Relaxation Of Electronic Excitations Research Articles

Multi-Spectroscopic and TD-DFT Studies on Chromogenic and Fluorogenic Detection of Cyanide in an Aqueous Solution.

Different spectroscopic techniques and Density Functional Theory (DFT)/Time-Dependent Density Functional Theory (TDDFT) calculations have been employed to investigate the dual channel CN- detection behaviour of the developed chemo-dosimeter (AK3). The CN- with AK3 reaction triggered a colour change from pale yellow to colourless and enhanced fluorescence. UV-Vis, fluorescence, 1H & 13C NMR and mass techniques coupled with theoretical calculations (Mulliken charges, dihedral angles) revealed that the CN- sensing process mechanism involves deprotonation of the N-H group followed by nucleophilic addition reaction. Detailed TD-DFT calculations showed that the relaxation of excited electrons from LUMO and to two different ground states is responsible for the weak/moderate fluorescence of AK3. Nucleophilic addition of CN- to the C-atom of the CH = CH bridge terminated the π-conjugation between donor and acceptor regions, reduced the coplanarity, decreased the ICT transition and consequently enhanced the fluorescence of the probe. The practical utility of the probe was demonstrated by detecting cyanide in food materials and determining CN- in environmental water samples.

Impurity, host and defect-related luminescence of CsLaSiS4 thiosilicate crystals doped with Ce3+ ions

Ce+3 doped CsLaSiS4 single crystals were synthesized using high-temperature flux synthesis. XRD analysis showed that the products are isostructural to CsLaSiS4 and crystallize in orthorhombic space group Pnma. Absorption spectra were studied at room temperature, and photoluminescence properties were examined in the temperature range of 5–310 K. The energy of interband transitions Eg = 3.75 eV was determined at room temperature in the Tauc model. At room temperature, only a non-elementary d → f emission band of Ce+3 ions was observed in the 520 nm region. The luminescence kinetics upon excitation by a pulsed cathode beam or X-ray synchrotron radiation exhibited a dominant nanosecond component. Decay time, as well as build-up time, decreased with increasing concentration of Ce+3 ions. Concentration quenching of Ce+3 emission was not observed up to 11.9 mol% of Ce+3 ions. At a low temperature of 5 K, new wide emission bands at 422 and 688 nm appeared in the photoluminescence spectrum. It is shown that the 422 nm band in the photoluminescence spectrum corresponds to host emission, specifically the luminescence of self-trapped excitons (STE). The 688 nm emission band corresponds to defect-related luminescence. STE emission is quenched according to the Mott law at temperatures above 26 K with an activation energy of 20 meV. An efficient energy transfer channel from the STE to the Ce+3 ion via the radiative resonance mechanism is observed. The emission of Ce+3 ions and defect-related luminescence can be excited by the intracenter way, due to electron-hole recombination, or by the creation of defect bound excitons. Based on the obtained spectroscopic data, a band scheme illustrating the processes of relaxation of electronic excitations at T = 5 K in Ce+3 doped CsLaSiS4 crystals is proposed.

Luminescence of MgF2-WO3 ceramics synthesized in the flux of 1.5 MeV electron beam

The results of comparative analysis of optical properties of MgF2 single crystals and MgF2 ceramics, non-doped and doped with tungsten trioxide obtained in the process of high-speed radiation synthesis in air under a flux of 1.5 МеV electrons, are presented.The synthesized ceramics are found to have a high degree of crystallinity, their elements composition is close to stoichiometric, and the phase composition is a rutile lattice.Using time-resolved spectroscopy, it was shown that the initial defectiveness of as-synthesized ceramics samples is represented by emission centers belonging to WO3 complexes and radiation defects of F2- types with spectral-kinetics parameters similar to those in irradiated by ionizing radiation MgF2 single crystals. The ratio between these types of defects is determined by the temperature and luminescence excitation method and the ratios between dopant and co-dopant (LiOH).In MgF2 ceramics samples, being exposed to ionizing radiation, two radiative channels of relaxation of electronic excitations were found: T-So (triplet-singlet) emission transition in self-trapped excitons (STE) at 3.2 eV and recombination of F–H Frenkel pairs accompanied with the emission at 4.0 eV. The temperature dependences of these channel efficiency in the range of 5–300 K are similar to those ones observed in MgF2 single crystals.

