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Background. The available data from clinical studies suggest the essential role of free radical processes in the pathogenesis of drug dependence syndrome. At the same time, there is a limited understanding of using markers of oxidative stress in laboratory monitoring and prediction of drug pathology.Objective. To characterize changes in promising indicators of oxidative stress in patients with psychostimulant and opioid dependence syndrome.Material and Methods. The total study population was divided into three groups of men aged 23–35: healthy controls (n = 20), patients with the opioid (n = 20) and psychostimulant (n = 20) dependence syndrome. Patients were analyzed for the oxidative stress markers while being in therapy for addiction syndrome aimed at treating mental disorders and detoxification.Results. The study of antioxidant activity and blood plasma thiol groups did not reveal any significant differences between patients suffering from opioid and psychostimulant addiction. The values of the parameters mentioned above were maintained by 20–30% lower than the control indices throughout the entire study. The nature of changes in erythrocyte suspension parameters was not so unambiguous. Thus, patients with opioid dependence syndrome were characterized by a 91% increase in thiobarbituric acid (TBA)-reactive materials in the setting of a slightly altered state of the glutathione system parameters. Psychostimulant dependent patients revealed relatively low level of the products of biomolecule oxidative modifications in the erythrocytes that is 52% higher compared to the control values. It decreased during the therapy, but glutathione concentration reduction by 33% and an imbalance of glutathione metabolism were determined.Conclusion. The course of the opioid dependence syndrome is characterized by a pronounced intensification of free radical processes while the common trait for psychostimulant abusers is significant changes in the antioxidant defense system. Therefore, in the first case, it is most justified to conduct a laboratory assessment of indicators of oxidative damage, and in the second one, it is also advisable to determine the markers of the state of individual links of the antioxidant system.

The temperature dependences of the specific contact resistance of silicon ρc with a doping step are measured experimentally and described theoretically. The measurements are performed in the temperature range from 4.2 to 380 K. It is established that the contacts of the studied Au–Ti–Pd–n+–n-Si structures are ohmic. It is shown that minimal ρc is implemented at T = 75 K. Its value rises both with a decrease in temperature (due to the freezing effect) and with an increase in temperature (due to the electron-enriched layer at the boundary with the bulk material). It is established that the bulk electron concentration strongly decreases in the near-contact region in a layer with a thickness on the order of one micron due to the compensation of silicon by deep acceptors appearing because of the formation of a rather high vacancy concentration during stress relaxation and the appearance of a high dislocation density, as well as due to their diffusion from the contact after heating to 450°C. The data on the occurrence of vacancy-type defects are confirmed by X-ray measurements. The dislocation density in the studied structures is also estimated from X-ray measurements.

We determine the electronic density of states for computationally-generated bulk samples of amorphous chalcogenide alloys As$_{x}$Se$_{100-x}$. The samples were generated using a structure-building algorithm reported recently by us ({J. Chem. Phys.} ${\bf 147}$, 114505). Several key features of the calculated density of states are in good agreement with experiment: The trend of the mobility gap with arsenic content is reproduced. The sample-to-sample variation in the energies of states near the mobility gap is quantitatively consistent with the width of the Urbach tail in the optical edge observed in experiment. Most importantly, our samples consistently exhibit very deep-lying midgap electronic states that are delocalized significantly more than what would be expected for a deep impurity or defect state; the delocalization is highly anisotropic. These properties are consistent with those of the topological midgap electronic states that have been proposed by Zhugayevych and Lubchenko as an explanation for several puzzling opto-electronic anomalies observed in the chalcogenides, including light-induced midgap absorption and ESR signal, and anomalous photoluminescence. In a complement to the traditional view of the Urbach states as a generic consequence of disorder in atomic positions, the present results suggest these states can be also thought of as intimate pairs of topological midgap states that cannot recombine because of disorder. Finally, samples with an odd number of electrons exhibit neutral, spin $1/2$ midgap states as well as polaron-like configurations that consist of a charge carrier bound to an intimate pair of midgap states; the polaron's identity---electron or hole---depends on the preparation protocol of the sample., submitted to J Phys Chem B

