Spectral–Luminescence Properties of Molecular Complexes of Photoconductive Polymers with Neutral Organic Acceptors

It has been found that in films of aryl– and alkyl–substituted polysilanes -- poly(methylphenylsilane) and poly(dihexylsilanes) -- the molecules of electron acceptors of tetracyanoquinodimethane, dinitrofluorenone, and trinitrofluorenone form weak complexes with charge transfer from the longest segments of the polymeric chain. The absorption and luminescence spectra of these complexes are similar to the spectra of complexes formed by the carbonyl groups of poly–N–epoxypropyl carbazole with the above acceptors and consist of wide bands situated in the visible range of wavelengths.     

Fluorescence Study of DNA–Dye Complexes Using One-Photon and Two-Photon Picosecond Excitation

Preliminary results of investigation of one-photon- and two-photon-induced fluorescence of acridine orange (AO), epirubicin (ER), hypericin (HYP), and ethidium bromide (EB) in complexes with DNA are presented. A spectrofluorometer based on a picosecond Nd:YAG laser was used for investigations of two-photon (1064-nm, 1-mJ, 40-ps) and one-photon (532- and 355-nm) dye excitation. The spectra of two-photon-induced fluorescence of dyes and their complexes with DNA as well as the kinetics of dyes' fluorescence intensification during their interactions with DNA in dependence on the biomacromolecule concentration were obtained. The intensities of AO, HYP, and EB fluorescence were increased 2.4, 3.2, and 8 times, respectively, after binding with DNA at two-photon excitation, while at one-photon excitation the corresponding values were 2.5, 3.7, and 10 times. The difference in fluorescence enhancement during DNA–dye complex formation at linear and nonlinear excitation may possibly be associated with the fact that the cross sections of one-photon and two-photon absorption, in general, change unequally during the binding of dyes to organic molecules and bathocromic shift of the electronic transitions. It was shown that the peak of AO fluorescence shifted to a longer wavelength on 10 nm after two-photon excitation at 1064 nm in comparison with one-photon excitation at 532 nm.     

Spectral–Luminescent and Generation Characteristics of a Gel–Like Gelatin Doped with a Dye

We present the results of comparative measurements of spectral–luminescent characteristics of Rhodamine 6G and Rhodamine C in water and water–gelatin solutions. We found that gelatin added to a water solution disaggregates the molecules of the dye. The maxima of the absorption and luminescence spectra of the solution shift then to the longwave region. The quantum yield of luminescence and the efficiency of generation of the solutions investigated are measured. The generation intensity of the gel is shown to depend on its composition and the dye selected. Measurement of spectral–luminescent characteristics of water–gelatin solutions makes it possible to select dyes that would generate most efficiently in gel solutions.     

Influence of nonradiative excitation-energy transfer on the dye laser active-medium gain-coefficient dichroism

The excitation power dependences of the luminescence polarization degree and of the anisotropy of the stimulated emission from glycerin solutions of rhodamine 6G at different dye concentrations have been obtained experimentally. It is shown that the anisotropy of the stimulated emission is determined by the dichroism of induced absorption in the range of low concentrations and by the development of excitation-energy migration into the generation channel. The complex character of the behavior of the fluorescence polarization degree is explained by the competition between several processes of excitation-energy transformation — nonradiative energy transfer and directed energy transfer to excimers with an increase in the dye concentration and in the excitation power.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 63–66, January–February, 2005.     

Time-resolved spectroscopy of soantaneous luminescence of CdSSe1−x quantum dots

Picosecond time-resolved spectroscopy of the edge luminescence band of CdS _x Se_1–x quantum dots with crystallite diameters as small as a few nanometers under band-to-band excitation reveals strong enhancement of the radiative recombination rate compared to bulk CdS owing to quantum confinement. The splitting of the luminescence band into two lines originates from near-band-gap absorption. Analysis of the temperature as well as the spectral dependence of the decay time (leading to a red shift of the luminescence with increasing time) and of the total-light-decay law result in a new model for the dominant radiative recombination channel: donor-acceptor pair recombination instead of an excitonic mechanism as claimed in previous publications.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.     

