Effect of Temperature on the Rate of Triplet–Triplet Annihilation of 1,2-Benzanthracene in a Polymer Matrix

The process of triplet–triplet annihilation (TTA) of 1,2-benzanthracene (1,2-BA) incorporated into polymer films of polyvinyl butyral has been investigated in the temperature interval 80–360 K. An analysis of the kinetics of the decay of delayed annihilation fluorescence (DAF) of 1,2-BA has shown that the process of the triplet excitation energy transfer in a disperse medium of the polymer at distant times of the DAF decay can be described using the approximation of random walks of the triplet energy in the inhomogeneous medium of the polymer. At low temperatures, at the initial times of DAF decay the TTA process is described with the aid of a model of static annihilation.     

Phosphorescence of Poly(Methylphenyl)Silane Films

In the 5–100 K temperature range, the photoluminescence spectra of poly(methylphenyl)silane (PMPS) and polystyrene films doped with acetophenone, propiophenone, benzophenone, diphenyl, and naphthalene, as well as the thermostimulated luminescence spectra of neat PMPS films, have been investigated. It has been shown that the PMPS film phosphorescence spectrum consists of a structureless band with a maximum near 450 nm, and the literature structural phosphorescence spectrum of this polymer may belong to an uncontrolled impurity or a defect with chromophore groups of acetophenone.     

Davydov splitting and two-photon excitation of cadmium tungstate (CdWO4) single crystal phosphorescence

Phosphorescence characteristics of CdWO₄ excited by one-photon (λ = 308 nm) and two-photon (λ = 570–590 nm) processes were measured. A Davydov splitting of 120 ± 20 cm⁻¹ was obtained in the phosphorescence spectra, suggesting a diffusion coefficient of about 1.2 × 10⁻² cm² s⁻¹, and a diffusion length of about 3.1 × 10⁻⁴ cm for the room temperature measured lifetime of 8μs. The phosphorescence quantum efficiency was less than 2% at low temperatures (only 0.25% at room temperature), indicating that the dominant decay mechanism was radiationless. The radiative lifetime was thus estimated as 1–2 ms. The two-photon phosphorescence excitation is characterized by an absorption cross-section of the order of 10⁻⁴⁹cm⁴s.     

Solvent and temperature dependence of the phosphorescence decay time of hexacyanochromate (III)

The temperature dependence of the ²E_g - ⁴A_2g phosphorescence of tetrabutylammonium hexacyanochromate (III) dissolved in various solvents has been measured within the liquid range of these solvents. In this limited temperature range, the reciprocal decay times obey Arrhenius relationship. From the absence of a solvent isotope effect, in ethanol and methanol and their monodeuterated analogues it is concluded that hydrogen bonds between the solvent and the complex ion do not provide promoting or accepting modes for the radiationless deactivation. The room temperature decay times have been found to correlate with the solvatochromic shift of the ⁴A_2g ? ⁴T_2g absorption bands and with Dimroth's E_T parameter. According to the selection rules for non-radiative decay a combination of CN stretching with either a Cr-C-N bending or a Cr-C stretching vibration is postulated as the promoting mode.     

Millisecond fluorescence in InAs quantum dots embedded in AlAs

The temperature dependence of steady-state and time-resolved photoluminescence from self-assembled InAs quantum dots embedded in AlAs has been studied. Millisecond-long nonexponential photoluminescence decay is observed in the temperature range of 4.2–50 K. At higher temperatures, the decay time decreases to a few nanoseconds. The experimental results are interpreted using a model of singlet–triplet splitting of exciton levels in small dots in a dense quantum dot system with local carrier transfer between dots.     

Computer calculation of positron diffusion and annihilation in atomic hydrogen

We have performed a computer-aided analysis of position behaviour in atomic hydrogen. Effect of electric, magnetic and temperature fields on the diffusion and annihilation of positrons has been studied. Electric field is varied over a wide range of 0–200 V cm⁻¹ amagat⁻¹, magnetic field over 0–30 kG while the temperature range considered is 300–10,000°K. The position decay rate decreases with electric and temperature fields but increases with magnetic fields. However, the effect of these fields is reversed on the diffusion coefficient.     

Delayed Luminescence of 8-Aza-D-Homogone-1,3,5(10),13-Tetraene-12,17a-Dione in the Gas Phase and Solid Solutions

It is shown that the delayed-luminescence spectrum of 8-aza-D-homogone-1,3,5(10),13-tetraene-12,17a-dione (8-aza-D-homogonane) in the gas phase comprises a band of thermally activated delayed fluorescence and a band of annihilation delayed fluorescence that are respectively due to the C- and D-centers representing products of photo-and thermotransformations of the initial steroid for which we failed to detect delayed luminescence. The triplet-triplet energy transfer from the C-centers to the D-centers has been detected and its efficiency has been determined. It has been established that, in a frozen hexane solution (T = 77 K) of the substance taken from a vacuum cuvette after the investigation of initial 8-aza-D-homogonane in the gas phase, only the C- and D-centers phosphoresce. For a frozen solution of the initial steroid in a vitrifying mixture of tetrahydrofuran and toluene, a phosphorescence of both the steroid itself (λ _max ^phos = 415 nm) and the C- and D-centers (λ _max ^phos = 498 and 532 nm, respectively) was detected. This indicates that even the initial steroid kept at room temperature contains products of its transformation whose concentration increases on irradiation and heating. The C- and D-centers are stable molecules.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 2, pp. 192–197, March–April, 2005.     