Protein crystallization is central to understanding of molecular structure in biology, a vital part of processes in the pharmaceutical industry, and a crucial component of numerous disease pathologies. Crystallization starts with nucleation and how nucleation proceeds determines the crystallization rate and essential properties of the resulting crystal population. Recent results with several proteins indicate that crystals nucleate within preformed mesoscopic protein-rich clusters. The origin of the mesoscopic clusters is poorly understood. In the case of lysozyme, a common model of protein biophysics, earlier findings suggest that clusters exist owing to the dynamics of formation and decay of weakly-bound transient dimers. Here we present evidence of a weakly bound lysozyme dimer in solutions of this protein. We employ two electrospray mass spectrometry techniques, a combined ion mobility separation mass spectrometry and a high-resolution implementation. To enhance the weak but statistically-significant dimer signal we develop a method based on the residuals between the maxima of the isotope peaks in Fourier space and their Gaussian envelope. We demonstrate that these procedures sensitively detect the presence of a non-covalently bound dimer and distinguish its signal from other polypeptides, noise, and sampling artefacts. These findings contribute essential elements of the crystal nucleation mechanism of lysozyme and other proteins and suggest pathways to control nucleation and crystallization by enhancing or suppressing weak oligomerization.

We develop a computationally efficient algorithm for generating high-quality structures for amorphous materials exhibiting distorted octahedral coordination. The computationally costly step of equilibrating the simulated melt is relegated to a much more efficient procedure, viz., generation of a random close-packed structure, which is subsequently used to generate parent structures for octahedrally bonded amorphous solids. The sites of the so-obtained lattice are populated by atoms and vacancies according to the desired stoichiometry while allowing one to control the number of homo-nuclear and hetero-nuclear bonds and, hence, effects of the mixing entropy. The resulting parent structure is geometrically optimized using quantum-chemical force fields; by varying the extent of geometric optimization of the parent structure, one can partially control the degree of octahedrality in local coordination and the strength of secondary bonding. The present methodology is applied to the archetypal chalcogenide alloys...

New high-molecular nonionic surfactant “SMATWEEN” was synthesized by catalytic esterification of styrene-co-maleic anhydride copolymer (styromal) with polyoxyethylene sorbitantrioleate (Tween-85) and used as a demulsifier and depressant for multiple high-paraffinic oils from Kazakhstan oilfields. The structure of the synthesized surfactant was studied and proven. The high demulsibility efficiency of new macromolecular surfactant was shown. It was found that it is necessary to enter the demulsifier SMATWEEN before the pour point. Rheological parameters of the reagent show improvement by introducing it in high-paraffinic oil before the crystallization of the most high-molecular paraffins.

Toward quantitative description of protein aggregation, we develop a computationally efficient method to evaluate the potential of mean force between two folded protein molecules that allows for complete sampling of their mutual orientation. Our model is valid at moderate ionic strengths and accounts for the actual charge distribution on the surface of the molecules, the dielectric discontinuity at the protein-solvent interface, and the possibility of protonation or deprotonation of surface residues induced by the electric field due to the other protein molecule. We apply the model to the protein lysozyme, whose solutions exhibit both mesoscopic clusters of protein-rich liquid and liquid-liquid separation; the former requires that protein form complexes with typical lifetimes of approximately milliseconds. We find the electrostatic repulsion is typically lower than the prediction of the Derjaguin-Landau-Verwey-Overbeek theory. The Coulomb interaction in the lowest-energy docking configuration is nonrepulsive, despite the high positive charge on the molecules. Typical docking configurations barely involve protonation or deprotonation of surface residues. The obtained potential of mean force between folded lysozyme molecules is consistent with the location of the liquid-liquid coexistence, but produces dimers that are too short-lived for clusters to exist, suggesting lysozyme undergoes conformational changes during cluster formation.

