Optics and Spectroscopy (v.117, #2)

The energy levels of the 3d 94l (l = 1–3) and 3p 53d 104l (l = 0, 1) configurations in the Kr IX ion and the probabilities of radiative transitions to the ground state from these excited states have been calculated within the relativistic perturbation theory using a zero-approximation model potential. The results are shown to be stable within this approximation. The well-known problem of anomalously low accuracy of the calculations of some higher lying singlet levels is considered.

Using the methods of a symmetry group chain, a rigorous algebraic model for describing the energy spectrum of the ammonia dimer (NH3)2 is constructed with the torsional and exchange nonrigid motions taken into account. The model is rigorous in the sense that its correctness is limited only by the correctness of the chosen symmetry of internal dynamics of the dimer.

Using the group chain methods, a rigorous algebraic model for describing the energy spectrum of the ammonia dimer (NH3)2 is constructed with an allowance for both the most important torsional and exchange nonrigid motions and the inversional nonrigid motion also taken into account. The model is rigorous in the sense that its correctness is limited only by the correctness of the chosen symmetry of internal dynamics of the dimer.

A technique for analyzing the extended X-ray absorption fine structure spectra of an atom in different structural states in a material under study is proposed. This technique makes it possible to determine the parameters of the nearest environment of absorbing atoms without applying Fourier filtering and related techniques. The proposed approach is tested by an example of the L3 absorption spectra of platinum, obtained by direct calculation for models of one-component platinum nanoparticles and bimetallic Pt-Ag nanoparticles with different core and shell structures. The error in determining the structural parameters is analyzed, and the range of applicability of the technique proposed is discussed.

Quantum-chemical methods are used to calculate electronic-absorption spectra, dipole moments, and atomic-charge distributions of aromatic amino acids in aqueous solutions at various concentrations. The electronic-absorption spectra are analyzed and compared with the spectra that are calculated for gas phase. The bathochromic shift of the spectral position is shown for the long-wavelength absorption peak of the singlet-singlet transition for aqueous solutions of amino acids.

By using the combination of density functional theory (DFT) B3LYP/6-311++G(d,g) within polarized continuum (PCM) model, solvent effects on 3-hydroxy-2-quinoxalinecarboxylic acid (3HQC) tautomers were investigated. Geometrical parameters, vibrational frequencies, electronic and energetic properties, and the nucleus-independent chemical shifts (NICS) were calculated in benzene, diethyl ether, and water. It has been seen that these molecular features of 3HQC are mostly solvent dependent.

The dependence of the luminescence quantum yield of linear polymer molecules that contain chromophores at the ends on the number of units of polymer chain is studied. Numerical calculations for the dipole-dipole energy transfer between chromophores show that an increase in the chain length by a factor of 2 leads to a several-fold increase in the luminescence quantum yield depending on the ratio of the Förster radius to the radius of polymer coil. An approximate formula is derived for the adequate analysis of the dependence at the Förster radius that is significantly less than the radius of polymer coil. The dependence of the luminescence quantum yield of linear polymer molecules on the length of the statistical segment (unit) at a constant total length of the chain is also studied.

We have studied the absorption and fluorescence spectra of Malachite Green and Crystal Violet in aqueous and alcoholic-aqueous solutions in which nanoparticles from Ln(III) and Sc(III) diketonates are formed at concentrations of complexes in a solution of 5–30 μM. We have shown that, if the concentrations of the dyes in the solution are lower than 0.5 μM, dye molecules are incorporated completely into nanoparticles or are precipitated onto their surface. The fluorescence intensity of these incorporated and adsorbed Malachite Green and Crystal Violet molecules increases by several orders of magnitude compared to the solution, which takes place because of a sharp increase in the fluorescence quantum yields of these dyes and at the expense of the sensitization of their fluorescence upon energy transfer from β-diketonate complexes entering into the composition of nanoparticles. We have shown that, if there is no concentration quenching, the values of the fluorescence quantum yield of the Crystal Violet dye incorporated into nanoparticles and adsorbed on their surface vary from 0.06 to 0.13, i.e., are close to the fluorescence quantum yield of this dye in solid solutions of sucrose acetate at room temperature. The independence of the fluorescence quantum yield of Crystal Violet on the morphology of nanoparticles testifies to a high binding constant of complexes and the dye. The considerable fluorescence quantum yields of triphenylmethane dyes in nanoparticles and sensitization of their fluorescence by nanoparticle-forming complexes make it possible to determine the concentration of these dyes in aqueous solutions by the luminescent method in the range of up to 1 nM.

