Optics and Spectroscopy (v.95, #4)

An approach to calculating the exitation cross sections and rate coefficients for a wide class of transitions in atoms and ions induced by collisions with heavy charged particles in considered. Calculations were carried out in terms of the Born approximation in the impact parameter representation with the normalization and Coulomb repulsion (in the case of an ionic target). Transitions between levels in He and He-like ions without changes of the principal quantum number are considered for n=2–4. Fitting formulas are suggested for transition rate coefficients. Tables of cross sections and parameters of fitting formulas are given for He and C4+.

We spectroscopically studied the population of the excited hydrogen atomic states with the principal quantum numbers n=3 and 4 in a decaying plasma produced by a pulsed discharge in a mixture of helium (p=40.4 Torr) with a small amount of hydrogen ([H2]≈1012 cm−3). Experiments on recording the response of the spectral line intensities to a short-duration electron temperature perturbation revealed the contribution of electron-ion recombination to the population of the H*(n=3) states in the early afterglow. The ions produced by collisions of hydrogen molecules with metastable He(23 S 1) atoms, whose density decreases relatively rapidly with time in the decaying plasma, were assumed to be involved in this process. No population of the H*(n=4) atomic levels due to electron-ion recombination was found. Our experimental results are consistent with the conclusions of previous studies that excitation transfer during collisions of metastable helium molecules with hydrogen molecules plays a major role in the population of the excited hydrogen atomic states both with n=3 and with n=4 during most of the afterglow.

The processes of population of excited levels of a krypton atomic ion via electron-stimulated desorption are studied. The radiation yield per single exciting electron in the corresponding krypton spectral lines is measured by the optical time-of-flight mass spectrometric method. The efficiency of the processes studied is compared with that of electron-impact excitation of the same levels.

Diffusion ionization of the Rydberg diatomic quasi-molecular complex formed upon collisions of rubidium atoms by N. N. Bezuglov; V. M. Borodin; V. Grushevskii; A. N. Klyucharev; K. Michulis; F. Fuzo; M. Allegrini (515-524).
The problem of impact ionization of two Rb atoms, one of which belongs to the Rydberg P series and the other of which is in the ground state, is studied and solved with the use of the methods of stochastic dynamics for description of the path instability of a Rydberg electron in a collision diatomic complex. The actual potential of the atomic framework was taken into account in terms of the single-channel quantum-defect method, whose accuracy was demonstrated on the example of determination of the photoionization cross sections in the quasi-classical approximation. A representation of the effective average lifetime of an external electron in the bound state that takes into account both the diffusion migration within the Coulomb crowding of the potential energy curves for the quasi-molecular complex and the direct quantum autoionization is obtained in analytical form. It is shown that the rate constants of diffusion collisional ionization calculated by us are in good agreement with the experimental data.

For the elements W, Re, Os, Ir, and Pt, we experimentally studied the relative intensities of the Lβ2 and Lβ3 lines, I2)/I3), in the x-ray emission L spectrum, as well as the relative intensities of the multiple-ionization satellites of the Lβ2 line, I(β2s )/I(β2+s ). Based on the data obtained, we determined the partial L 1 level width related to the Coster-Kronig (CK) L 1L 3 M 5 transition (Γ13M ). We found that the width Γ13M in the series W, Re, Os, Ir, and Pt monotonically increases, remaining (with the exception of Ir) a factor of 2.3–1.3 smaller than that predicted by a calculation performed in the approximation of “frozen” orbitals. We discuss the possible causes of this discrepancy. The derived widths Γ13M were used to improve the published total L 1 level widths Γ(L 1), CK yields f 12 and f 13, and fluorescence yields ω1.

Precision measurements of optical excitation functions for the mercury atom by N. M. Érdevdi; O. B. Shpenik; V. S. Vukstich (529-536).
We describe a computer-based facility for studying the excitation of atoms by ultramonochromatic electrons and give optical excitation functions for the 12 mercury spectral lines that originate from the n 1 S 0, n 1 P 1, n 1 D 2, n 3 S 1, n 3 P j , and n 3 D j levels. We detected about 100 features in the energy dependences measured from the excitation threshold to 15.5 eV. The previously found positions of the features on the energy scale are in good agreement with our results. Most of the resonant features are shown to be mainly attributable to the decay of short-lived states of the negative mercury ion. We detected a postcollision interaction effect in the optical excitation functions of the lines that originate from the n 1 S 0 levels at energies of about 11 eV.

