Optics and Spectroscopy (v.110, #3)

Excitation cross sections at slow collisions of hydrogen and sodium atoms are calculated based on two sets of quantum-chemical data. The results of calculations permit one to conclude that, upon the excitation of the sodium atom from the ground state in the region of near-threshold energies, the cross sections are highly sensitive to matrix nonadiabaticity elements. In addition, the matrix nonadiabaticity element was varied for the transition 3s → 3p of the sodium atom at fixed collision energy near the reaction threshold. It was found that the variation leads to a significant change in the excitation cross section 3s → 3p, and the range of the energetic dependence of this cross section was determined.

4s-1s two-photon decay in hydrogen atom with allowance for cascades by T. A. Zalyalyutdinov; D. A. Solovyev; L. N. Labzovskii (328-334).
The 4s-1s two-quantum decay in the hydrogen atom is calculated in the framework of quantum electrodynamics with allowance for cascades. It is shown that the pure two-photon contribution cannot be separated from the contribution of cascade processes. Interference between the transitions of these two types is comparable in magnitude with the pure two-photon contribution. The separation of pure two-photon contributions is discussed recently in connection with the role of two-photon decays in the separation of radiation from matter in the early Universe.

We discuss the population transfer to the D 0 u + and F 0 u + ungerade ion-pair states of the I2 molecule upon optical excitation of the E0 g + and f 0 g + gerade states by the optical-optical double resonance method. This effect is explained by multiphoton ionization of iodine vapor and subsequent collisions with photoelectrons.

The coefficients γ of broadening by self-pressure, and pressure of nitrogen, oxygen, and air are calculated for absorption lines of the rotational band and for the ν2 band of the ozone molecule for temperatures 296, 252, and 212 K. The calculations are performed by the semiclassical method using rectilinear and exact trajectories for interacting molecules. It is shown that the experimental data obtained for the two bands at T = 296 K can be reconstructed better using different isotropic intermolecular interaction potentials. The experimental and calculated broadening coefficients of ozone absorption lines for the rotational band and for the ν2 and ν1 + ν3 vibrational bands were used to determine the parameters of an analytical model, which permits one to calculate γ in a wide range of rotational quantum numbers, 0 ≤ J ≤ 45, 0 ≤ K a ≤ 20, and temperatures of 200–296 K.

The optical excitation functions (OEFs) for four series of spectral lines of the Zn atom at the transitions 4sn 1 s 1 S 0 → 4s4p 1 P 1 (n 1 = 6–10), 4sn 2 s 3 S 1 → 4s4p 3 P 2 (n 2 = 6−8), 4sn 3 d 1 D 2 → 4s4p 1 P 1 (n 3 = 4−6), 4sn 4 d 3 D 1, 2, 3 → 4s4p 3 P 2 (n 4 = 4−7) excited by a monoenergetic (0.04–0.08 eV) electron beam with energies from the excitation threshold to 18–19 eV are measured. In the energy dependences of the excitation cross sections, the effect of post-collision interaction of slow (scattered) and fast (emitted after the autoionization state decay) electrons is observed and described. This interaction leads to the capture of a slow electron into the listed discrete levels and the appearance of maxima in the excitation functions. The energies of the maxima are used to determine the energies and widths of single autoionization states or their groups by approximate formulas of the classical approach. The autoionization levels of the zinc atom responsible for the maxima observed are determined. Possible transfer of the orbital angular momentum between electrons is analyzed.