The structural glass transition is often regarded as purely a problem of the classical theory of liquids. The dynamics of electrons enters only implicitly, through the interactions between ionic cores or molecules. Likewise, zero-point effects tied to the atomic masses hardly affect the typical barriers for liquid rearrangements. Yet, glasses do exhibit many quantum phenomena—electronic, optical, and cryogenic peculiarities that seem to have universal characteristics. These anomalies of the glassy state are uncommon or strongly system dependent in crystals and amorphous solids not made by a quasi-equilibrium quench of a melt. These clearly quantum phenomena include midgap electronic states in amorphous semiconductors, the two-level systems, and the Boson peak. Here, we discuss how these quantum phenomena found in glasses are not merely consequences of any kind of disorder but have universal characteristics stemming from the structural dynamics inherent in the glass transition itself. The quantum dynamics ...

We compute, on a molecular basis, the viscosity of a deeply supercooled liquid at high shear rates. The viscosity is shown to decrease at growing shear rates, owing to an increase in the structural relaxation rate as caused by the shear. The onset of this non-Newtonian behavior is predicted to occur universally at a shear rate significantly lower than the typical structural relaxation rate, by approximately two orders of magnitude. This results from a large size—up to several hundred atoms—of the cooperative rearrangements responsible for mass transport in supercooled liquids and the smallness of individual molecular displacements during the cooperative rearrangements. We predict that the liquid will break down at shear rates such that the viscosity drops by approximately a factor of 30 below its Newtonian value. These phenomena are predicted to be independent of the liquid's fragility. In contrast, the degree of nonexponentiality and violation of the Stokes–Einstein law, which are more prominent in fragile substances, will be suppressed by shear. The present results are in agreement with existing measurements of shear thinning in silicate melts.

This article provides a brief, up-to-date review of the theory of activated relaxations in supercooled condensed systems, of Wolynes and coworkers. Supercooled liquids and frozen glasses are truly frustrated systems whose microscopic dynamics consist of cooperative motions, high in the energy spectrum of structural states, and well above the defect states of an ordered solid. There is a temperature below which activated processes dominate dynamics and thus a free energy landscape emerges, many of whose microscopic attributes can be computed. In addition to signature phenomena in supercooled liquids, we apply this microscopic picture to the phenomenon of slaving of large scale conformational changes of a folded protein, imbedded in a glass-forming solvent, to the solvent motions. The size of the basin encompassing the corresponding conformational substates will be deduced, based on kinetic measurements alone. Implications of the theory for interpreting computer simulations of glass-forming systems are discussed.

Using recent advances in the Random First-Order Transition (RFOT) Theory of glass-forming liquids, we explain how the molecular motions of a glass-forming solvent distort the protein's boundary and slave some of the protein's conformational motions. Both the length and time scales of the solvent imposed constraints are provided by the RFOT theory. Comparison of the protein relaxation rate to that of the solvent provides an explicit lower bound on the size of the conformational space explored by the protein relaxation. Experimental measurements of slaving of myoglobin motions indicate that a major fraction of functionally important motions have significant entropic barriers.

We formulate a theory of reactive transport in the presence of environmental fluctuations that restart a transfer attempt, such as, for instance, during charge transfer through an intermediate state on a fluctuating bridge. It is shown that the rate and statistics of the fluctuations affect the steady-state populations of the reactants and products and the rate at which the steady state is approached. We give explicit examples in which the reactant/product populations can be inverted by varying the rate of environmental fluctuations. We thus propose a design principle of a controllable molecular scale device. We speculate that when noise-controlled charge transfer takes place in nature, as during electron transport in bacterial photosynthetic reaction centers, such a design can be used to maintain functionality over a wide temperature range and would be favored by evolution.