In the density functional theory approximation, we have calculated the phonon dispersions and the densities of vibrational states of the 2H, 3C, 4H, 6H, and 8H hexagonal polytypes of diamond. We have found that a one-dimensional incommensurate modulation of the structure arises along the hexagonal axis, the parameter of which not only exceeds the parameters of basic translations of the crystal lattices of polytypes, but also is not a multiple of them. Based on the estimation of interactions between bilayers of carbon atoms in the structure of polytypes, we have assumed that competing interactions between bilayers are the main mechanism by which incommensurability arises. We have shown that optical measurements of vibrational frequencies in the center of the Brillouin zone of polytypes make it possible to retrieve the dispersion of acoustic branches of cubic diamond.

Reflective photoluminescence fiber temperature probe based on the CdSe/ZnS quantum dot thin film by Helin Wang; Aijun Yang; Zhongshi Chen; Yan Geng (235-239).
A reflective fiber temperature sensor based on the optical temperature dependent characteristics of a quantum dots (QDs) thin film is developed by depositing the CdSe/ZnS core/shell quantum dots on the SiO2 glass substrates. As the temperature is changed from 30 to 200°C, the peak wavelengths of PL spectra from the sensing head increase linearly with the temperature, while the peak intensity and the full width at half maximum (FWHM) of PL spectra vary exponentially according to the specific physical law. Using the obtained temperature-dependent peak-wavelength shift, the average resolution of the designed fiber temperature sensor can reach 0.12 nm/°C, while it reaches 0.056 nm/°C according to the FWHM of PL spectrum.

Conformational analysis of 5,7-Dimethoxycoumarin was performed and two stable conformers were obtained. The difference between the total energies of these conformers was 1.4698 kcal/mol and the difference between the zero point corrected energies was nearly zero. Vibrational frequencies of these conformers were calculated by B3LYP method using 6-311++G(d, p) basis sets and compared with experimentally recorded FT-IR and Raman spectra. Vibrational assignments were made by calculated total energy distributions. Time dependent density functional theory calculations were done by the same level of theory in order to investigate low-lying excited state and obtained results were compared with the maximum absorbtion peak value of experimental UV-visible spectrum.

Simple colorimetric detection of doxycycline and oxytetracycline using unmodified gold nanoparticles by Jie Li; Shumin Fan; Zhigang Li; Yuanzhe Xie; Rui Wang; Baoyu Ge; Jing Wu; Ruiyong Wang (250-255).
The interaction between tetracycline antibiotics and gold nanoparticles was studied. With citrate-coated gold nanoparticles as colorimetric probe, a simple and rapid detection method for doxycycline and oxytetracycline has been developed. This method relies on the distance-dependent optical properties of gold nanoparticles. In weakly acidic buffer medium, doxycycline and oxytetracycline could rapidly induce the aggregation of gold nanoparticles, resulting in red-to-blue (or purple) colour change. The experimental parameters were optimized with regard to pH, the concentration of the gold nanoparticles and the reaction time. Under optimal experimental conditions, the linear range of the colorimetric sensor for doxycycline/oxytetracycline was 0.06–0.66 and 0.59–8.85 μg mL−1, respectively. The corresponding limit of detection for doxycycline and oxytetracycline was 0.0086 and 0.0838 μg mL−1, respectively. This assay was sensitive, selective, simple and readily used to detect tetracycline antibiotics in food products.

Modulation instability of homogeneous resonant excitation regimes of molecular J aggregates by N. A. Veretenov; L. A. Nesterov; N. N. Rosanov; S. V. Fedorov (256-263).
We have performed a linear analysis of the stability of stationary states of homogeneous resonant excitation regimes of molecular J aggregates with coherent sustaining radiation. In the analysis, we have taken into account not only two-particle but also three-particle interactions of molecules, which are related to the exciton-exciton annihilation mechanism. Our detailed investigation of the stability of stationary states has allowed us to determine stability and instability regions and correlate them with the existence of bistability boundaries.

Generation of terahertz surface polaritons in homogeneous round cross-section plasma waveguides upon nonlinear optical rectification of femtosecond laser pulses is analyzed theoretically. It is assumed that nonlinear polarization inducing a surface electromagnetic wave is formed at the waveguide boundary in a thin layer of the nonlinear dielectric that surrounds the waveguide. The efficiency of the femtosecond radiation conversion into surface polaritons is studied as a function of the waveguide radius and duration of the exciting laser pulse.

We have studied the locking effect of photon-echo responses in a three-level system and the information reproducibility upon coding the information in the temporal shape of the object laser pulse. We have shown that these effects differ from their analogs in the two-level system.