Luminescence of Hf-=SUB=-x-=/SUB=-Zr-=SUB=-1-x-=/SUB=-O-=SUB=-2-=/SUB=- thin films upon excitation with vacuum ultraviolet synchrotron radiation

Using the methods of low-temperature luminescence spectroscopy and vacuum ultraviolet synchrotron radiation (MAX IV synchrotron, Lund, Sweden), charge transfer processes and the nature of charge carrier traps responsible for leakage currents in nanometer dielectric films of hafnium-zirconium-oxygen HfxZr1-xO2 solid solutions on silicon substrate, as well as in films doped with La-ions and annealed in argon atmosphere. The photoluminescence spectra, photoluminescence excitation spectra of the 2.7 eV emission band and previous simulation data within the framework of density functional theory confirm the presence of an oxygen vacancy in the studied films. Based on the study of competing channels for the relaxation of electronic excitations due to the radiative decay of self-trapped excitons (4.35 eV emission band) and the luminescence of defects (2.7 and 3.5 eV emission bands), a conclusion was made about the efficiency of transport and capture of excitons and separate charge carriers in films with varying degrees of defectiveness and films doped with lanthanum. The obtained data confirm the conclusion that oxygen vacancies are traps responsible for leakage currents in the films under study Keywords: Luminescence, synchrotron radiation, defects, energy transfer.

Люминесценция тонких пленок Hf-=SUB=-x-=/SUB=-Zr-=SUB=-1-x-=/SUB=-O-=SUB=-2-=/SUB=- при возбуждении синхротронным излучением вакуумного ультрафиолетового диапазона

Using the methods of low-temperature luminescence spectroscopy and vacuum ultraviolet synchrotron radiation (MAX IV synchrotron, Lund, Sweden), charge transfer processes and the nature of charge carrier traps responsible for leakage currents in nanometer dielectric films of hafnium-zirconium-oxygen solid solutions on silicon substrate, as well as in films doped with La-ions and annealed in argon atmosphere. The photoluminescence spectra, photoluminescence excitation spectra of the 2.7 eV emission band and previous simulation data within the framework of density functional theory confirm the presence of an oxygen vacancy in the studied films. Based on the study of competing channels for the relaxation of electronic excitations due to the radiative decay of self-trapped excitons (4.35 eV emission band) and the luminescence of defects (2.7 and 3.5 eV emission bands), a conclusion was made about the efficiency of transport and capture of excitons and separate charge carriers in films with varying degrees of defectiveness and films doped with lanthanum. The obtained data confirm the conclusion that oxygen vacancies are traps responsible for leakage currents in the films under study.

Radiation defects in NaCl matrix with reduced lattice symmetry caused by light cation doping and elastic uniaxial deformation

The processes of radiation defect creation and radiative relaxation of electronic excitations under applied local or/and uniaxial elastic deformation have been studied in NaCl crystals by means of optical absorption, luminescence and thermoactivation spectroscopy methods. In NaCl:Li at 80 K, X-ray-induced absorption bands peaked around 3.35 and 4.6 eV have been detected and ascribed to interstitial halide atoms located nearby Li impurity cations, HA(Li) centres. Subsequent thermal annealing of HA(Li) centres leads to the formation of polyhalide centres responsible for the absorption band at 5.35 eV. In an X-irradiated and stressed NaCl:Li crystal (degree of uniaxial elastic deformation of ε = 0.9%), the peak of thermally stimulated luminescence at ~115 K is composed of the ~2.7-eV emission appearing, in our opinion, due to the recombination of the electron, thermally released from an F′ centre, with a hole-type HA(Li) centre. The applied uniaxial elastic stress facilitates the self-trapping of anion excitons in regular regions of a NaCl lattice and impedes the energy transfer by mobile excitons to impurities/defects and, in turn, attenuates the Br-related luminescence peaked at 3.95 eV with respect to the π-emission of self-trapped excitons (~3.35 eV). The 3.95 eV emission has been detected in a natural NaCl crystal containing homologous Br impurity ions.

Relaxation of electronic excitations in K2GeF6 studied by means of time-resolved luminescence spectroscopy under VUV and pulsed electron beam excitation

Highly crystalline K2GeF6 micropowders were prepared by co-precipitation in the hydrofluoric acid-water-alcohol solution and studied by means of the cathodoluminescence and time-resolved luminescence spectroscopy under synchrotron radiation excitation at 7 K. The nature of various intrinsic emissions was revealed. The luminescence band at 510 nm detected under excitation by photons with the energy above 9 eV is related to the radiative decay of self-trapped excitons. Fast emission bands with ~400 ps decay time found in the spectral range from VUV to visible (8–2.4 eV) are assigned to the cross-luminescence transitions from K 3p hole states and intraband luminescence transitions due to the presence of the Ge 4s, 4p valence states. The creation of cation excitons was detected in the energy range 18–20 eV. Their non-radiative decay results in the formation of hole states in the Ge energy bands, which finally recombine with electrons from the above lying valence states and provide fast intraband luminescence. Important electronic band structure parameters including the energy gap width of 11 eV, cation exciton formation energy 18.1 eV and ionization energy of cation states 20.0 eV were determined from the luminescence excitation spectra.