Point defects in BaS:Bi phosphors

For this study in phosphorescence, thermoluminescence(TL), thermally stimulated conductivity (TSC) and dielectric constant (k'), samples of polycrystalline barium sulphide phosphors doped with Bi were prepared. These samples were studied after exciting them with UV, X-rays, γ-rays and low-energy electrons. It has been observed that the phosphorescence of these phosphors exhibits a two-stage exponential decay. Complex TL patterns with overlapping glow peaks are obtained and the magnitude of TL output for X-ray irradiated samples is the biggest when compared with that for other ionizing radiations. The TL emission in each peak observed in the 350 to 600 nm wavelength range consists of three overlapping bands. These samples exhibit high dielectric constants in the frequency range 10²–10⁵ Hz and at RT (30 °C). It is noticed that k' decreases with increase in temperature up to 150°C, but increases thereafter with further increase in temperature. Trap depth (E) and frequency factor(S) are calculated from the experimental data. Attempts have been made to explain these results with impurity complex formation.     

Kinetics of the Decay of Prolonged Luminescence of Disordered Chrysene under the Conditions of Nonequilibrium Transfer of Energy

We present the results obtained in investigation of the decay of the prolonged luminescence of disordered chrysene in a microsecond range. At the temperature of liquid nitrogen, a highly nonexponential decay of both phosphorescence and annihilation of delayed fluorescence is observed. The observed character of decay is due to relaxation of the energy of electronic excitation in a system of energydisordered centers, when the value of kT is much smaller than the width of the excitedstate energy distribution. At the same time, in the system investigated there is a classical relationship between phosphorescence and delayed fluorescence, i.e., the rate coefficient for the reaction of triplettriplet annihilation is a constant.     

Some luminescent properties of additively colored KBr · Ag crystals

The luminescent characteristics of additively colored KBr · Ag crystals are investigated. In particular, the decay of the phosphorescence, the flash of emission due to IR light in relation to the silver concentration, and the temperature quenching of the steady luminescence are discussed. It is shown that the investigated crystals have a complex system of shallow and deep traps. The thermal activation energy for nonradiative transitions in the B centers is found. It is concluded that the luminescence is produced by an electron-recombination mechanism.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 81–85, November, 1973.     

Investigation on the improvement of red phosphorescence in CaTiO3:Pr3+ nanoparticles

Red phosphorescence in nanosized CaTiO₃:Pr³⁺ phosphors has been investigated by measuring the spectra of emission and excitation, as well as the charging and decay processes. The emission intensity and the persistent time have been improved. More energy traps have been observed in nanoparticles compared to the bulk powders. The integral emission intensity from the nanoparticles is remarkably greater than that from the bulk. A phosphorescence recombination mechanism through tunneling in nanoparticles is discussed.     

Excited State Proton Transfer Reaction of 4-Methyl-2,6-dicarbomethoxyphenol in Some Protic Solvents at Room Temperature and 77K: Interaction with Base

The ground and excited state proton transfer reactions of 4-methyl-2,6-dicarbomethoxyphenol (CMOH) have been studied in water and some alcoholic solvents both in the presence and in the absence of base by means of absorption, emission, and nanosecond spectroscopy at room temperature and 77 K. Solute–solvent interaction appears to play a major role in determining the nature of absorbing and fluorescing species in these solvent media. The emission properties of CMOH have been examined in relation to those of 4-methyl-2,6-diformylphenol (MFOH). The spectral characteristics obtained indicates that CMOH in methanol, ethanol, and water exists in more than one structural form in both the ground and excited states. At 77 K the emission spectra show phosphorescence only in the presence of a base. The fluorescence decay rates of CMOH are relatively slower than those of MFOH and nonradiative decays are dominant in the decay process of CMOH. It is proposed that intramolecular hydrogen bond in CMOH is stronger compared to that in MFOH. From our theoretical calculation at AM1 level of approximation suggested that the process is thermodynamically endothermic in the ground state and also encounter high barriers. However, the process becomes exothermic in the excited state.     

Temperature Dependence of the Efficiency of Collisional Transfer of Vibrational Energy in Mixtures of Vibrationally Excited Triplet Molecules with Foreign Gases

By the pressure dependences of the decay rates of delayed fluorescence activated by vibrational excitation of triplet molecules of benzophenone and anthraquinone, the efficiencies of collisional transfer of vibrational energy (V–V-transfer) in the vibrational quasi-continuum of the triplet state have been estimated. It is shown that the efficiencies of the process in mixtures with foreign gases increase with increasing dipole moment and polarizability of colliding molecules. In the temperature range 433–513 K, we obtained an inverse temperature dependence of the V–V-transfer efficiency, which is satisfactorily described by empirical relations taking into account long-range attractive forces. The results obtained point to the determining role of long-range attractive forces in quasi-resonance V–V-transfer of vibrational energy by molecules excited in vibrational quasi-continuum.     