Dynamics of ultrathin laser targets with optimal parameters by A. A. Andreev; K. Yu. Platonov; V. I. Chestnov; A. E. Petrov (276-286).
The set of equations describing the motion of a thin (compared to the wavelength) target in the field of a laser pulse that takes into consideration separate motion of the electron and ion layers is derived. In the case of strong Coulomb coupling between the layers, the set of equation of motions of the layers is reduced to the well-known light-sail equation containing a self-consistent coefficient of nonlinear reflection of laser radiation by a moving target. The optimal thickness of the laser target at which the target acquires maximum energy for given laser-pulse parameters is determined. It is shown that this thickness depends not only on laser intensity, but also on laser-pulse duration and the ratio of electron and ion masses. The growth rates of transverse instability of optimal targets under their intense acceleration are analyzed. It is demonstrated that instability does not develop in the currently experimentally accessible range of laser intensities and pulse durations between 100 and 200 fs.

Generation and propagation of fast electrons in laser targets consisting of thin nanofilaments are studied numerically and analytically. Such targets completely absorb laser radiation and exhibit a large coefficient of laser-energy conversion to kinetic energy of a flow of fast electrons. Analytical estimates show that the optimal thickness of the filament is on the order of the skin depth of the laser plasma, while an optimal distance between filaments is on the order of the Debye radius of hot electrons. A bunch of relativistic electrons can propagate as far as several hundred micrometers in such targets, while the fastest electrons can propagate several millimeters. Upon bending of filaments, the flow of electrons propagates along the filaments and can be focused by bringing the filaments together. Laser targets of the discussed composition are used as sources of dense bunches of relativistic electrons and subsequent generation of high-intensity X-ray radiation with their help.

Applicability of the Rayleigh-Gans-Debye (RGD) approximation for describing light scattering by nanoparticles with large dielectric losses (such as carbon nanotubes) is analyzed. By a comparison of the approximate results with exact ones, it is shown that the presence of dielectric losses expands the range of applicability of the RGD approximation. This conclusion is illustrated by a differential cross-section diagram of scattering by a multiwall carbon nanotube.

A non-Gaussian model for estimating the radial velocity of turbulent flows in the atmosphere for coherent detection of scattered optical radiation is proposed. The model was obtained based on a theoretical approach that includes results of the statistical analysis of a pulse Doppler lidar signal in a turbulent medium, as well as on the perturbation-theory methods that have been developed in the theory of probability and mathematical statistics. It is shown that the estimate of the Doppler shift in the first-order perturbation theory is a sum of a regular component and two conditional fluctuation components—Gaussian and non-Gaussian ones. In the case of a homogeneous and isotropic turbulence, the estimate of the radial wind velocity is approximately equal to its true average value. The statistical uncertainty in measurements of the average radial wind velocity is determined by the behavior of conditional Gaussian and non-Gaussian components and significantly depends on the state of atmospheric turbulence. It is shown that basic equations of the non-Gaussian model in the limit case coincide with formulas of the local and nonlocal models, as well as with those of the Gaussian model.

Photorefractive light scattering in lithium niobate crystals doped with Mg2+, B3+, Y3+, and Ta5+ by N. V. Sidorov; M. N. Palatnikov; A. V. Syuy; E. A. Antonycheva; A. A. Yanichev; A. A. Gabain; A. A. Kruk (315-319).
We have investigated the photorefractive (photoinduced) light scattering in lithium niobate single crystals: LiNbO3, LiNbO3:B, LiNbO3:Y(0.46 mas %), LiNbO3:Y(0.24):Mg(0.63 mas %), and LiNbO3:Ta(1.13):Mg(0.0109 mas %) that were grown from congruent melts. We have found that the shape of the speckle structure of this scattering and the kinetics of the development of its indicatrix depend substantially on the type of the impurity dopant in the lithium niobate crystal. We have observed that, upon laser irradiation of crystals doped with Y3+, Ta5+:Mg2+, and Y3+:Mg2+, the shape of their scattering indicatrix changes with time. At the same time, the LiNbO3:B crystal is characterized by a complete absence of time changes in its speckle structure, which indicates that the photorefractive effect in this crystal is substantially lowered.

We review the studies of nanoripples formation on various semiconductor species using ultrashort pulses and different experimental conditions. We present recent studies in this field and discuss the fabrication of 2D periodic nanostructures formed on the surface of semiconductor crystals applying a method of two-beam interference of femtosecond laser. Further, we discuss the short-period ripples formation and present the studies of the semiconductors with different bandgaps potentially suitable for generation of sub wavelength nanoripples. We also show the method of formation of the nanoholes and nanodots on the surface of different semiconductors. Ripples formation using different laser parameters and ambient media is analyzed with the objective of identifying conditions of forming short-period nanoripples. Finally, we discuss the formation of extended homogeneous laser-induced periodic surface structures using few-cycle laser pulses.