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In the paper the properties of Triplet Charge Transfer Excitons (TCTEs) in a linear stack with inversion of Donor and Acceptor (DA) molecules are analyzed. On the basis of two-electron approximation and spin Hamiltonian, the expressions for transfer parameters and energy levels of the three branches of TCTEs are calculated. The dispersion and the width of the excitonic bands are obtained. The vibronic spectra of TCTEs with intramolecular phonons are studied using vibronic approach. The coupling of TCTEs with triplet Frenkel excitons is described. The model of TCTEs in DA media confirms their weak coupling with other electronic excitations and can be useful in applications connected with their big relaxation time. [ABSTRACT FROM AUTHOR]

Anaerobic digestion is an established technology for distillery effluent treatment witch seems to be a promising alternative for Bulgarian industry. In this study the methanogenic activity of two different naturally formed microbial consortiums was compared. The better one was used to start continuous anaerobic digestion of high-strength distillery wastewater (COD 85 520 mgO2 . l-1) in laboratory scale anaerobic baffled reactor system. The average applied organic loading rate and hydraulic retention time were 4.28 kg COD m-3 . d-1 and 20 d respectively. A COD reduction of about 98 % and specific methane production of 0.39 m3 . kg-1 CODremoved were reached. Effects of different inhibitory factors such as low pH and presence of oxygen were investigated. In spite of unfavorable factors were applied simultaneously after an adaptation period the reactor showed stable response. The results obtained show the feasibility of this anaerobic process for distillery effluent treatment, representing a valid option to up-grade the existing wastewater treatment processes.

The paper treats the excitonic model of channelrhodopsin (ChR) which consists of dimers, each one connected with two-fold symmetry axis. By using the general theory of molecular excitons and the studies of bacteriorhodpsin (BR), the tensors of dielectric permeability and the gyration tensor are calculated including the contributions of the molecular gyrotropy and of the excitonic coupling. Two cases of refraction in the membrane of ChR are studied: a) normal incidence; b) oblique incidence. The components of refracted waves and the shape of the signal of circular dichroism (CD) are analyzed. The CD spectrum of ChR exhibits a negative couplet in those two cases unlike the case of BR in which couplet disappears at normal incidence. [ABSTRACT FROM AUTHOR]

Two-quanta anharmonic spectra, namely overtones and combination bands with degenerate vibrations, in uniaxial gyrotropic crystals from point group of symmetry C3, D3, C4, D4, are investigated, with due account taken of the time inversion. The symmetry of two-phonon states is studied. A general model of the anharmonicity and the influence of the crystal chiral structure on intermolecular exchange of the circularly polarized degenerate vibrations is elaborated. The contribution of overtones and of the combination bands to dielectric permittivity tensor, and to the gyration tensor is studied too. The paper also deals with the IR and vibrational circular dichroism (VCD) spectra near the frequencies of the unbound and of the bound two-phonon states as well as the general features of two-phonon circularly polarized states appearing in the linear and in nonlinear optical processes in gyrotropic crystals. [ABSTRACT FROM AUTHOR]

The paper studies vibronic spectra (exciton+intramolecular vibrational quanta) of helical polymers in solutions. The theoretical model employed here makes it possible to describe the ultraviolet (UV) absorption and circular dichroism (CD) spectra in the conditions of linear and quadratic exciton–phonon interaction. On the basis of the expanded Moffitt exciton model the structure of degenerate and nondegenerate one-phonon vibronic spectra containing one-particle and two-particle (unbound) exciton–phonon states is found. The frequencies and the shape of UV and CD spectral lines are calculated for typical values of the exciton–phonon interaction constants. [ABSTRACT FROM AUTHOR]

The possibility to investigate optical activity of uniaxial media by measuring the difference between the intensities of reflected electromagnetic waves (emw) of different circular polarization (reflection optical activity—ROA) is discussed. The components of the gyration tensor and the polarization of the normal emw in uniaxial media are found. The reflection of emw is studied in two cases: (a) optical axis normal to the boundary; (b) optical axis normal to the plane of incidence. Three modifications of differential spectroscopy are proposed: (1) left and right circularly polarized waves—usual reflection; (2) total reflection–circular polarization (TROA); (3) total reflection of linearly polarized waves with different azimuths of vibration. The specific features of the proposed methods (ROA and TROA) are discussed as well as the possibility of studying the optical activity induced by nondegenerate and degenerate excitations (vibrations). [ABSTRACT FROM AUTHOR]

