Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations to investigate the effects of ion atmosphere on the dynamics of water-water hydrogen bonds at room temperature. The average number of hydrogen bonds per water molecule decreases with increase of ion concentration. The dynamics of hydrogen-bond breaking is found to accelerate somewhat and that of hydrogen-bond structural relaxation, which occurs at a longer time scale, is found to slow down with increasing ion concentration for both NaCl and KCl solutions.     

Effect of point defects on luminescence characteristics of ZnO ceramics

Photo- and thermally stimulated luminescence of ZnO ceramics are produced by uniaxial hot pressing. The luminescence spectra of ceramics contain a wide band with a maximum at 500 nm, for which oxygen vacancies V_O are responsible, and a narrow band with a maximum at 385 nm, which is of exciton nature. It follows from luminescence excitation spectra that the exciton energy is transferred to luminescence centers in ZnO. An analysis of the thermally stimulated luminescence curves allowed detection of a set of discrete levels of point defects with activation energies of 25, 45, 510, 590 meV, and defects with continuous energy distributions in the range of 50–100 meV. The parameters of some of the detected defects are characteristic of a lithium impurity and hydrogen centers. The photoluminescence kinetics are studied in a wide temperature range.     

Mesodroplet Heterogeneity of Low-Concentration Aqueous Solutions of Polar Organic Compounds

It is found experimentally that a mesoscopic droplet phase is formed in low-concentration aqueous solutions of various polar organic compounds, which are considered in the chemical literature as infinitely soluble in water. The content of dissolved organic molecules in droplets is much higher than in the ambient solution. The droplet size increases with temperature. Theory can explain the mesodroplet formation by the phase separation of a binary mixture affected by the dichotomous noise of twinkling hydrogen bonds between molecules of organic compound and water. The Snyder polarity index, which is used by chemists as a miscibility criterion for molecular compounds, depends in the model on the dipole moments of mixed molecules and the energy and number of hydrogen bonds. With this refinement, it can be used as an estimation criterion for the existence and intensity (i.e., the number of droplets per unit volume of organic aqueous solution) of mesodroplet separation.

     

Effect of C-H bonds on the quenching of luminescent lanthanide chelates

The quenching of europium(III) and terbium(III) chelate luminescence by high-energy C-H vibrational manifolds was studied with two types of stable chelates, i.e., a seven-dentate phenylethynylpyridine derivative and a nine-dentate terpyridine derivative. The replacement of C-H bonds by C-D bonds in the chelating parts of the ligands had a clear positive effect on Eu³⁺ luminescence but a negligible effect on Tb²⁺ luminescence. In aqueous solution, however, the positive effect was undetectable, if the chelating ligand did not create complete shielding of the ion against aqueous quenching. In chelates, where the coordination of water molecules to the inner sphere is prevented, the residual quenching through C-H vibrational quanta can be avoided by replacement of all C-H bonds in the vicinity of the emitting ion by C-D bonds.     

Effect of the carrier gas composition on the electrical and luminescent properties of CVD-grown zinc oxide films

The electrical and luminescent properties of zinc oxide films grown by CVD on sapphire substrates with different carrier gases are investigated. It is found that hydrogen, oxygen, and ammonia used as a carrier gas in the growth process insignificantly influence the defect structure and impurity content in the crystal. Only hydrogen decreases the density of radiative recombination centers responsible for the green luminescence band typical of zinc oxide.     

Scientific Review: Interactions of PEO with Monovalent Electrolytes in Solvents of Different Hydrogen Bonding Capacities

Small-Angle Neutron Scattering (SANS) has been used in order to investigate the ion binding capacities of PEO in different solvents. Aprotic (acetonitrile) as well as protic (methanol and water) solvents, have been studied where methanol and water form an average two and four tetrahedrally oriented hydrogen bonds per molecule, respectively [1]. Unusual behavior of PEO has been observed in aprotic solvents and solvents with moderate hydrogen bonding where monovalent ions associate to the polymer backbone leading to a polyelectrolyte-like (where a certain fraction of monomers are charged) behavior. This is in marked contrast to behavior in aqueous solutions where water molecules associate via hydrogen bonding to the polymer and the ions are more preferentially coordinated by the solvent than the polymer.     

Creation and optical properties of stable and metastable hole-trapped centres in beryllium oxide

Abstract

The paper is devoted to study of formation mechanisms and optical absorption of the hole-trapped centers in neutron, electron-impulse and X-irradiated BeO crystals. V⁰ and V^? centers are found out to be formed as a result of neutron irradiation creating cation Frenkel pairs. Within the transient absorption decay kinetics, we registered a component whose thermal-time properties coincide with those of the luminescence of triplet self-trapped excitons. A number of absorption bands from the V_B center and exciton hole nucleus are interpreted as transitions between the O^? ion p-levels splitted by the crystal field, as well as polaron transitions and transitions into the valence band.     