Formation mechanisms of equilibrium component composition of molecular layers of polymethine dyes by E. N. Kaliteevskaya; V. P. Krutyakova; T. K. Razumova; A. A. Starovoytov (363-370).
We study the concentration dependences for the absorption spectra and component composition of molecular layers from three homologous series of symmetric polymethine dyes of different electron-donating ability of their terminal heterocyclic groups. We find that a change in the layer thickness leads to a change in the width and position of the spectrum due to a change in the number of absorption bands. The number of bands of monomers increases with increasing chain length and electron-donating ability. The concentration ratio of monomers and associated forms depends on the spatial orientation of molecules in the layer. The electron-donating ability of terminal groups affects the angle between the chromophores of molecules that form a dimer and the intensity ratio between the short- and long-wavelength absorption bands of dimers. We conclude that the effect of the thickness of the layer on its spectral parameters is determined by the degree of intramolecular electron asymmetry that arises as a result of the interaction of chemically symmetric molecules with charges of the substrate surface and upon intermolecular interaction. This asymmetry leads to changes in free energies of ground states of monomeric molecules and, as a consequence, to an increase in equilibrium concentrations of cis-stereoisomeric forms in the layer.

Spectral changes of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy), observed as a function of solute concentration, external quenching by aniline (one of the fragments of the investigated molecule), and temperature, were interpreted in terms of the selective quenching of different EAADCy conformers. It was shown that molecules with the donor and acceptor moieties oriented orthogonally are considerable more self and external quenched than the coplanar conformers. On the other hand, results of performed studies indicate that in the studied system static (mainly) and dynamic processes of quenching take place.

The [Pd(C∧N)(4,4′-bpy)Cl], [Pd(C∧N)NO3]2(μ-4,4′-bpy) and [Pd(C∧N)(μ-4,4′-bpy)]4(NO3)4 complexes (C∧N are deprotonated forms of 2-phenylpyridine (ppy), 2-(2′’-thienyl)pyridine (tpy), and 2-phenylbenzothiazole (bt); bpy is 4,4′ bipyridyl) are synthesized and characterized by 1H NMR spectroscopy, electronic absorption and emission spectroscopy, and cyclic voltammetry. The upfield shifts of the chemical shift of a proton in the ortho-position to the donor carbon atom of the cyclopalladated ligand of complexes (Δδ = −(1.1–1.5) ppm) is assigned to the anisotropic effect of the circular current of the pyridine rings of 4,4′-bipyridyl orthogonal to the coordination plane. The characteristic long-wavelength absorption and phosphorescence bands of the complexes are assigned to the chromophore metal-complex fragment {M(C∧N)}. The quasi-reversible reduction waves of complexes are assigned to the ligand-centered processes of successive electron transfer to the π*-orbitals localized mainly on the coordinated pyridine components of 4,4′-bipyridyl.

Photonics of zinc complexes of 3,3′-bis(dipyrrolylmethenes) by R. T. Kuznetsova; T. N. Kopylova; G. V. Mayer; O. O. Sikorskaya; E. G. Ermolina; G. B. Guseva; L. A. Antina (385-392).
We measure the spectral luminescence characteristics (absorption, fluorescence, lasing, phosphorescence, and luminescence excitation spectra; fluorescence and phototransformation quantum yields; lifetimes of long-lived radiation) and estimate the fluorescence radiation constants and lifetimes for new synthesized binuclear bihelical complexes of zinc(II) with 3,3′-bis(dipyrrolylmethenes) of different structure in liquid and solid solutions. We discuss possible applications of these compounds.

The structure and spectral properties of two organic ruthenium complexes used as sensitizing dyes for solar batteries (well-known N3 dye and its selenophene-conjugated analogue C105 ([Ru(bpy)(bpysef)(COOH)2(NCS)2] (bpy = 2,2′-bipyridine, bpysef = 4,4′-bis(5-hexylselenophene-2-yl)2,2′-bipyridine)) are comparatively studied within the density functional method. It is shown that the conjugation of the bipyridine ligand with selenophene affects the electronic structure of the C105 dye. A multilevel model for interpreting the electronic spectra of dyes is proposed based on the analysis of the shapes of molecular orbitals. The nature of the absorption bands of these ruthenium complexes in the region of 300–800 nm is explained. It is found that, in the polar acetonitrile solvent, these dyes are negatively solvatochromic, which agrees with the current classical views on the effect of the solvent on the shape of electronic absorption spectra of related compounds.