We explore several potential issues that have been raised over the years regarding the "entropic droplet" scenario of activated transport in liquids, due to Wolynes and coworkers, with the aim of clarifying the status of various approximations of the random first order transition theory (RFOT) of the structural glass transition. In doing so, we estimate the mismatch penalty between alternative aperiodic structures, above the glass transition; the penalty is equal to the typical magnitude of free energy fluctuations in the liquid. The resulting expressions for the activation barrier and the cooperativity length contain exclusively bulk, static properties; in their simplest form they contains only the bulk modulus and the configurational entropy per unit volume. The expressions are universal in that they do not depend explicitly on the molecular detail. The predicted values for the barrier and cooperativity length and, in particular, the temperature dependence of the barrier are in satisfactory agreement with observation. We thus confirm that the entropic droplet picture is indeed not only internally-consistent but is also fully constructive, consistent with the apparent success of its many quantitative predictions. A simple view of a glassy liquid as a locally metastable, degenerate pattern of frozen-in stress emerges in the present description. Finally, we derive testable relationships between the bulk modulus and several characteristics of glassy liquids and peculiarities in low-temperature glasses., minor corrections made, appeared in J. Phys. Chem. B, vol. 118, pp. 13744-13759

By chemical modification based on waste of polyolefins production, new reagents of complex action for oil production and transportation were developed. The structure of the obtained products was confirmed by H NMR and IR 1 spectroscopy. Developed reagents were tested as inhibitors of ARPD and depressants. The assay object was highly paraffinic Kumkol oil that was characterized by low content of sulfur and asphaltene components. The depressor activity of reagents was determined by standard ASTM D585309. The maximum decrease of the pour point and improvement of rheological properties was observed with the amount of the reagent 200–300 ppm; depression was 21°C. The effectiveness of inhibitors of ARPD was analyzed by the “Cold rod” method according to RD 39-3-1273-85. The results indicated sufficiently high inhibitory activity of reagent: the inhibition rate was 73% at joint heat treatment and entering additives into oil. With the use of the X-ray diffraction method, the effects of inhibitors on the structure of the samples of asphaltene deposits were controlled. It was revealed that the modified polyolefins were able to change the phase composition of deposits, thereby improving the rheological properties of oil.

Kinetics and nonlinear optical properties of a long range electron transfer complex in a polar solvent are studied in the presence of two perpendicularly directed, independent (incoherent) lasers with electric fields aligned along the electron path. The rate constant and absorption/emission cross sections in both direction are computed on the basis of the channel approach, whose validity is justified by a rigorous derivation. It is found that despite strong dissipation due to interaction with the polar environment, emission can be stimulated in one (“primary”) direction. The emission spectra are found to depend on the frequency of the auxiliary laser propagating perpendicular to the direction of the primary laser, the intensities of both lasers, the frequency and reorganization energy of quantum modes in the solvent, and the reaction heat. The emission spectrum is only slightly sensitive to the laser intensities in certain high frequency regimes. The latter property is important for potential applications...

A theory of multiphoton absorption of mixed valence compounds in polar solvents is developed. When these systems interact with a strong laser field, there exist various absorption ‘‘channels’’ which correspond to one, two, three,..., absorbed photon quanta. The probability of each channel to be switched on or off is dependent on the laser intensity, which results in strong intensity dependence of the absorption spectrum. Bands can be eliminated from the spectrum simply by changing the laser intensity. The physical picture of channels is justified by a derivation based on a kinetic master equation for the flow of electronic population. Calculations are carried out for the case where the polar environment can be modeled by a single high frequency (‘‘quantum’’) vibrational mode plus a classical bath represented by a collection of low frequency harmonic oscillators. The spectrum is found to be only weakly sensitive to the quantum vibrational mode. Suggestions for experimental verification are made.

Classical electron transfer in polar glasses is described by a theory based on a model microscopic Hamiltonian which includes the discreteness and randomness of the glassy polar modes with distinct orientation. When configurational dynamics is fast, the reaction is described by exponential kinetics with a rate constant of non‐Arrhenius type. The temperature dependent rate constant resembles the tunneling rate, despite the classical transfer of the electron. This effect is called ‘‘false tunneling.’’ In this limit the possibility of a self‐acceleration of the reaction is pointed out. When configurational dynamics is very slow the reaction kinetics are nonexponential with multirelaxation time behavior. The reaction is shown to be almost insensitive to temperature change pointing out on a possible explanation of a broad temperature‐independent range in the ‘‘rate constant’’ in an electron transfer in cytochrome c oxidation in chromatium. At short times, the reaction accelerates compared to the exponential behavior, while at long times it becomes slower. For strongly exothermic reactions the kinetics are always slower than an exponential decay.