Photo- and thermostimulated processes in α-Al2O3

We reported on the recombination processes determined by the release of electrons from defects connected with the dosimetric 430 K thermostimulated luminescence (TSL) peak as well as with the 260 K TSL peak. These TSL peaks appear in thermochemically reduced α-Al₂O₃ crystals containing hydrogen and emission of these TSL peaks corresponds to luminescence of the F-center. The X-ray exposure or UV excitation in the absorption band of F-centers at 6.0 eV of reduced α-Al₂O₃ crystals doped with acceptor impurities results in the appearance of a broad anisotropic complex absorption band in the spectral region 2.5–3.5 eV and in the appearance of a predominant TSL peak at 430 K. Above 430 K the above-mentioned broad absorption band disappears. Optical bleaching of the 2.5–3.5 eV band is accompanied by the disappearance of the 430 K TSL peak and results in F-center emission. The X-ray or UV excitation of reduced α-Al₂O₃ crystals with donor-type impurities results in the appearance of an anisotropic absorption band at 4.2 eV and the appearance of a dominant TSL peak at 260 K. Above 260 K the 4.2 eV absorption disappears and photostimulated luminescence (PSL) of the F-center recombination luminescence in the 4.2 eV region is no longer observed. Optical bleaching of the 4.2 eV absorption band is accompanied by the disappearance of the 260 K TSL peak. The successful use of reduced α-Al₂O₃ in dosimetry needs the optimization of the concentration of all components (acceptors, hydrogen, intrinsic defects) involved in the thermo- and photostimulated processes.     

Controlled Dissolution of Phenytoin by Hybridizing with Silica Nanoparticles

A sparingly soluble model drug, phenytoin (5,5-diphenyl-hydantoin, denoted as PT), was incorporated during or after hydrolysis and polycondensation of tetra orthoethyl silicate (TEOS) to obtain silica–drug hybrids. We also compare the hybrids obtained by sol–gel process with those obtained by simple adsorption on nonporous silica particles. The initial rate of dissolution in water increases by a factor of 40 with respect to the intact PT by aging silica before drug addition. The IR results show that C=O in the position 2 of PT and N–H shift toward the higher wavenumber, showing that intermolecular hydrogen bonds between C=O and N–H are loosened or broken to form new hydrogen bonds between C=O in PT and Si–OH in silica. The dissolution rate of PT is determined by the degree of the breakage of hydrogen bonds between PT molecules and the intensity of the interaction between silica and PT.     

Luminescence of Europium–Doped Anodic Oxide Films on Multilayer Titanium–Tantalum Composites

Luminescence of europium in anodic oxide films on thin–film Ti–Ta composites and the process of anodic oxidation of these by the method of chronovoltammetry are investigated for different orders of arrangement of metallic layers and temperatures of heat treatment. A correlation between the luminescence spectra and chronovoltammetric diagrams is noted. The luminescence is shown to be an efficient method for studying the complex phase composition of anodic oxide films. The luminescence spectra of samples are characterized by the presence of several types of luminescence centers inherent in the Eu³⁺ ion and the base. Conditions for the formation of Eu–doped anodic oxide films on a diffusion alloy, which are of interest for microelectronics as a new dielectric material, are determined.     

Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

Spectroscopic studies of real space indirect symmetric GaAs/AlAs short period superlattices

The band structure and the optical transitions of symmetric ultra-short period (GaAs) _m /(AlAs) _n superlattices (n=m is the number of monolayers) have been studied using different spectroscopic techniques, namely photoacoustic spectroscopy, reflectivity and luminescence, photoluminescence excitation and high excitation intensity time-resolved luminescence. Direct observation of the transition from type I to type II energy band alignment is reported for superlattices whose configuration consists of more than 12 monolayers (i.e. m=n>12). The radiative recombination processes associated with the real space indirect transitions have been investigated as a function of the density of photogenerated electron-hole pairs, revealing an unusual density-dependent behaviour. A tentative interpretation in terms of a condensed electron-hole state at the indirect gap is given, which accounts for the long decay time of the type II luminescence at high excitation rate.     

Ultrafast nonlinear absorption and reflection of ZnS/ZnSe periodic nanostructures

ZnS/ZnSe heterostructures under condition of ZnSe interband excitation by a 150 fs laser pulse exhibit strong narrow-band modification of absorption and wide-band modification of reflection. Mean decay time for nonlinear reflection in heterostructures ranges from 2 to 3 ps whereas in bare ZnSe monolayer it exceeds 5 ps. Possible physical processes responsible for nonlinear refraction in the transparency region include interplay of absorption driven nonlinear refraction via Kramers–Kronig relations and intrinsic dielectric properties of dense electron–hole plasma. For nonlinear absorption at ZnSe band edge, interplay of plasma screening effects and states filling effects are relevant.     

Femtosecond laser spectroscopy of In0.53Ga0.47As/InP multiple quantum wells: interfacial roughness and photoexcited carrier relaxation

Interfacial properties and relaxation dynamics of photoexcited carriers in In_0.53Ga_0.47As/InP multiple quantum wells (MQWs) have been investigated by means of cross-sectional scanning tunneling spectroscopy and optical spectroscopy methods (luminescence, absorption and pump–probe experiments). The MQW structure consists of 125 periods of 10-nm-wide well layers and 40-nm-wide barrier layers on an InP(0 0 1) substrate. The observed interfacial roughness of the InGaAs-on-InP is 1–2 monolayers (ML), while that of the InP-on-InGaAs is 3–4 ML. The Stokes shift observed in luminescence and absorption spectra at 77 K corresponds well to the well-width distribution observed by the cross-sectional STM. Differential absorption spectra measured by pump–probe spectroscopy show that relaxation of hot carriers in conduction and valence bands followed by exciton formation takes place in 17–30 ps depending on the excitation photon energy. The excitons formed at the band bottom are localized at thicker areas within a quantum well in 500 ps.     

