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The relationships governing variation of the photoluminescence of graphitic carbon nitride synthesized by heat treatment of melamine in a closed air medium containing oxygen in the temperature range of 10–300 K were investigated. It was shown that the concentration of oxygen in the obtained material 4–5 at.% increases with increase of temperature and decreases with increase in the duration of the synthesis process. By measurements at reduced temperatures right down to 10 K it was possible to resolve bands due to radiative recombination processes in the photoluminescence spectra of the graphitic carbon nitride. It was found that increase of the synthesis temperature from 500 to 600°C and also increase of the duration at the given temperature from 30 to 240 min shift the maximum in the photoluminescence spectrum from 2.74 eV into the region of lower energies to 2.71–2.67 eV. This is due to the bigger role of the molecular system formed by the π bonds of carbon and nitrogen atoms with sp2 hybridization and characterized by a smaller forbidden band width in the emission of light. Transitions due to recombination through oxygen-induced levels in the forbidden band of the semiconductor lead to the appearance of a "tail" in the photoluminescence spectra in the region of low energies (2.40–2.33 eV). Increase of the carbon nitride synthesis temperature to 600°C leads to a change in the structure of the energy bands and to increase of the energy of the radiative transitions as a result of increase in the degree of doping with oxygen atoms and thermal stratifi cation. [ABSTRACT FROM AUTHOR]

Absorption, fluorescence, and phosphorescence spectra of metal complexes of 4-tert-butylphenyl-substituted tetra(1,4-diazepino)porphyrazine MPADz4Ph8′ (M = Mg, Zn; PA = porphyrazine; Dz = diazepine; Ph = phenyl; Ph′ = 4-tert-butylphenyl) were recorded in DMF at 293 and 77 K. The Q(0–0) absorption bands of MgPADz4Ph8′ and ZnPADz4Ph8′ did not show signs of splitting, i.e., corresponded to the monomeric form. Singlet–triplet gaps of 4700 cm–1 for the Mg complex and 4800 cm–1 for the Zn complex were determined from fluorescence and phosphorescence spectra. Fluorescence quantum yields and lifetimes were measured. Rate constants of S1,2 → S0 radiative transitions and total rate constants of nonradiative deactivation of the S1,2 states were determined. Internal conversion rate constants were estimated. Conformer geometries of MgPADz4Ph8 (in the 6H tautomeric form) were calculated by density functional theory in the PBE/TZVP version. The conformer of symmetry S4v (D2d) had the lowest energy. Nonplanar diazepine rings induced small but perceptible out-of-plane distortions in the central porphyrazine ring of MgPADz4Ph8. This feature correlated with the observed Stokes shifts of 400 and 350 cm–1 (at 293 K) for MgPADz4Ph8′ and ZnPADz4Ph8′ whereas the metal phthalocyanines had shifts of ~50 cm–1. Excited electronic states of MgPADz4Ph8 were calculated using INDO/Sm. The Q-state energy of 15,200 cm–1 agreed well with the observed value of 14,800 cm–1. Strong featureless absorption in the range 330–450 nm (Soret band analog) with maxima at ~25,000 and ~29,000 cm–1 was due mainly to two strong electronic transitions with calculated energies of 24,100 and 31,500 cm–1; a shoulder on the long-wavelength side of the Soret band at ~23,000 cm–1, to a transition of calculated energy 23,800 cm–1. [ABSTRACT FROM AUTHOR]

The chemical composition of Pr3+-doped strontium–aluminum–bismuth–borate glasses with different concentrations, that is, (50 – x)B2O3 + 20Bi2O3 + 7AlF3 + 8SrO + 15SrF2 + xPr2(CO3)3 (where x = 0.1, 0.5, 1.0, and 1.5 mol.%), have been prepared by the melt quenching technique. XRD, SEM, and EDS studies reveal the structural and amorphous properties of the prepared glasses. Different borate groups are analyzed by FTIR measurements. Judd–Ofelt (J–O) intensity parameters, Ωλ (λ = 2, 4, and 6) are calculated from the absorption spectrum to identify the nature of Pr3+ ions with their surrounding ligands. Using the J–O parameters, the radiative properties such as radiative transition probabilities AR and radiative lifetimes τcal are evaluated for various excited transitions of Pr3+ ions. Stimulated emission cross sections σ p E and branching ratios βR for the emission transitions are evaluated by the photoluminescence spectra. The higher values of σ p E and βR are identified for the emission transitions 3P0 → 3H4 and 1D2 → 3H4 of SABiBPr15 and SABiBPr01 glasses; these glasses are suitable for light emitting materials. The chromaticity color coordinates are calculated to find the average color of the emission spectra. [ABSTRACT FROM AUTHOR]

Experimental luminescence and selective-emission (SE) spectra of YAG:Er3+ (10 at.%) and CaF2:Er3+ (1 at.%) single crystals and Er2O3 polycrystal under laser and laser thermal excitation of the Er3+-ion multiplets are compared. Luminescence spectra under resonant excitation are determined by multiplet population relaxation with the corresponding radiative and nonradiative probabilities. The form of the SE spectra is determined by the thermal population of the multiplets and the probabilities of only radiative transitions. The SE band at 800 nm (4I9/2 → 4I15/2) is an indicator of high-temperature thermal emission of Er3+ ions. The absence of this band in luminescence spectra is explained by the short lifetime of the τ(4I9/2) level of 53 ns at T = 300 K. [ABSTRACT FROM AUTHOR]