A phase transition from a liquid to glass state is studied for polar solvents in the presence of ions. The entropy and free energy of the system above and below the transition temperature, Tc, is found on the basis of the theory of random dipole–dipole and charge–dipole interactions . The dependence of the transition temperature on the concentration of charged impurities, c, is found. It is shown that a relative charge concentration of only 10−5 is needed to increase Tc by 20 K. The transition temperature is also dependent on the value of the charge. For a Fe+3/Fe+2 reactive complex the transition temperature is changed by 10 K for c=10−6. For dissolved salts we describe the relationship between Tc and the separation distance of the ion pair. The glass transition is insensitive to the size of dissolved ions. The theory proposed has only one adjustable parameter, the elementary dipole moment of the solvent. The theory allows to explain a peak dependence in Tc measured as a function of the alcohol content Y...

Metastable clusters of mesoscopic dimensions composed of protein-rich liquid exist in protein solutions, both in the homogeneous region of the solution phase diagram and in the region supersaturated with respect to an ordered solid phase, such as crystals; in the latter region they are crucial nucleation sites for ordered solids. We monitor, using three optical techniques, the long-term evolution of the clusters in lysozyme solutions at conditions where no condensed phases, liquid or solid, are stable or present as long-lived metastable domains. We show that cluster formation is a reversible process and that the clusters are in near equilibrium with the solution, up to a capillary correction. In contrast to classical phase transformations, the solution concentration at cluster-solution equilibrium is close to its initial value; this is akin to chemical reaction equilibria and demonstrates the complex chemical composition of the clusters. However, similar to classical phase transformations, en route to full equilibration, the average cluster size grows with time following a universal law t(0.26±0.03), independent of the cluster volume fraction; the cluster size distribution is scale-invariant at all stages of cluster evolution. Despite the correspondence of these behaviors to the Lifshitz-Slyozov-Wagner (LSW) theory predictions, the cluster sizes are about 10× smaller than the LSW prediction, likely due to the complex cluster composition. The observed cluster evolution helps us to understand nucleation mysteries, such as nucleation rates lower by orders of magnitude than classical theory predictions, nucleation rate variable under steady conditions, and others.

Long-living mesoscopic clusters of a dense protein liquid are a necessary kinetic intermediate for the formation of solid aggregates of native and misfolded protein molecules; in turn, these aggregates underlie physiological and pathological processes and laboratory and industrial procedures. We argue that the clusters consist of a nonequilibrium mixture of single protein molecules and long-lived complexes of proteins. The puzzling mesoscopic size of the clusters is determined by the lifetime and diffusivity of these complexes. We predict and observe a crossover of cluster dynamics to critical-like density fluctuations at high protein concentrations. We predict and experimentally confirm that cluster dynamics obey a universal, diffusion-like scaling with time and wave vector, including in the critical-like regime. Nontrivial dependencies of the cluster size and volume fraction on the protein concentration are established. Possible mechanisms of complex formation include domain swapping, hydration forces, dispersive interactions, and other, system-specific, interactions. We highlight the significance of the hydration interaction and domain swapping with regard to the ubiquity of the clusters and their sensitivity to the chemical composition of the solvent. Our findings suggest novel ways to control protein aggregation.

