of the far-red emission from 77 to 550 K, which accounts for the above contrast. Near the LNT, the far-red Fe³⁺ photoluminescence is at a maximum and is dominant over other emissions in the spectrum. However, as the temperature increases, the efficiency decreases, falling to well below one percent, whereas the ‘blue’ emissions remain stable. This thermal quenching effect in photoluminescence is paralleled in TL. After irradiation and during storage at RT, whereas the ‘blue’ emission in volcanic feldspars is affected by ‘fast’ anomalous fading, charge trapped at Fe³⁺ centers as latent far-red emission is almost stable. As the TL evolves and the extant ‘blue’ emission is emitted, more and more of the trapped charge associated with far-red emission recombines non-radiatively, resulting in an efficiency for emission in natural TL that is less than one percent. A modified band model, which calls for 'hopping' conductivity during the storage, accounts for the anomalous fading. Trap emptying at lower temperatures should lead to better use of the stable latent far-red stored charge for the dating of volcanic deposits.     

Investigation of regeneration effect of blue luminescence in NaCl using variable delay optically stimulated luminescence (VD-OSL)

Optically stimulated luminescence (OSL) of pure analytical sodium chloride (NaCl) was studied. Performing series of CW-OSL (continuous wave OSL) readouts it was found that subsequent CW-OSL decay, after some delay, starts from significantly higher intensity level than the final intensity of the previous readout. To study this ‘regeneration effect’ a new type of measurements was implemented – the variable delay OSL (VD-OSL) method. The idea of VD-OSL is explained – it allows to study OSL kinetics at very long time scale. VD-OSL data confirm coexistence of two effects in NaCl – regeneration and fading of the OSL signal. After partial CW-OSL readout, regeneration predominates in the short time scale of the order of 10³ s. Then, fading becomes dominant. The same measurements were applied to commonly used Al₂O₃:C material as well. Nevertheless, the results do not show such behavior at the studied time scale. Theoretical arguments are presented that the regeneration mechanism cannot be explained on the basis of the simple trap model.     

Exciton luminescence of 8-azasteroid microcrystals

Luminescence of microcrystals of 2,3-methoxy-8-azagon-1,3,5(10),13-tetraene-12,17-dion of the class of molecules of biologically active steroids is detected at room temperature (293 K). It represents fast fluorescence of free and self-localized excitons and prolonged phosphorescence of triplet excitons. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 440–443, May–June, 1999.     

Polarised hot electron luminescence in p-GaAs: Electric field effects

Electrons photo-excited to high-energy conduction band states of GaAs exhibit complex energy and momentum distributions determined by the anisotropic valence band structure and the optical matrix elements. In p-type GaAs a fraction of these hot electrons combine with localised acceptor states, producing a hot electron luminescence (HEL) spectrum with a cascade of peaks corresponding to discrete energy losses resulting from LO-phonon emission. The highest peak involves unscattered electrons, and their energy distribution is due to warping of the initial heavy-hole (HH) bands. We report measurements of the line shape of this 0-HH peak, and its polarisation profile which identifies emission from electrons along particular directions. An applied electric field of 1 kV cm⁻¹ distorts the hot electron momentum distribution, and this is reflected in the polarisation profile. These line shapes and profiles, with and without field, are calculated using a computer model incorporating a band structure and optical matrix elements, the effect of electric field being included using a k-broadening model. The data and model are in good quantitative agreement assuming an electron lifetime of 100 fs, and confirm the expected differences in the profiles for different excitation polarisation states and applied field directions.     

Laser-modified luminescence for optical data storage

The yearly growing quantities of dataflow create a desired requirement for advanced data storage methods. Luminescent materials, which possess adjustable parameters such as intensity, emission center, lifetime, polarization, etc., can be used to enable multi-dimensional optical data storage (ODS) with higher capacity, longer lifetime and lower energy consumption. Multiplexed storage based on luminescent materials can be easily manipulated by lasers, and has been considered as a feasible option to break through the limits of ODS density. Substantial progresses in laser-modified luminescence based ODS have been made during the past decade. In this review, we recapitulated recent advancements in laser-modified luminescence based ODS, focusing on the defect-related regulation, nucleation, dissociation, photoreduction, ablation, etc. We conclude by discussing the current challenges in laser-modified luminescence based ODS and proposing the perspectives for future development.     

Radiation induced luminescence processes in c-BN

Spectral properties of cubic boron nitride have been studied using methods of photoluminescence (PL), X-ray excited luminescence (XL), thermoluminescence (TL) and optically stimulated luminescence. It is found that emission of cubic boron nitride is presented by 4 subbands, their relative yield is determined by the excitation type: blue, green (dominant) and red bands are observed in PL, ultraviolet, blue (dominant), green and red bands—in XL. Three thermal peaks are found in TL curves in the 0–700°C temperature range, their presence and intensity depend on radiation type used. A tentative correspondence between thermal peaks and emission bands is found.