Phosphorescence and phase structure of benzophenone—organomontmorillonite nanocomposites

We have investigated the temperature dependences (in the 4.2–180 K range) of the phosphorescence spectra of benzophenone (BP) in the presence of nanodisperse particles of the clay mineral montmorillonite (MMT) modified by different surface-active substances: dioctadecyldimethylammonium (DODM) and octadecylbenzyldimethylammonium (OBDM) chlorides. The MMT-OBDM sample has a stronger affinity for the given organic medium (BP) in the composite compared to the MMT-DODM sample due to the presence of a benzyl radical in the composition of the first modifier molecule. It has been shown that in the investigated heterosystems BP is present in both the crystal phase and the amorphous phase, and the latter thereby is formed in the near-surface layers of nanoparticles in the form of a thin shell due to the disordering action of inorganic particles on their adjacent BP layers. An increase in the chemical affinity between the organic clay surface and the BP molecules promotes the relaxation processes in the amorphous films of BP and increases the decay rate of its phosphorescence. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 809–814, November–December, 2005.     

Temperature dependence of the phosphorescence of pure and impure benzophenone crystals

The spectrum and quantum yield of the phosphorescence of pure and impure benzophenone crystals have been examined from 77 to 293 °K. The molecular quantum yield has a negative temperature coefficient, as does the transfer of triplet energy. The latter effect is due mainly to reduction in the lifetime of triplet excitons.Translated from Izvestiya VUZ. Fizika, Vol. 11, No. 8, pp. 106–110, August, 1968.     

Triplet‐Triplet Electron Excitation Energy Transfer in the Gas Phase for the Carbazol–Diacetyl Donor‐Acceptor Pair

For the carbazol–diacetyl donoracceptor pair, the features of exothermal triplettriplet (TT) transfer of electron excitation energy in the gas phase upon excitation of carbazol by the nitrogen laser radiation which is not absorbed by diacetyl have been investigated. The luminescence spectra of carbazol in the gas phase have been studied. It is shown that carbazol phosphorescence in the gas phase is absent. Therefore, for estimating the TT transfer efficiency and its temperature dependence, in the kinetic approximation the dependences of the intensity of timeresolved delayed fluorescence of carbazol and sensitized phosphorescence of diacetyl on diacetyl pressure have been analyzed. It has been found that in the gas phase the features of the electronic structure of the molecules under investigation are responsible for the low efficiency of transfer (2·10^–4 at 453 K), which increases by an order of magnitude with increasing temperature in the 453–513 K range despite the increasing influence of the back transfer.     

A novel structure of visible semiconductor lasers

Terraced substrate inner current stripe lasers emitting in the range of 750–780 nm are developed with very simple fabrication processes. The lasers have good performance of cw room temperature, linear light-current output over 30 mW per facet and maintain stable fundamental transverse and longitudinal mode of 2–4 times current threshold. The current threshold of 25 mA under cw room temperature operation has been achieved.     

Photophysical Processes in the Vapor of Carbazole

The characteristics of the rapid and retarded fluorescence of the vapor of carbazole excited by the radiation of a nitrogen laser are studied. The dependences of the intensity and rate of decay of the retarded fluorescence on the exciting-radiation intensity, the temperature, and the pressure of the vapor and foreign gases are used to determine the predominant mechanisms of relaxation of triplet molecules. It is shown that the decay of the retarded luminescence is governed by the competition between the processes of triplet-triplet annihilation that lead to the emission of annihilation retarded fluorescence and intercombination conversion of the triplet molecules to the ground electronic state. The characteristic times of these processes and the lifetimes of the triplet state in the vapor _T are evaluated; the temperature dependence of _T is analyzed in a wide interval of temperatures (77–573 K). It is inferred that in all aggregative states, the reduction in _T with increase in the temperature has a common nature and reflects an increase in the rate of intercombination transition T ₁ — S ₀ with increase in the content of vibrational energy.     

Fluorescence probes of viscosity: A comparative study of the fluorescence anisotropy decay of perylene and 3,9-dibromoperylene in glycerol

The authors compare the results of fluorescence anisotropy decay measurements for glycerol solutions of perylene with those of 3,9-dibromoperylene (DBP). For both molecules a good linear dependence is observed between the glycerol viscosity (varied by temperature) and the longer rotational correlation time obtained as a result of a global (using data obtained at 256- and 430-nm excitation wavelengths) biexponential analysis of the fluorescence anisotropy decay, at least in the range of 7–60 P for perylene and 4–60 P for DBP. This significantly extends the reported range of 0.5 to 150 cP investigated by Williams and Ben-Amotz [1] with the probe BTBP.     

Recombination centres and traps in ZnIn2S4

Luminescence intensity, phosphorescence decay and thermoluminescence glow have been measured for ZnIn₂S₄ and the results have been compared with previous data on the same material. Superlinear luminescence is found and it is interpreted in terms of the presence of several recombination centres in this crystal. From the analysis of the phosphorescence decay and thermoluminescence glow an exponential trap distribution is deduced with a slope of 70 meV/decade.