Radiative recombination of an electron with multiply charged uranium ions by M. B. Trzhaskovskaya; V. K. Nikulin (537-545).
Precise relativistic calculations of the total and differential cross sections of radiative recombination of an electron with hydrogen-, helium-, and lithium-like uranium ions with energies in the range of 1–1000 MeV/amu1 for the cases of capture of an electron to the K and L shells are performed in terms of the Dirac-Fock method. The influence of electron-electron interactions, including the magnetic Breit interaction, on this process is studied, as well as the effect of the basic quantum electrodynamic corrections. The discrepancy between the results obtained and data on the total and differential cross sections calculated for the case of recombination of an electron in the Coulomb field of a bare nucleus, which were reported in most previous publications and used to interpret the results of experiments, is considered. It is found that, for the uranium ions under consideration, the contribution of electron-electron interaction and other corrections may be as high as 11%. This contribution is larger for similar ions of lighter elements; it also increases with decreasing degree of ionization of the uranium ions. The calculated angular distribution of unpolarized photons emitted in the case of capture of electrons to the K and L shells of the U91+, U90+, and U89+ ions is in good agreement with the experimental data.

On the basis of symmetry methods, a rigorous model is, for the first time, obtained for describing the spectrum of the ethanol molecule CH3CH2OH in a given vibronic state with allowance for the simultaneous torsional motion of the hydroxyl group OH and the methyl group CH3.

Spin-orbit coupling in oxygen near the dissociation limit by B. F. Minaev; L. B. Yashchuk (553-559).
By using the method of configuration interactions in the valence basis with triple exponentials, the spectrum of the oxygen molecule is calculated in a range of internuclear distances from 1.2 to 2.45 Å, with the matrix of configuration interactions being diagonalized with regard to the spin-orbit coupling. The matrix elements of the spin-orbit coupling are presented, along with the spin splittings of triplet and quintet states. Particular attention is given to the intermediate range for the breaking of the O=O bond (1.8–2.45 Å), where the strong mixing of multiplets and rearrangement of the valence bonding to the atomic limit O(3 P)+O(3 P) occur. Other dissociation limits up to O(1 D)+O(1 D) are also taken into account. The results obtained are discussed in the context of the theory of the chemical bond, catalysis, radiation collisions, and the theory of spectral bands for high vibrational quantum numbers.

Monte Carlo simulations of chain conformations and the diffusion equation were used to analyze the fluorescence kinetics of short polymer chains labeled with a probe and a quencher at opposite ends. In simulations, three chain models were considered: an ideal chain (without volume interactions); a self-avoiding chain taking into account the exclusive volume effect; and a self-avoiding chain with limited flexibility between nearest segments. For each model, end-to-end distance distribution functions were obtained, which were different from Gaussian ones. The distribution functions were used in a diffusion equation to simulate the fluorescence kinetics of the probe affected by intramolecular end-to-end collisions of short chains. The kinetics has been numerically calculated for a representative experimental system in a nonviscous solution. The simulated time-resolved fluorescence decays were monoexponential except at very short times (<2 ns). Diffusion coefficients were calculated for different chain models and different chain lengths. The experimental data could be reproduced by assuming systematically smaller end-to-end diffusion coefficients for the shorter chains.