We develop a systematic description of spectral diffusion of ideal chromophores interacting with incoherently relaxing two-state, localized environmental degrees of freedom ("spins") for general initial environment configurations. We remedy the existing, incomplete treatments by formulating the problem in terms of the proper correlation function and by obtaining an accurate solution for generic aperiodic arrangements of environmental spins, nearly free of the customary simplifying assumptions on the multiparticle spin coordinate distribution. We report and estimate, for the first time, the effects of the drift and distortion of a narrow spectral line that arise when the line is not in the center of the inhomogeneous band. While the drift turns out to be modest in most ensemble measurements, accounting for its effects is imperative in analyzing single chromophore spectral jumps, to which end the authors propose a novel experiment. Further, we argue that by employing a sufficiently large chromophore one can decouple the concentration of the fluctuating centers from the strength of their interaction with the chromophore. Finally, the additional line broadening, owing to a distribution of the central chromophore frequencies, is evaluated. Upper estimates for an analogous broadening stemming from a nonequilibrium environment are made.

Melting is analyzed dynamically as a problem of localization at a liquid-solid interface. A Lindemann-like criterion of melting is derived in terms of particular vibrational amplitudes, which turn out to equal a universal quotient (about one-tenth) of the molecular spacing, at the interface. The near universality of the Lindemann ratio apparently arises owing to strongly overdamped dynamics near melting, and despite the anharmonic interactions' being system-specific. A similar criterion is derived for structural displacements in the bulk of the {\em solid}, in particular the premelted layer; the criterion is no longer strictly universal, but still depends only on the harmonic properties of the solid. We further compute the dependence of the magnitude of the elemental molecular translations, in deeply supercooled fluids, on the temperature and the high frequency elastic constants. We show explicitly that the surface tension between distinct liquid states, near the glass transition of a supercooled liquid, is nearly evenly split between entropic and energetic contributions., Comment: to appear in J. Phys. Chem. B

The purpose of this study was to determine the changes in concentrations of the two Russian photosensitizers in blood plasma of patients under Photodynamic therapy. In this work were used two sensitizers of domestic production: Photohem (hematoporphyrin derivative) and Photosense (sulfonated aluminium phtalocyanine). It was found that one month after injection the concentrations of the photosensitizers in blood plasma remained high enough. Was shown alteration of level of apo-(beta) -lipoproteins oxidation and antioxidant activity of blood plasma under the influence of Photodynamic therapy.

The goal of this work is to study the laser-induced photosensitized oxidation of lipids of the artificial (lyposomes) and cell's (erythrocytes, leukocytes) membranes and human blood lipoproteins. As a exogenous photosensitizers we are used hematoporphyrin derivatives (HPD) and sulfonated phthalocyanine aluminum (Pc). Irradiation was performed by helium-neon laser (632.8 nm). It was found out the increasing of the products of lipid oxidation in the suspensions of a lyposomes, human blood apo-(beta) -lipoproteins, erythrocytes and leucocytes. Accumulation of the lipid oxidation products depend on as irradiation dose as HPD or Pc concentrations. Interaction of photosensitizers with the lyposomes, lipoproteins and erythrocytes was investigated.

Erratum: “Universality of the onset of activated transport in Lennard-Jonesliquids with tunable coordination: Implications for the effects of pressureand directional bonding on the crossover to activated transport,configurational entropy, and fragility of glassforming liquids” [J. Chem.Phys. 136, 084504 (2012)]

We establish, via classical density functional theory, that the crossover to activated transport in liquids takes place when the depth of the metastable minimum in the free energy corresponding to long-lived aperiodic structures reaches a certain near universal value. We show that the particle vibrational displacement is strongly correlated with this depth in a broad range of pressure and temperature, thus providing basis for a Lindemann-like criterion for the onset of activated transport in liquids. The configurational entropy at the crossover temperature T(cr), too, is found to be nearly system-independent, consistent with the random first order transition theory. We show that to reproduce existing data for the pressure dependence of T(cr), the liquid must increase its coordination with pressure. Upon increasing pressure at fixed coordination, the liquid's fragility is predicted to exhibit re-entrant behavior. This prediction is consistent with glycerol data but is in contrast with data in several organic liquids and polymers, whose fragility monotonically decreases with pressure in the so far accessed pressure range. Allowing for increase in coordination with pressure mitigates the disagreement, owing to the resulting decrease in the thermal expansivity. Finally, we rationalize the correlation between the isobaric and isochoric fragilities put forth by Casalini and Roland [Phys. Rev. E 72, 031503 (2005)] and make predictions on the limiting behavior of the fragility at high pressure.