A theory is developed for the IR absorption and vibrational circular dichroism (VCD) spectra of helical polymers in the frequency range of the frequency sum for two vibrational excitons. Bound and unbound two-exciton states are shown to exist, using the model for first nearest neighbors interactions (applicable to the amide II vibration). The dielectric permittivity tensor and the gyration tensor are found in the following cases: (a) oriented helices; (b) isotropic solutions of helical polymers. The frequency dependence of the various VCD terms is studied in the vicinity of the bound two-exciton states and in the band of unbound two-exciton states. [ABSTRACT FROM AUTHOR]

Theoretical model of the excitonic spectra in hexagonal 2D structures of two components (donors (D) and acceptors (A) respectively) has been explored. The A molecules occupy the nodes of a hexagonal lattice and D molecules are located in the centers of hexagons and create a trigonal lattice. 2D compounds of similar structure DA2 have been realized in MXenes - transition metal carbides, nitrides, or carbonitrides. Two types of molecular excitons and their coupling have been treated: (a) Charge Transfer Excitons (CTEs) of ionized DA pair of neighbor molecules (six branches of CTEs); (b) Frenkel excitons with the origin of electronic excitations of neutral excited D or A molecule. The symmetry analysis and Green functions methods are the basis of calculations of the excitonic spectra and of the levels of linear absorption in polarizations perpendicular and parallel to the molecular layer. [ABSTRACT FROM AUTHOR]

The paper treats a model of Charge Transfer Excitons (CTEs) in a simple hexagonal two-dimensional lattice. The symmetry governed combinations of CTEs and the excitonic spectra of pure CTEs have been analyzed. The coupling of CTEs and Frenkel excitons and corresponding equations for the spectra of the coupled excitons with nonvanishing transition dipole moments have been studied. [ABSTRACT FROM AUTHOR]

This paper explores the excitonic spectra and their manifestation in the linear absorption in the 3D model of a hexagonal donor-acceptor crystal of the crystallographic point group 6. A similar two-component crystal of alternative parallel layers with hexagonal order in each layer has been synthesized. Two types of excitons, Frenkel excitons (FEs) and charge-transfer excitons (CTEs) have been considered along with their coupling and the exciton-phonon coupling with one mode of intramolecular vibrations. We find also the symmetry allowed combinations of FEs and CTEs. The excitonic and vibronic spectra have been calculated and numerically simulated. [ABSTRACT FROM AUTHOR]

The excitonic and vibronic spectra of quadratic and hexagonal two-dimensional (2D) lattices of regularly arranged donor and acceptor molecules and the manifestation of those spectra in the linear absorption are modeled and numerically simulated. The splitting of the spectra caused by Frenkel excitons (FEs) and charge-transfer excitons (CTEs) coupling as well as by the anisotropy of the model has been studied analytically and displayed by simulations. The excitonic density of states, their renormalization by the FE-CTEs coupling and the shape of absorption lines in the excitonic, one-phonon, and two-phonon vibronic spectra have been calculated. The main results can be used in interpreting the absorption spectra of 2D symmetric donor-acceptor lattices. [ABSTRACT FROM AUTHOR]

In this paper, we consider a model of 2D molecular crystal of square symmetry with one molecule per unit cell. Results of the studies of the linear absorption in excitonic and vibronic spectra have been summarized for the following cases: (a) charge-transfer excitons (CTEs) in quadratic lattice; (b) Frenkel excitons (FEs) in that lattice; (c) coupled CTEs and FEs in quadratic lattice. The impact of the anisotropy inside the lattice plane and perpendicular to it on the linear absorption is analytically calculated and numerically simulated. The linear absorption spectra strongly depend upon transition dipole moments of Frenkel and charge-transfer excitons and also on the availability of one-particle and many-particle exciton-phonon states. 2D Van Hove singularities in the excitonic density of states occur in the present model. [ABSTRACT FROM AUTHOR]