The broadband luminescence of chromium optical centers with strongly overlapping spectral lines and similar emission probabilities from excited 4 T 2 states of red and green Cr3+ centers in stoichiometric magnesium-doped lithium niobate crystals has been separated for the first time. The spectral-luminescence characteristics and parameters of intracenter interaction between red and green optical Cr3+ centers in stoichiometric lithium niobate have been calculated. The luminescence quantum efficiencies of red and green chromium centers are determined.

We study the effect of an external magnetic field on annihilation of triplet excitons of aromatic molecules in Langmuir-Blodgett films. Differences in the magnetic effect in relation to the number of layers, temperature, and recording time of annihilation delayed fluorescence are determined.

Cherenkov-like mechanism of surface waves excitation by V. S. Zuev; A. M. Leontovich; V. V. Lidsky (411-417).
The theory of the excitation of surface waves by a fast charged particle moving though a thin homogeneous metal film surrounded by a dielectric medium is proposed. The Vavilov-Cherenkov effect is shown to occur for surface waves at the particle velocity one or two orders of magnitude lower than the corresponding velocities in a homogeneous medium.

Based on the radiative transfer theory for a simple model of a discrete system containing optically large inclusions, an analytical relationship is established between the absorption and scattering indices and phase function integral parameter of an elementary volume of the medium and the corresponding characteristics of inclusions. It is shown that, in order to retain the form of the Bouguer-Beer-Lambert law, i.e., direct proportionality between the above phenomenological parameters and volumetric concentration of inclusions, one must introduce certain correcting factors into the law. An analytical description of these corrections and their analysis are presented. It is established that the indices of the absorption, scattering, attenuation, and backscattering of an elementary volume of the medium with inclusions are always smaller than similar characteristics of the disperse system with homogeneously distributed absorbing and scattering centers of the same mass. The correcting coefficients for these indices always decrease with increasing optical dimensions of inclusions. The unusual behavior of the forward scattering index of a medium with inclusions, which may increase with their optical dimensions, is quantitatively interpreted. The conditions for this anomaly are found and its cause is shown to be related to the multiple scattering of light within the same inclusion.

Bragg reflection waveguide: Anti-mirror reflection and light slowdown by G. G. Kozlov; V. S. Zapasskii; Yu. V. Kapitonov; V. V. Ovsyankin (425-431).
The effect of the light group velocity reduction in dielectric Bragg reflection waveguide structures (SiO2/TiO2) in the vicinity of the cutoff frequency is studied experimentally. The effect of anti-mirror reflection specific to Bragg reflection waveguides is described and employed to detect slow light. Experiments were performed using Ti:sapphire laser pulses ∼100 fs in length. The group index n g ∼ 30 with a fractional pulse delay (normalized to the pulse width) of ∼10 is demonstrated. The problems and prospects of the implementation of slow-light devices based on Bragg reflection waveguide structures are discussed.

Higher-order modes of photonic-crystal fibers by A. B. Sotsky; O. A. Bel’skaya; L. I. Sotskaya (432-437).
The propagation constants and the field distribution of the higher-order modes in photonic-crystal fibers have been studied. It is shown that the standard single-mode operation condition of a fiber, in which the first higher-order mode is cut-off, is not applicable to photonic-crystal fibers because the latter can guide higher-order eigenmodes, even when this condition is fulfilled.

We report a two dimensional Stokes vector imaging technique for transamination measurements of the polarization state of scattering medium. Measurement of the depth resolved Stokes parameters allows determination of the degree of polarization, birefringence, retardation, optical activity and characterization of the medium. The polarized light preserved and degree of polarization very with scatterer concentration. The transmitted intensity patterns by varying a polarization state of the incident laser light (λ = 632.8 nm) and changing analyzer configuration provides a useful information about concentration, orientation, and shape of the sample under investigation. The results are important for the understanding of polarization phenomenon in turbid media, like biological tissues.