The polymerization of sickle cell hemoglobin (HbS) in the erythrocytes of sickle cell anemia patients is the primary event in the pathophysiology of this debilitating and deadly disease. Correspondingly, the majority of the current clinical treatments rely on delaying HbS polymerization. In search of pathways towards novel, more efficient treatment strategies, we explore the mechanism of nucleation of the HbS polymers. Previous work has shown that this nucleation follows a two-step mechanism, whereby the polymers nucleate inside dense liquid clusters suspended in the solution and occupying about 10−5 of the solution volume. We show that free hematin, which is spontaneously released by HbS due to its intrinsic instability to autoxidation, accelerates by ∼100× the rates of both nucleation and growth of the polymers and that its removal leads to complete arrest of HbS polymerization. Exploring the mechanism underlying these hematin effects, we show that hematin enhances the attraction between the hemoglobin molecules in the solution and this yields ∼100× higher volume of the dense liquid clusters in which nucleation occurs. These findings suggest that the nucleation of sickle cell hemoglobin polymers and the ensuing pathology can be suppressed by controlling the release and concentration of hematin in the red blood cells.

Semiconductor glasses exhibit many unique optical and electronic anomalies. We have put forth a semi-phenomenological scenario (J. Chem. Phys. 132, 044508 (2010)) in which several of these anomalies arise from deep midgap electronic states residing on high-strain regions intrinsic to the activated transport above the glass transition. Here we demonstrate at the molecular level how this scenario is realized in an important class of semiconductor glasses, namely chalcogen and pnictogen containing alloys. Both the glass itself and the intrinsic electronic midgap states emerge as a result of the formation of a network composed of $\sigma$-bonded atomic $p$-orbitals that are only weakly hybridized. Despite a large number of weak bonds, these $pp\sigma$-networks are stable with respect to competing types of bonding, while exhibiting a high degree of structural degeneracy. The stability is rationalized with the help of a hereby proposed structural model, by which $pp\sigma$-networks are symmetry-broken and distorted versions of a high symmetry structure. The latter structure exhibits exact octahedral coordination and is fully covalently-bonded. The present approach provides a microscopic route to a fully consistent description of the electronic and structural excitations in vitreous semiconductors., Comment: 22 pages, 17 figures, revised version, final version to appear in J. Chem. Phys

The theory presented in [J. Chem. Phys. 104, 669 (1996)] deals with the effect of separated charged impurities on the glass transition temperature Tg. To explain experimental data on the effect of salt on the glass transition temperature, it is necessary to include mechanisms of aggregation and dissociation of salt molecules. Due to aggregation and dissociation, the concentration of charges can be different than the salt concentration. It is pointed out that this might explain the discrepancy between some experimental data and the theory.

The temperature dependence of the curves of absorption and dispersion of light of ionic crystals in the exciton region is investigated taking into account the interaction of excitons with acoustic and optical phonons. It is shown that the shape of the curves depends on the sign of an effective exciton mass m. In the approximation linear in the phonon operators the maxima of the curves shift with increasing temperature towards lower energies when m > 0, and towards higher energies when m < 0. Es wird die Temperaturabhangigkeit der Absorptions- nnd Dispersionskurven des Lichtes von Ionenkristallen im Exzitonenbereich untersucht, wobei die Wechselwirkung der Exzi-tonen mit akustischen und optischen Phononen berucksichtigt wird. Es wird gezeigt, das die Kurvenform vom Vorzeichen einer effektiven Exzitonenmasse m abhangt. In der in den Phononenoperatoren linearen Naherung verschieben sich die Maxima der Kurven mit stei-gender Temperatur zu niedrigeren Energien, wenn m > 0, und zu hoheren Energien, wenn m < 0 ist.