The theory of nonradiative transitions, based on the model of orientational broadening of electronic levels, is used to interpret known data on the effect of the polarity of the medium and temperature on the lifetime of the S( A) state for 12′-apo-β-caroten-12′-al and 8′-apo-β-caroten-8′-al. The effect of promoting vibrations on the rate constant for nonradiative transitions is considered. The results make it possible to hypothesize that the internal electric field created by the environment strongly perturbs the pigment molecules. Consequently, in the excited S( A) state, both pigments exist as different conformers I and II. Their structures differ by a 180° rotation of the cyclohexene ring relative to the polyene chain. Conformer II predominates in polar media, and is found in an intramolecular charge-transfer state S( A/ ICT). In addition, as a result of fluctuations in the internal electric field, it can also be found in a charge-separated state S( A/ CS). [ABSTRACT FROM AUTHOR]

Luminescence spectra of CaPrF solid solutions are studied. It is found that, depending on the excitation energy, different kinds of emission centers appear in these spectra. An interconfigurational 4f 5d →4f luminescence is typical for single Pr ions in tetragonal sites. Data on the structure of the solid solutions show that the emission centers involved in S → 4f transitions can be attributed to Pr ions contained in clusters. [ABSTRACT FROM AUTHOR]

Absolute quantum yields and luminescence lifetimes were measured for Eu, Tb, Gd, formates and Y formate activated by Eu as functions of temperature, deuterium substitution in ligands, and Eu concentration in Y formates. Radiative and nonradiative transition probabilities in the lanthanide formates were determined using the quantum yield φ and luminescence lifetime τ. The probabilities of radiative transitions in Eu were also found by employing corrected luminescence spectra. Relaxation mechanisms of electronic excitation energy in the studied crystals were discussed. [ABSTRACT FROM AUTHOR]

We present a simulation model for a stream of photon counts that can be detected in fluorescence fluctuation spectroscopy systems from diffusing molecules undergoing spontaneous transitions between radiative and nonradiative states. Using our model, we can obtain various characteristics of a random stream of photon counts which allows us to test fluorescence fluctuation spectroscopy methods and to predict the results of full-scale experiments. [ABSTRACT FROM AUTHOR]

We have studied the sensitized phosphorescence spectra of coronene in toluene and its delayed annihilation fluorescence spectra in n-decane at 77 K. We show that as a result of perturbation of electronic states by exchange interactions, intensity redistribution occurs in both the sensitized phosphorescence spectrum and the delayed annihilation fluorescence spectrum. The nonzero probability of the symmetry-forbidden radiative 0-0 transition is explained by the lower symmetry of the bimolecular system compared with the symmetry of the coronene molecule. [ABSTRACT FROM AUTHOR]

frequency stabilized, tuneable CO laser is used to measure the unsaturated absorption coefficients of pure carbon dioxide gas at pressures of 1 and 100 Torr and temperatures of 296-700 K. The radiative transition probability and the temperature dependence of the collisional self-broadening coefficient are determined for the R22 absorption line of the 100-001 transition of the CO molecule. The exponent on the temperature is found to depend on the method used to determine the collisional self-broadening coefficient. [ABSTRACT FROM AUTHOR]

system of radiative transfer equations is used to calculate the loss coefficient for amplified luminescence fluxes propagating along and transverse to the cavity axis in the active layer of high-power laser diode arrays taking the spreading of charge carriers in the cladding and contact layers of InGaAs/GaAs/AlGaAs heterostructures into account. It is shown that the spreading of charge carriers leads to a significant change in the amplified luminescence flux which can contribute up to 18% to the lasing threshold of these laser diode arrays. The calculated loss coefficients can greatly simplify the determination of the amplified luminescence fluxes in laser diode arrays with an error of less that 16% and can be used to determine how much the amplified luminescence affects the power and dynamic characteristics of diode arrays. [ABSTRACT FROM AUTHOR]

To a first approximation in perturbation theory, we have obtained expressions for the rate constants for intersystem crossings in acceptor molecules. We show that exchange interactions between the components of the donor-acceptor pair can change the probability of intersystem crossings for both radiative T → S0 and nonradiative S1 → T transitions. The theoretical conclusions are supported by the results of experimental studies. [ABSTRACT FROM AUTHOR]

It has been shown that the low-frequency periodic processes in spectroscopy and chemical transformations that proceed without a change in the total energy of a complex molecular system can be explained as a result of synchronized radiationless transitions between the resonant ground states of quantum systems of a different structure. [ABSTRACT FROM AUTHOR]

A theory of interaction of a three-level V-type quantum system with a bichromatic field of two waves, one of which is weak, has been developed. It is suggested that the upper two levels of this system are closely spaced and, therefore, both waves interact simultaneously with both allowed transitions. An expression for nonlinear macroscopic polarization of the V-system has been obtained by the density-matrix method. [ABSTRACT FROM AUTHOR]