For the first time, direct experimental evidence of a new mechanism for the quenching of fluorescence of organic ligands (L) in complex compounds with lanthanide ions (Ln3+) is obtained. By analogy with the mechanism of luminescence quenching upon pair interactions of Ln3+ ions in inorganic systems, this mechanism is called the cross-relaxation mechanism. The experiments are performed with complexes of Tb3+ with dianions of halogen-substituted fluoresceins (HSFs): 4,5-dibromo-and 4,5-diiodofluorescein, eosin B, eosin, erythrosin, and Rose Bengal in dimethyl sulfoxide. In accordance with this mechanism—exchange energy transfer, L2−(*S 1), Tb3+(7 F 6)→L2−(T 1), Tb3+(7 F 5, 4), allowed by the spin selection rules—an increase in the quantum yield of formation of the triplet state (ΦT) of a ligand L2− and a decrease in the quantum yield of fluorescence (Φfl) are found to take place upon complexation. The efficiency of this process amounts to ∼1 in accordance with the equality ΦflT=1, valid for solutions of HSFs. The possibility of other processes leading to a similar effect, specifically, recharging of the system (as for complexes of HSFs with Eu3+ and 3+) is considered. An example of inductive resonance interactions in complexes of HSFs with Pr3+ is given. The manifestation of equilibrium between outer-sphere and inner-sphere complexes in the photophysics of complexes of metals with HSFs is discussed.

The spectral and luminescent properties of Pt(II) and Pd(II) complexes with heterocyclic imine ligands—1-phenylpyrazolate, 2-phenylpyridinate, and 2,2′-bipyridyl—are studied. It is found that the field strength of these ligands satisfies the following relation: Ppy>Bipy≈Ppz.

An icelike state of water supercooled to −1°C was found using Raman spectra excited by second harmonic pulses from a Nd:YAG laser. The spectrum of the OH stretching vibrations of this state was found to be similar to the spectrum of hexagonal (Ih) ice. The icelike state is destroyed by laser pulses of a moderate intensity (1 MW/cm2) with a repetition frequency of 1 Hz, and the water returns to its initial state, with no icelike clusters, within five minutes through an aperiodic process of variation in the concentration of hydrogen bonds without changing its macroscopic temperature.

Mechanisms of femtosecond energy transfer upon strong excitation in crystals by V. I. Baryshnikov; T. A. Kolesnikova (594-598).
It is shown that the fast temperature-independent shift of the long-wavelength edge of fundamental absorption observed upon electron bombardment of alkali halide, alkaline-earth, and oxygen-containing crystals is caused by the impact mechanism. In the course of the impact shift of lattice ions, short-lived perturbation (mainly, of their valence shells) occurs. The perturbation fields in the vicinity of impact-shifted anions act for no more than 100 fs. As a result, the distribution of the density of states in the valence band of a crystal is distorted, and the long-wavelength shift of the fundamental-absorption edge may increase for less than 100 fs.

Optical activity of γ1-(GaxIn1−x )2Se3 crystals by I. P. Studenyak; M. Kran’chets; L. M. Suslikov (599-602).
The optical activity of uniaxial γ1-(GaxIn1−x )2Se3 crystals (x=0.1, 0.2, 0.3, 0.4) is studied at T=295 and 77 K in the spectral range 0.5–0.8 πm. It is found that the cationic substitution In → Ga leads to a nonlinear increase in the specific rotation of the plane of polarization ρ and the component g 33 of the gyration tensor. It is shown that the gyrotropy of the crystals studied is determined by high-energy transitions whose energy exceeds the energy of the edge transitions and that the gyrotropy observed has a molecular origin.

Numerical simulation of random inhomogeneities in a strongly anisotropic single-mode optical fiber (SMOF) forming a loop in a fiber ring interferometer (FRI) designed according to the minimal scheme is used to obtain the dependences of an interference signal at the FRI output and the zero shift and drift of an interference pattern of counterpropagating waves on the angles of orientation of the axes of linear birefringence in an SMOF at both inputs of the FRI loop. It is shown that the use of a superluminescent diode as a radiation source makes it possible to increase the sensitivity of fiber-optic gyroscopes fabricated on the basis of FRIs by 1–2 orders of magnitude and to obtain the values of real sensitivity required for navigation purposes.

Specific features of the propagation of light pulses with frequency lying in the range of autoionization resonances of the medium are analyzed. It is shown that the interference nature of the autoionization spectra makes it possible to combine a strong frequency dispersion of the refractive index with small absorption when the radiation frequency approaches the spectral range of the "transparency window" of an isolated resonance or of a series of overlapped autoionization resonances. This allows one to control the time delay of the pulses and their group velocity. Depending on the parameters of the medium, the pulse group velocity may be maximally reduced by a factor of 104–105.