Photostimulated formation of sensitized anti-Stokes luminescence centers in AgCl(I) microcrystals

The low-temperature photostimulated activation of sensitized anti-Stokes luminescence in heterogeneous systems based on AgCl(I) microcrystals with adsorbed organic dye molecules and their aggregates is investigated. It is shown that the observed considerable (by more than an order of magnitude) enhancement of the intensity of this luminescence is caused by the formation of silver atoms and few-atom clusters on the surface of AgCl(I) microcrystals, which increase the efficiency of a two-quantum excitation of sensitized anti-Stokes luminescence by optical radiation in the range 630–730 nm with the flux density 10¹³–10¹⁵ quantum cm^?2s^?1. Analysis of all the experimental results indicates that the excitation mechanism of anti-Stokes luminescence is based on successive electron transfer or electron-excitation energy transfer from a dye molecule to an atomic-molecular dispersive silver center.     

Relative quantum yield of the green photoluminescence of cadmium sulfide for a complicated absorption mechanism

Because of the complicated mechanism for light absorption in the spectral range 4580–4910 Å, two quantum yields have been introduced for the green photoluminescence of polycrystalline CdS films. One of them is due to intrinsic interband absorption, while the other is due to absorption at superstoichiometric cadmium atoms. In a calculation of these quantum yields at 77 °K for two groups of polycrystalline films, it is shown that the spectral distribution of neither contains selective bands characteristic of the absorption, reflection, and excitation spectra of the green luminescence.Translated from Vysshikh Uchebnykh Zavedenii Fizika, N0,6, pp. 54–59, June, 1970.     

Photoinduced Effects in the Luminescence Spectra of Chalcogenide Glasses

We investigate photostructural transformations in glassy Ge–S films by the method of luminescence and Raman scattering of light. The appearance of a new shortwave band in the luminescence spectra of sulfurenriched Ge–S specimens as a result of irradiation by a He–Cdlaser light and subsequent relaxation is discussed. To explain the results obtained, a model of photostructural transformations is invoked, in which the decisive role is assigned to solitary electrons of the chalcogen atoms.     

Luminescence of orthovanadates M 3 + M3+(VO4)2 and M 2 + M4+ (VO4)2

The absorption, excitation, and luminescence spectra of vanadates of type M ₃ ⁺ M³⁺(VO₄)₂ and M ₂ ⁺ M⁴⁺(VO₄)₂ are studied, where M⁺ is Na, K, Rb, Cs; M³⁺ is Al, Sc; M⁴⁺ is Zr, Sn, Ti. The luminescence spectra maxima are located at 490–510 nm, while those of the excitation spectra are at 360–375 nm. Temperature characteristics of luminescence and thermostimulated luminescence are studied. The question of activation of complex vanadates by rare-earth ions is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 24–28, March, 1976.     

Determination of the Gamow–Teller strength function for the neutron-deficient isotopes In

The Gamow–Teller β decays of the neutron-deficient indium isotopes ^104–107In have been investigated by using total absorption γ-ray spectrometry on mass-separated sources. The experimental Gamow–Teller strength, deduced as a function of the excitation energy in the daughter nuclei ^104–107Cd, is compared to shell-model predictions.     

Synthesis, Characterization, and Luminescence Spectroscopic Accessibility Studies of tris(2,2′-Bipyridine)Ruthenium(II)-Labeled Inorganic-Organic Hybrid Polymers

The modified tris(2,2-bipyridine)ruthenium(II) complex 2 was anchored via a sol-gel process to different polysiloxane matrices to give a series of novel inorganic–organic hybrid polymers. One of the bipyridine ligands of 2 was provided with a long-chain spacer carrying a triethoxysilyl function (T group) at the end, which enables almost free mobility of the transition metal center. The polymers were swollen in several organic solvents of different polarity to investigate the luminescence behavior in the presence of quencher molecules. The luminescence of 2 was quenched using anthracene and atmospheric oxygen in appropriate concentrations of 5 · 10^–4 to 5 · 10^–3 and 3 · 10^–3 M, respectively. The luminescence behavior of 2 was determined by steady-state and time-resolved luminescence experiments. Translational mobilities of molecular species dissolved in the liquid phase were investigated by the kinetics of luminescence quenching after laser excitation. Both the translational mobility of anthracene and atmospheric oxygen, specified with the rate constant k₂ and the percentage of accessible luminescent centers were determined. Translational mobilities and the accessibility for anthracene and atmospheric oxygen in hybrid materials are significantly higher than in conventional Q type polysiloxanes.