Chalcogenide glasses as a medium for controlling ultrashort IR pulses: Part I by A. I. Konyukhov; E. A. Romanova; V. S. Shiryaev (442-448).
Chalcogenide glasses are ideal materials for developing fiber lasers and amplifiers, remote sensors, high-speed switches, and other devices that operate in the IR range of 1–10 μm. The nonlinear refractive index of chalcogenide glasses may exceed that of quartz glass by a factor of 100–1200 or even more. The data on the dispersion properties of some chalcogenide glass compositions in the IR range are presented. The possibility of forming waveguide structures with specified dispersion properties (in particular with a fixed wavelength at which the group velocity dispersion is zero) from these glasses is numerically investigated. It is shown by the example of completely glassy periodic waveguide structures with planar geometry that the use of photonic band gap modes makes it possible the change the position of zero dispersion in a wide wavelength range. In the calculations the contrast of waveguide structures was varied using parameters of glasses of different composition.

Technique for obtaining and processing spectral information with static fourier spectrometer by K. V. Glagolev; Ig. S. Golyak; Il. S. Golyak; A. A. Esakov; V. N. Kornienko; I. V. Kochikov; A. N. Morozov; S. I. Svetlichnyi; S. E. Tabalin (449-455).
We propose a technique and algorithm for processing interferograms obtained with a static Fourier spectrometer. Spectra of secondary radiation from certain compounds are obtained in the visible and near-UV spectral ranges upon excitation by different radiation sources. We present and discuss results of experimental data processing.

Optical vortices and topological effects in coiled fibers with combined anisotropy by K. N. Alekseyev; B. P. Lapin; M. A. Yavorsky (456-463).
The propagation of optical vortices in coiled anisotropic elliptical fibers is studied. The structure of high-order modes and their propagation constants are determined. The possibility of the generation of TE and TM modes from optical vortices with a unit topological charge in weakly anisotropic coiled fibers is demonstrated. The topological effects that arise upon the propagation of radiation along strongly anisotropic elliptical coiled fibers are investigated. It is established that topological effects are suppressed by anisotropy in the case of the propagation of l = 1 modes; the effects are manifested in these fibers in the form of the rotation of the field intensity distribution by a solid angle spanned by the light trajectory only for certain combinations of l > 1 modes.

The features of the postexposure relaxation of holographic gratings recorded in inhomogeneous polyvinyl acetate latex films with photosensitive agents (photochromic molecules of fulgide dyes and phenanthrenequinone) have been considered. The diffusion coefficients and rms displacements of izomerized probe in polymer latex particles and aqueous environment are determined within the model of two diffusion states. The effective diffusion coefficient of the molecular probe, which is responsible for the relaxation of gratings, increases with an increase in their period in wet films, whereas in dry films, this parameter is independent of the grating period. In the films subjected to high-temperature treatment the effective diffusion coefficient decreases with an increase in the grating period. The successive stages of grating relaxation in latex films with phenanthrenequinone are related to the diffusion of free molecules, radicals, and polymer chains, as well as to the local displacement of macromolecular segments at distances of 5–25 nm.

A method for retrieving the vertical profiles of atmospheric aerosol concentration from the results of lidar sensing at Ng:YAG laser wavelengths is developed based on the found multiple regressions between the optical location characteristics of aerosol at wavelengths of 0.355, 0.532, and 1.064 nm, as well as between the aerosol backscattering coefficient at these wavelengths and the concentration of aerosol particles. The method does not require solving ill-posed inverse problems and minimizes the use of a priori information. The reliability and generality of regressions obtained are confirmed by their good agreement with the AERO-NET data. The method efficiency is demonstrated by numerical experiments on retrieving profiles of back-scattering coefficients and concentration that corresponds to different optical models of aerosol.

Improving image quality in reflection confocal microscopy involving gold nanoparticles and osmotically active immersion liquids by B. A. Veksler; A. Lemelle; I. S. Kozhevnikov; G. G. Akchurin; I. V. Meglinski (483-488).
We consider the possibility of using gold nanoparticles to improve the image contrast of biotissue structures in reflection confocal laser microscopy. We present the results of experimental studies using gold nanospheres with a diameter of 60 nm compared to osmotically active immersion contrast agents based on glycerol.