An approximate operator approach that is equivalent to the mathematical Wentzel-Kramers-Brillouin method and allows one to solve the set of constitutive equations for an N-level quantum medium resonantly interacting with multifrequency optical pulses is proposed. The transitions occurring via a common level with the ordinal number j are considered in detail. It is shown that, for j=1, the self-induced transparency regime is realized, whereas, at j=N, the regime of electromagnetically induced transparency is realized. Within the framework of the WKB approximation, an expression that represents the area theorem for multifrequency resonant pulses is derived.

On accounting for the effect of particles of a condensed dispersed phase on radiant energy transfer in gaseous media by N. N. Bezuglov; A. K. Kazanskii; A. N. Klyucharev; K. Michulis; F. Fuzo; M. Allegrini (631-637).
Questions concerning the formation of the optical properties of dense gaseous and plasma media in relation to the specific features of radiant energy transfer are considered. The integral equations describing the radiation trapping are investigated as a new class of generalized wave equations of Schrödinger type. Starting from the methods of quantum mechanics, original analytical and numerical approaches are suggested for solving problems of the radiative kinetics of both spatially homogeneous and inhomogeneous absorbing media containing dispersed particles. In terms of the quasi-classical approximation, two classes of reference problems for determination of phase factors are formulated. Solutions for a number of model problems are presented that demonstrate the efficiency of the methods developed.

Free polarization decay in media with a short-range dipole-dipole interatomic interaction by O. Kh. Khasanov; G. A. Rusetskii; A. A. Afanas’ev; R. A. Vlasov; T. V. Smirnova; O. M. Fedotova (638-644).
The effect that may be exerted by an interatomic dipole-dipole interaction upon optical transient processes in dense resonance media, in particular, free polarization decay, is analyzed. The behavior of the macroscopic polarization after a single-pulse excitation of a dense ensemble of two-level atoms is considered. It is shown that the free polarization signal is of oscillatory nature, with the oscillation frequency varying in time and being dependent on the dipole-dipole interaction constant, the intensity and duration of the exciting pulse, and the detuning of its carrier frequency from the resonance. The free polarization signal decay, which depends on the magnitude and sign of the sum of the detuning of the exciting pulse carrier frequency from the resonance and the Lorentz frequency, may obey either a power or an exponential law. The signal decay rate is determined not only by the inhomogeneous broadening, but also by the ratio of the above parameters.

Speckle-pattern rotation in a few-mode optical fiber in a longitudinal magnetic field by L. I. Ardasheva; N. D. Kundikova; M. O. Sadykova; N. R. Sadykov; V. E. Chernyakov (645-651).
The process of rotation of a speckle pattern at the output of a few-mode optical fiber in a longitudinal magnetic field is mathematically modeled. It is shown that the deviation from a linear relation between the rotation angle of a speckle pattern at the output of a few-mode fiber and the fiber length in the presence of a longitudinal magnetic field results from a specific dependence of polarization corrections to the propagation constants of TE and TM modes. The calculation results are compared with the results of an experiment on determining the rotation angle of a speckle pattern as a function of magnetic field in a fiber of constant length.

Application of correlation speckle photography for measuring contact pressures by A. V. Osintsev; Yu. P. Presnyakov; V. P. Shchepinov (652-657).
A method for measuring contact pressures based on the measurement of irreversible changes in a surface microrelief by the speckle photography technique is presented. The analytic dependence between the change in the contrast of the carrying Young’s fringes and the relative area of change in the microrelief is considered. An example of a contact interaction between a cylinder end and a flat slab, the latter with a specially produced regular roughness on its contact surface, is used to experimentally verify the relation between the irreversible change in the roughness, the change in the contrast of the holographic interference fringes, and the change in the contrast of the carrying Young’s fringes. It is shown that the sensitivity of correlation speckle photography is higher than the sensitivity of the correlation holographic interferometry technique.

An optical method of detection of rheumatoid factor in blood serum is demonstrated. The method employs the diffraction of light from a grating on a polystyrene plate formed as a result of biochemical reaction between rheumatoid factor and stained antibodies.