Vol 122, No 6 (2017)

Equilibrium nuclear configurations of the planar formaldehyde homodimer (H₂CO)₂ and the (H₂CO)₂···HF complex are determined in the MP2/6-311++G(3df, 3pd) approximation taking into account the superposition error of basis sets of monomers. Harmonic values of the frequencies and intensities of fundamental transitions between vibrational states of these hydrogen-bonded complexes were calculated using the Gaussian 09 package of programs. Anharmonic values of the frequencies and intensities of the ν(H–F) stretching vibration and several intermolecular vibrations in the (H₂CO)₂···HF trimer were obtained from variational solutions of one-, two-, and three-dimensional vibrational Schrödinger equations. The anharmonic influence of the C=O and hydrogen bond O···H–F stretching vibrations, as well as of librational vibrations of monomers, on the spectral parameters of the strongest ν(H–F) absorption band of trimer was studied.

New sub-Doppler resonances at central frequencies of atomic (molecular) transitions that appear in the spectrum of absorption of the probe optical radiation under the influence of optical pumping propagating in the orthogonal direction through a relatively narrow area of a cylindrical cell containing dilute gas medium are discovered and analyzed. These resonances are induced by specific optical pumping of atoms as they fly freely from the inner cell surface through the pumped region toward the probe optical beam. The obtained mathematical relations are used to investigate the dependence of the discussed resonances on the intensity and spatial distribution of the localized optical pumping. The proposed method could allow reducing the Doppler broadening of the detected spectral lines by the factor equal to the ratio of the effective width of the narrow pumped region to the cell radius. The obtained results may find application in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization by using the discovered sub- Doppler resonances.

Optical characteristics of polymerization products under variable degrees of neutralization of acrylic acid (AA) and cross-linking agent and at a fixed content of redox components have been studied by IR spectroscopy of frustrated total internal reflection and THz spectroscopy. The results of the analysis show that absorption of IR radiation and dispersion of THz radiation for the samples under study depend on the degree of AA neutralization, which predetermines the mechanism of structure formation of hydrogels, refractive index, and degree of their limited swelling. It is found that the dependences of the refractive index of dry compact hydrogels and the degree of limited swelling of ground polymer samples on the content of neutralizing agent in solutions of the reaction systems have a symbatic character.

Infrared (IR) spectra of a number of crystals with proton conductivity have been investigated. The width of the band gap is determined, and most lines of the IR absorption spectra are identified. A direct proof of the existence of protons, defects ОН–and Н₃О⁺, molecules of adsorbed and crystallization water in the crystals grown in ordinary and in heavy water is also carried out. The transparency of the rectangular potential barrier for protons is calculated, and the possibility of tunneling and translational diffusion of protons in wide-band-gap crystals is shown.

The mechanism of coloration of alkaline-earth fluoride crystals CaF₂, SrF₂, and BaF₂ in calcium vapors in an autoclave with a cold zone is studied. It was found that the pressure in the autoclave upon constant evacuation by a vacuum pump within the temperature range of 500–800°C increases due to evaporation of metal calcium. In addition to the optical-absorption bands of color centers in the additively colored undoped crystals or to the bands of divalent ions in the crystals doped with rare-earth Sm, Yb, and Tm elements, there appear intense bands in the vacuum ultraviolet region at 7.7, 7.0, and 6.025 eV in CaF₂, SrF₂, and BaF₂, respectively. These bands belong to the H_a^- hydrogen centers. The formation of hydrogen centers is also confirmed by the appearance of the EPR signal of interstitial hydrogen atoms after X-ray irradiation of the additively colored crystals. Grinding of the outer edges of the colored crystals leads to a decrease in the hydrogen absorption-band intensity with depth to complete disappearance. The rate of hydrogen penetration inside the crystal is lower than the corresponding rate of color centers (anion vacancies) by a factor of tens. The visible color density of the outer regions of the hydrogen-containing crystals is several times lower than that of the inner region due to the competition between the color centers and hydrogen centers.

The luminescence spectra of Cr³⁺ and Eu³⁺ ions embedded in mullite ceramic samples are studied. The samples were annealed at temperatures from 600 to 1200°С. Beginning at a temperature of 890°С, the spectra demonstrated the presence of a newly formed crystal phase. At temperatures from 920 to 1200°С, the crystalline form manifested itself as a stable mullite phase. A temperature of 920°С corresponded to a sharp change in the symmetry of the crystal field around Eu³⁺ (phase transition). The parameters of the spectral lines of ions were used to estimate the mullite crystal-lattice disorder and the residual deformations in ceramic samples. The spread of the mullite crystal-lattice parameter is determined as |Δа/а| ≈ 0.028 and is independent of the annealing temperature.