Excited state relaxation in cationic pentamethine cyanines studied by time-resolved spectroscopy

In this work, we examined the nature and pathways of electronic excitation relaxation at low temperature in cationic pentamethine pyrylocyanine dye, which in the ground state is characterized by symmetric and non-symmetric localization of cationic charge on the carbon chain between the terminal groups. The carbon chain in such symmetric and non-symmetric charge localizations is close to polymethine and polyene conformations, respectively. The effect of symmetry breaking during the two-way transition polymethine – polyene in the optically excited state on the electronic properties of dyes is considered. Combined quantum-chemical calculations and time-resolved spectral measurements allowed us to characterize the symmetric and non-symmetric structures in the ground and excited states with specific peaks of transitions and lifetimes. The features of the polymethine structure of the dye are attributed to large-radius polaron formation, whereas for the polyene structures the model of excitons formed on the double bonds of the carbon chain can be used to analyze the relaxation pathways. The energy diagram model for relaxation pathways of optically excited electronic states has been proposed. The proposed model can be characteristic for pentamethine cyanine dyes with the symmetric and non-symmetric distribution of cationic charge at the carbon chain between the terminal groups in the ground and excited states.

On the limit of the accuracy of timе stamp at the detection of annihilation γ-quanta with a scintillation detector

An influence of the various relaxation processes of the electronic excitations causing the scintillation in the crystalline compounds under ionising radiation is analysed. It was found that the intracenter relaxation of electronic excitations in the luminescence ion forms a physical limit for the time resolution of the scintillation detector. The limit of the time resolution, which can be provided when measuring the ionising radiation with a scintillation detector, has been established by simulation. A comparison of the time resolution limits for various errors by the electronic part of the ionising radiation detector is performed. It is shown that inorganic scintillation materials based on single crystals activated by cerium ions have a limit of 10 ps, while self-activated scintillators with low yield and short scintillation kinetics may show results not worse than 20 ps. It has been demonstrated that a further increase in the scintillation yield while keeping the short kinetics in self-activated materials can provide a better time resolution in comparison with Ce-activated materials in future detectors.

Local Electric Field Controls Fluorescence Quantum Yield of Red and Far-Red Fluorescent Proteins.

Genetically encoded probes with red-shifted absorption and fluorescence are highly desirable for imaging applications because they can report from deeper tissue layers with lower background and because they provide additional colors for multicolor imaging. Unfortunately, red and especially far-red fluorescent proteins have very low quantum yields, which undermines their other advantages. Elucidating the mechanism of nonradiative relaxation in red fluorescent proteins (RFPs) could help developing ones with higher quantum yields. Here we consider two possible mechanisms of fast nonradiative relaxation of electronic excitation in RFPs. The first, known as the energy gap law, predicts a steep exponential drop of fluorescence quantum yield with a systematic red shift of fluorescence frequency. In this case the relaxation of excitation occurs in the chromophore without any significant changes of its geometry. The second mechanism is related to a twisted intramolecular charge transfer in the excited state, followed by an ultrafast internal conversion. The chromophore twisting can strongly depend on the local electric field because the field can affect the activation energy. We present a spectroscopic method of evaluating local electric fields experienced by the chromophore in the protein environment. The method is based on linear and two-photon absorption spectroscopy, as well as on quantum-mechanically calculated parameters of the isolated chromophore. Using this method, which is substantiated by our molecular dynamics simulations, we obtain the components of electric field in the chromophore plane for seven different RFPs with the same chromophore structure. We find that in five of these RFPs, the nonradiative relaxation rate increases with the strength of the field along the chromophore axis directed from the center of imidazolinone ring to the center of phenolate ring. Furthermore, this rate depends on the corresponding electrostatic energy change (calculated from the known fields and charge displacements), in quantitative agreement with the Marcus theory of charge transfer. This result supports the dominant role of the twisted intramolecular charge transfer mechanism over the energy gap law for most of the studied RFPs. It provides important guidelines of how to shift the absorption wavelength of an RFP to the red, while keeping its brightness reasonably high.