An investigation is made of the optical absorption and the shape of the dispersion lines due to impurity centres in a solid for the case of weak electron-phonon coupling and high impurity concentration. It is shown that the shape of the absorption lines may be Lorentzian or Gaussian depending upon the magnitude of the shift in the electronic levels of the impurities due the interactions between impurity centres. Der optische Absorptionskoeffizient und die Form der Dispersionslinien, die durch Verunreinigungen in einem Festkorper hervorgerufen werden, werden fur den Fall einer schwachen Elektron—Phonon-Kopplung und hoher Verunreinigungskonzentration untersucht. Es wird gezeigt, das die Dispersionslinien in Abhangigkeit von der Grose der Verschiebung der Niveaus der Verunreinigungszentren, die durch deren gegenseitige Wechselwirkung verursacht wird, Lorentz- oder Gauss-Form besitzen konnen.

The Green's function method has been used to investigate the optical activity in the impurity absorption region of solids, taking account of anharmonicity. The host crystal containing the impurity centres is assumed to be optically non-active. It is shown that the dependence of the optical rotation on the frequency of the exciting light is the same as that of the refractive index of the impurity centres, and the frequency dependence of the circular dichroism is the same as that of the absorption coefficient. [Russian Text Ignored].

A two-particle Green's function method is used to calculate the dispersion and absorption of light by impurity centres in solids, assuming the electron-level separation to be of the same order of magnitude as the maximum phonon energy and assuming a weak electronphonon interaction. The absorption spectrum and corresponding dispersion curve are shown to contain phonon and non-phonon contributions. The forms of these contributions are calculated and their variation with temperature and the electronic structure of the impurity centre is investigated. [Russian Text Ignored].

The two-particle Green's function method is used to investigate the dependence of light absorption and dispersion due to substitutional impurity centres upon their concentration in the case of weak electron-phonon coupling, taking into account the change of the quasi-elastic constants due to the substitution. At low impurity concentrations the half-width of the zero-phonon line and the corresponding dispersion curve as well as their position in the spectrum are shown to vary linearly with concentration; at higher concentration the absorption coefficient may deviate from linearity. [Russian Text Ignored].

An investigation is made of the shape of the emission spectra due to impurity centres in a solid, taking into account the variation of its quasi-elastic constants during the phototransitions. A formula for the short-wavelength edge of the emission spectrum similar to the Urbach expression for absorption is obtained. Es wird die Linienform der Emissionsspektren von Fremddefektzentren in Festkorpern untersucht, wobei die anderungen der quasielastischen Konstanten wahrend des Photoubergangs berucksichtigt werden. Es wird eine Formel fur die kurzwellige Kante des Emissionsspektrums erhalten, die der Urbach-Formel fur die Absorption ahnlich ist.

The spectra of resonant, Raleigh, and Raman scattering of light by impurity centres are calculated by the method of Green's functions and operator arrangement along the integration contour, taking account of anharmonicity. A general expression is obtained for the scattering spectrum. The dependence of the phonon and non-phonon parts of the spectrum on temperature and the nature of the host crystal are discussed in detail. As the frequency of the exciting light approaches the impurity absorption region, the integral scattering intensity increases with the absorption coefficient of the impurity centres. [Russian Text Ignored.]

An investigation is made of the shape of the absorption spectra due to impurity centres in a solid, taking into account the variation of its quasi-elastic constants during phototransitions in impurity centres. An analytical expression for the long-wavelength edge of the absorption curve, similar to the well-known Urbach rule, is obtained. Es wird die Form der Absorptionsspektren von Verunreinigungszentren in Festkorpern untersucht, wobei die Anderung der quasi-elastischen Konstanten wahrend der Photo-Ubergange in den Zentren berucksichtigt wird. Ein analytischer Ausdruck wird fur die langwellige Kante der Absorptionskurve erhalten, der der gut bekannten Urbach'schen Regel ahnlich ist.