The results of studying optical-absorption and spectral curves of photoconductivity in MnGaInS₄ single crystals are presented for two light polarizations (E || C and E ⊥ C). The intrinsic absorption edge and the band-gap width of MnGaInS4 single crystals in polarized light are determined. The anisotropy of optical absorption and photoconductivity spectrum of MnGaInS₄ single crystals is observed. It is suggested that the polarization splitting of the absorption edge is related to the splitting of the MnGaInS₄ valence band.

It is demonstrated that the reason for SERS on dielectric and semiconductor substrates is enhancement of the electric field in the regions of the tops of surface roughness with a very small radius or a very large curvature. The enhancement depends on the dielectric constant of the substrate and is stronger for a larger dielectric constant. It is indicated that the enhancement on dielectrics and semiconductors is weaker than on metals with the same modulus of the dielectric constant. The result obtained is confirmed by experimental data on the enhancement coefficients obtained for various semiconductor and dielectric substrates.

We have examined examples of nonlinear-photonics devices that are based on the coherent interaction of light with matter. Such interaction takes place if the duration of a light pulse is shorter than the relaxation times T₁ and T₂ in a resonant medium and if the strength of the light field is so high that Rabi oscillations arise. Theoretical analysis shows that these systems have a number of advantages compared to similar devices that operate under incoherent interaction conditions of light with matter.

Various recently emerged applications of mid-infrared pulses for high-order harmonic generation in laser plasmas produced on the surfaces of various materials are reviewed. Among them are (a) the highorder harmonic generation in Ag, Sn, fullerene, and graphene nanoparticle-containing plasmas using tunable two-color mid-infrared pulses, (b) efficiency dependence on the generating particle properties in the case of two-color high-harmonic generation of the longer-wavelength radiation in the plasmas, and (c) high-order sum and difference frequency generation using tunable two- and three-color commensurate and incommensurate mid-infrared pumps of graphite plasma. These studies demonstrate the perspectives of the frequency conversion of mid-infrared radiation in the extended laser-produced plasmas.

An optico-mechanical model describing the coherent (directed) transmittance and the degree of polarization of forward-transmitted light by a polymer film with elongated liquid-crystal (LC) drops has been developed. This model, based on the Foldy–Twersky and anomalous-diffraction approximations, makes it possible to analyze the optical response of a film under extension as a function of the film thickness, refractive index of the polymer, the sizes and anisometry parameters of liquid-crystal drops, their concentration, internal structure, polydispersity, and orientation of optical axes. The model is verified based on the comparison of numerical and experimental data for the inverse modification of interfacial anchoring by an ion-forming surfactant. The internal drop structure is determined by solving the problem of minimizing the volume free energy density. A comparative analysis of the calculated transmittance and degree of polarization for films with uniform homeotropic and modified inhomogeneous interfacial anchoring is performed. The spectral polarization characteristics of a film with elongated LC drops and single-domain internal structure, formed under mechanical extension with the aid of surfactants, are investigated.

The results of an experimental investigation of a sensor intended for detection and measurement of concentration of nanoparticles in an aqueous medium, which is based on optical-dielectric whispering-gallery-mode microcavities, are presented. Variation of the frequency and Q-factor of the eigenmodes of the microcavity upon its interaction with silver nanoparticles is studied. It is demonstrated that this type of sensor can be used for measurement of infinitesimally low concentrations of nanoparticles.

The results of an investigation of three-dimensional polymer microcavities doped with inorganic luminescent particles are presented. Microlasers in the form of rectangular parallelepipeds were fabricated based on the SU8 2025 photoresist by means of compact UV lithography. Luminescent particles containing erbium oxide were obtained by low-temperature synthesis of the corresponding chlorides in a nonaqueous medium. The obtained spectra confirm the presence of a narrowband laser radiation exhibiting a Stokes shift.

A comparative analysis of the spectral characteristics of the surface of muscular tissue of animal origin (pork) and its main components has been performed by the methods of diffuse reflection electronic spectroscopy (DRES) and frustrated total internal reflection IR spectroscopy. The experiments have shown that the application of the DRES method makes it possible to detect more pronounced changes in the surface optical characteristics of muscular tissue and obtain electronic spectra containing information about the component composition of its main parts under successive extraction of sarcoplasmic materials, myofibrillar proteins of the actomyosin complex, and stroma mucopolysaccharides.