Mechanisms of Relaxation of Electronic Excitation of Triazido-S-triazine

Mechanisms of Relaxation of Electronic Excitation of Triazido-S-triazine

Role of inter- and intraconfigurational transitions of Pr3+ ion in nonequilibrium carrier relaxation in garnet-type scintillators

Abstract Transient optical absorption induced by selective photoexcitation is exploited for a subpicosecond study of the electronic excitation relaxation in Pr3+ acting as an activator ion in garnet-type scintillating materials. Both intra- and interconfigurational (4f2–4f15d1) channels are shown to be important. The latter one limits the rate of population of the emitting level of Pr3+ ion and, consequently, the luminescence rise time. The relaxation time of nonequilibrium carriers to the radiating level of 4f15d1 configuration after selective excitation of Pr3+ ion to higher excited levels is found to be ~4 ps. The energies of the Pr3+ levels involved in the excitation relaxation are estimated in (Lu–Y)3Al5O12:Pr. The obtained results show that the relaxation rate might be decreased by engineering of the crystal field in the vicinity of activator ion.

Study of electronic excitation relaxation at cerium ions in Gd3Al2Ga3O12 matrix using multipulse transient absorption technique

A method for investigation of excitation relaxation—the multipulse transient absorption technique—has been applied for studying carrier relaxation and recombination processes of excited Ce3+ ions in Ce-doped and Mg-codoped gadolinium aluminum gallium garnet (Gd3Al2Ga3O12, GAGG) single crystals with different codoping levels. The method enabled deeper insight into the complex dynamics of electron transitions between higher excited levels and the emitting state of Ce3+, which are difficult to observe in conventional pump–probe experiments because of overlapping absorption bands arising from different structural units. Fitting the experimental results to calculations based on the rate equations for the population levels of Ce3+ ions and traps enabled us to estimate the relaxation times from these levels. In particular, it was shown that the electron relaxation time between doublet eg and triplet t2g components of the split level 5d of Ce3+ is faster than 300 fs. Substantial influence of electron trapping is demonstrated, and the potential fluctuations due to intrinsic composition fluctuations in multicomponent GAGG crystals are shown to be the predominant origin of the traps.

Relaxation of Excited Electrons on Carbon Nitrides Investigated by Electrochemiluminescence and Photoluminescence Spectra

Deep insights into the photoexcitation and relaxation of carbon nitrides are vital to developing highly efficient photocatalysts and luminescence-based applications. In this work, carbon nitride na...

Relaxation of electronic excitations in ions in cold krypton plasma

Reaction rate constants have been calculated for electronic transitions in ions and for their decay as induced by collisions with krypton atoms and/or spontaneous radiation processes. The rate constants have then been used in a series of modelings of electronic relaxation in the ions in cold krypton plasmas with the main focus on relaxation times and final states. It has been shown that the collision-induced (non-radiative) relaxation is much faster than the radiative one and completely dominates with typical relaxation times ranging between nanoseconds and microseconds. The relaxation always ends up in the electronic ground state, high electronic excitations survive for longer times than lower excitations due to spin–orbit coupling effects.

Monte Carlo studies of thermalization of electron–hole pairs in spin-polarized degenerate electron gas in monolayer graphene

Monte Carlo method is applied to the study of relaxation of excited electron–hole (e–h) pairs in graphene. The presence of background of spin-polarized electrons, with high density imposing degeneracy conditions, is assumed. To such system, a number of e–h pairs with spin polarization parallel or antiparallel to the background is injected. Two stages of relaxation: thermalization and cooling are clearly distinguished when average particles energy $$\langle E\rangle$$ and its standard deviation $$\sigma _E$$ are examined. At the very beginning of thermalization phase, holes loose energy to electrons, and after this process is substantially completed, particle distributions reorganize to take a Fermi–Dirac shape. To describe the evolution of $$\langle E \rangle$$ and $$\sigma _E$$ during thermalization, we define characteristic times $$\tau _\text {th}$$ and values at the end of thermalization $$E_\text {th}$$ and $$\sigma _\text {th}$$. The dependence of these parameters on various conditions, such as temperature and background density, is presented. It is shown that among the considered parameters, only the standard deviation of electrons energy allows to distinguish between different cases of relative spin polarizations of background and excited electrons.