Time-resolved luminescence spectroscopy of LiF(U) crystals

We studied the luminescence spectra and decay kinetics of LiF(U) crystals excited by pulsed laser radiation (λ = 337 nm) in temperature range of 15–300 K. Two groups of lines with different exponential decay times are revealed in spectra. Based on comparative analysis of fast and slowly decaying luminescence spectra, temperature dependences of decay time constants, and line intensities, we conclude that electronic transitions from radiation levels of the impurity luminescence center can occur, not only in a vibrationally relaxed state, but also in a state with an excited high-frequency mode in O-U-O bonds.     

Effects of concentrated NaCl and KCl solutions on the behaviour of aqueous peptide bond environment: single-particle dynamics and H-bond structural relaxation

The effects of salt concentrations on the structure, dynamics and hydrogen bond structural relaxation properties of ～1.10 M aqueous N-methylacetamide (NMA) solution at 308 K are studied by classical molecular dynamics simulations. We have considered the concentration range of salts solution from 0.222 to 3.756 M to investigate the behaviour of aqueous environment of peptide bonds in the presence of concentrated NaCl and KCl solution. It is found that the addition of salt solution facilitates the structural breaking of aqueous NMA hydrogen bonds, as a result the number of hydrogen bonds per NMA molecule and their stability decreases. The water and NMA molecule shows slower translational and rotational dynamics with increasing salt concentrations due to additional ion atmospheric friction. Our result shows that the cation–O_NMA radial distribution function decreases whereas the Cl^?─H_NMA radial distribution function increases with ion concentration. On the other hand, the cation–O_water and Cl^?─H_water radial distribution function shows very negligible change with respect to ion concentration. We have also calculated NMA–water and water–water hydrogen bond structural relaxation times. It is observed that the hydrogen bond structural relaxation of O_NMA─H_water is comparatively slower than the H_NMA─O_water hydrogen bond, which can be attributed to higher number and greater stability of the former hydrogen bond than the latter. The change of the dynamical quantities observed here is more prominent in addition of NaCl rather than the KCl solution.     

Spectroscopy of silver complexes in solutions of inorganic compounds

The absorption, photoluminescence, and photoexcitation spectra of a number of inorganic solid solutions with a silver ion impurity have been examined. The influence of the temperature on the spectral characteristics of haloid and oxygen-containing solutions activated with Ag⁺ ions has been investigated. The temperature dependences of the luminescence quantum yield of solid solutions with Ag⁺ impurity in the temperature range 77-150 K have been studied. It is shown that the spectra under observation are conditioned by electron transition between energetic levels of Ag⁺ ion which are deformed because of the interaction with environment.     

Electronic excitation energy transfer and nonstationary processes in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript>:Tl crystals

We report the results of our experimental study and numerical simulation of the electronic excitation energy transfer to impurity centers under conditions where nonstationary processes take place in the hydrogen sublattice of potassium dihydrogen phosphate (KH₂PO₄) single crystals doped with mercury-like Tl⁺ ions (KDP:Tl). We present the experimental results of our investigation of the decay kinetics of the transient optical absorption (100 ns–50 s) of intrinsic defects in the hydrogen sublattice of KDP:Tl obtained by pulsed absorption spectroscopy and the results of our study of the dynamics of the change in steady-state luminescence intensity with irradiation time (1–5000 s). To explain the transfer of the energy being released during electron recombination involving intrinsic KDP:Tl lattice defects, we formulate a mathematical model for the transfer of this energy to impurity Tl⁺ luminescence centers. Within the model being developed, we present the systems of differential balance equations describing the nonstationary processes in the electron subsystem and the hydrogen sublattice; provide a technique for calculating the pair correlation functions Y(r, t) of dissimilar defects based on the solution of the Smoluchowski equation for the system of mobile hydrogen sublattice defects; calculate the time-dependent reaction rate constants K(t) for various experimental conditions; and outline the peculiarities and results of the model parametrization based on our experimental data. Based on our investigation, the dramatic and significant effect of a gradual inertial increase by a factor of 50–100 in steady-state luminescence intensity in the 4.5-eV band in KDP:Tl crystals due to the luminescence of mercury-like Tl⁺ ions has been explained qualitatively and quantitatively.     

Optical and luminescence properties of CdWO4 and CdWO4:Mo single crystals

Luminescence properties of a pure CdWO₄ crystal and a CdWO₄:Mo crystal doped with molybdenum in different concentrations have been investigated. The effect of molybdenum impurity on the intrinsic luminescence of CdWO₄ has been found, and the role of the impurity in the formation of new luminescence centers has been investigated. The features of the formation of the intrinsic and impurity luminescence excitation spectra of CdWO₄ and CdWO₄:Mo crystals in the fundamental-absorption region have been considered. The reflection spectra of the CdWO₄ crystal have been investigated taking into account the crystal structure anisotropy.     