A luminescence spectroscopy study of new Li2BaAl2F10 single crystal

Large Li2BaAl2F10 single crystals of optical quality were grown using the vertical Bridgman method. X-ray diffraction method was used to determine the crystal structure (orthorhombic symmetry Cmc21), lattice parameters, atomic coordinates. The luminescent properties were investigated using selective photoexcitation by synchrotron radiation (E = 3.7–21 eV, T = 8 K, time integrated and time-resolved spectra) as well as upon excitation with unfiltered X-ray beam (synchrotron radiation or X-ray tube). We revealed both the broadband luminescence at E m = 4.0 eV (E ex=11.72 eV) attributed to the radiative annihilation of self-trapped excitons (STE) and the excitonic-type near-defect luminescence at E m = 3.0–3.2 eV (E ex=11.25 eV) attributed to radiative relaxation of electronic excitations in nonequivalent structural units of the crystal lattice. The fast exponential component with lifetime of 5.6 ns, a low-intensity intermediate component with a lifetime of 75–100 ns, a constant level — pedestal (sum of the micro- and millisecond decay components) were revealed in luminescence decay kinetics. The electronic structure parameters (bandgap E g = 13.0 eV, low-energy onset of the intrinsic host absorption E c = 11.2 eV), the energy threshold for the excitation of STE-luminescence (E th = 11.2 eV) are determined from spectroscopic data. Thermoluminescence (TL) has been studied (90–350 K) using spectral-integral regime. Four partially overlapping TL glow peaks were revealed, their deconvolution was done and thermal activation parameters were determined using TGCD method.

Radiation-thermal decomposition of lignin: Products and the mechanism of their formation (Review)

The mechanism of the radiation-thermal conversion of lignin including the formation of aromatic radical cations and their fragmentation resulting in the appearance of phenoxyl radicals is considered. The multipath formation of phenoxyl radicals occurs with the participation of the reactions of molecules with electrons and small radicals (•Н and •СН3) and electronic excitation relaxation processes. Phenoxyl radicals are characterized by smaller thermal stability in comparison with that of parent macromolecules. The further thermally stimulated decomposition of these radicals results in the release of monohydric and dihydric phenols from a polymeric chain. The most effective liberation of phenols takes place on the surface of lignin particles, whereas the formation of wood charcoal with the participation of unsaturated products dominates in the bulk. The formation of dihydric phenols is intensified in the presence of alkanes in the irradiated sample; this fact is indicative of an important role of •Н and •СН3 radicals in the formation of monomeric phenol products.

One-pot photo-synthesis and in-situ generation of hydrogen by silver/strontium titanate photocatalyst under visible or near-UV light and role of midgap states: Experiment and DFT computations

Abstract We report “one-pot” photo-synthesis of nanocrystalline Ag(0)/SrTiO3 photocatalysts in aqueous suspension followed by in-situ testing in the same suspension to generate hydrogen under visible (λ > 400 nm) or near-UV/visible (λ ≥ 366 nm) light. The Ag(0)/SrTiO3 showed a significant increase of the photocatalytic rate vs. SrTiO3 i) under near-UV/visible light with very small amount of silver promoter at Ag0.001/SrTiO3 and ii) under visible light with Ag0.05/SrTiO3. The Ag(0)/SrTiO3 was characterized by several complementary structural and spectroscopic methods; no bandgap narrowing in the nanocrystalline Ag(0)/SrTiO3 has been observed. Mechanism of photocatalytic hydrogen generation with Ag(0)/SrTiO3 under visible light involves relaxation of excited electron through the set of distinct electronic midgap states and includes preferential quenching of one of the two specific midgap transitions. The DFT computations of SrTiO3 with small cluster model for TiO2- and SrO-terminated surfaces result, for the first time, in optical bandgaps very close to experimental values. Geometry and binding energies were calculated for silver clusters attached to TiO2- and SrO-terminated surfaces. The DFT calculations suggest the mechanism of operation of Ag(0)/SrTiO3 under the UV light.

Influence of peculiarities of electronic excitation relaxation on luminescent properties of MgWO4

Luminescent properties of magnesium tungstate monocrystals grown by two different methods are studied. Only the exciton luminescence of these crystals themselves is observed. Temperature dependence of the low-energy range in the luminescence excitation spectra is described by the Urbach rule. Slope coefficient σ0 = 0.74 obtained from this dependence implies autolocalization of the excitons in MgWO4. The processes of electronic excitations relaxation are considered depending on the structure of valence band in MgWO4 and in other wolframites, ZnWO4 and CdWO4. In contrast to ZnWO4 and CdWO4, the d-states of the cation do not participate in formation of the MgWO4 valence band. Using the excitation spectra measured in the range of the fundamental absorption (4–20 eV), it is shown that this difference manifests itself in relaxation of electronic excitations and may be the cause of the relatively low light yield of MgWO4.