Surface states and its influence on luminescence in ZnS nanocrystallite

In this pape,r the influence of surface effects on the self-activated (SA) luminescence in ZnS nanoparticles prepared by the wet-chemical method is presented. It is observed that the luminescence of SA decreases dramatically by rinsing with methanol. In the rinsed sample, the luminescence of SA increases more by ultraviolet (UV) light irradiation. To clarify its origin, the Raman spectra and electron paramagnetic resonance (EPR) are studied. The results demonstrate that the vibrational modes assigned to organic functional groups of -OH and -COO and -CH₃ decreases remarkably by rinsing, while the EPR signal originated from the unpaired electrons of some transition metal impurity ions including Mn²⁺ increases. It is suggested that the SA centers prefer to occupy the sites near the surface and that the donor of SA emission may be partly related to the organic functional groups of -OH and -COO adsorbed on the surface. The surface-dangling bonds caused by unpaired electrons of some transition metal impurity ions play a role of surface states, leading to the quenching of the SA emissions. The organic functional groups chemically combine with these surface-dangling bonds leading to the decrease in surface states and surface nonradiative relaxation channels and to the increase in the SA emissions.     

Photoluminescence of the ZnSe single crystals doped by thermal diffusion of nitrogen

Photoluminescence (PL) spectra of ZnSe single crystals annealed in different ambients containing molecular nitrogen are investigated. The compensating activity of N impurity in n-ZnSe crystals is shown. It is caused by the formation of N_Se acceptor centers, having 101-108 meV activation energy. The intensity of amplification of both long-wave luminescence spectra bands and the edge luminescence spectra bands caused by the presence of nitrogen in annealing medium is investigated. The presented results allow one to assign the long-wave luminescence to deep acceptors caused by uncontrollable impurities, and the relevant bands of the edge luminescence spectra to the excitons bound with the same deep acceptors. The model explaining the transformations of the luminescent properties of ZnSe crystals by means of nitrogen impurity doping is proposed. The model considers the presence of donors having 75 meV activation energy, acceptors having 220-720 meV activation energy and centers having levels localized near the middle of the band gap.     

Photoluminescence of traces of aromatic compounds in aqueous solutions upon excitation by a repetitively pulsed laser

A method is suggested for quantitative and qualitative analysis of aromatic compounds in water at extremely low concentrations (<1 ng/l). The method is based on excitation of luminescence by short (20 ns) UV laser pulses with a peak power of 10⁴ W at 255.3 nm and a pulse repetition rate of 10 kHz. The shape of luminescence spectra of benzene, benzoic acid, saccharin, aspirin, and L-tryptophan at low concentrations in water is analyzed. The luminescence kinetics of these compounds is studied at short delay times (10–20 ns). The lifetimes of the electronic excited states of the aromatic compounds are measured. The applications of the method for studies of low-concentration aqueous solutions of biological and medicine compounds are analyzed.     

NTO及其铷盐水溶液光谱研究

用Raman光谱和FTIR光谱对NTO晶体和不同pH值的水溶液以及配合物Rb(NTO)·H₂O的水溶液进行了研究。从已测定的单晶结构表明金属离子与NTO-的CNO,CO基团和水分子中的氮、氧原子键合。Raman和FTIR光谱特征的主强峰也表明是在NTO-的CNO₂和CO基团的氧和氮原子与金属离子形成弱配位键。该配合物饱和水溶液的振动光谱峰与晶体的振动光谱峰基本上吻合。表明该配合物在水中键合未发生变化,因而NTO的碱金属配合物在水溶液中的振动光谱峰归属可以用晶体中键的形成来说明。讨论了金属离子与NTO必需在碱性水中才能形成NTO的金属配合物机理。     

Effect of copper on dislocation luminescence centers in silicon

The effect of copper on dislocation luminescence centers in silicon has been investigated using photoluminescence and transmission electron microscopy. It has been demonstrated that there exist two main mechanisms responsible for quenching of dislocation luminescence by the copper impurity. The first mechanism is dominant at high copper concentrations and associated with the decrease in the time of nonradiative recombination of nonequilibrium charge carriers due to the formation of copper precipitates in silicon. This leads to the quenching of the entire dislocation luminescence and the edge exciton luminescence. The second mechanism is associated with the interaction of individual copper atoms with deep dislocation centers D1/D2, which results in the passivation of the recombination activity of these centers. This mechanism takes place even at room temperature and is highly effective at low copper concentrations.     

Mechanisms of ZnO Luminescence in the Visible Spectral Region

Existing models of zinc oxide luminescence in the visible spectral region are considered and comparatively analyzed. Experiments are performed with ceramics obtained from initial ZnO powder and ZnO powders annealed in vacuum (ZnO-vac) and in air (ZnO-air). The luminescence characteristics of the ceramics, namely, emission and excitation spectra, kinetics, and temperature stability, are studied. The ZnO-vac ceramics exhibits green luminescence caused by neutral oxygen vacancies. The luminescence of ZnO-air ceramics is shifted to the red and is presumably related to residual lithium impurity. The ceramics differ in the luminescence excitation spectra, time characteristics, and temperature stability.     

Ion-Beam-Induced Luminescence of LiF Using Negative Ions

Negative ion-beam-induced luminescence(IBIL) measurements of a pure LiF crystal using 20 keV H~-are performed to monitor the formation and annihilation of luminescence centers during ion irradiation. Several emission bands are observed in the IBIL spectra and the evolvement mechanisms of the corresponding centers are identified. The difference between the IBIL measurements using positive ions and negative ions is that the intensities of luminescence centers can reach the maxima at lower fluences under negative-ion irradiation due to free charge accumulation.