Calculation of conformations and vibrational spectra of a fragment of the 2,6-hydroxyethyl cellulose molecule

We have used the molecular mechanics method for conformational analysis of a fragment of the molecule of a water-soluble cellulose ether: 2,6-hydroxyethyl cellulose (2,6-HEC). As a result of rotation about the C-C and C-O bonds within the bulky side substituents on the C5 and C2 backbone atoms in the fragment of the 2,6-HEC molecule, we have obtained 12 of the most stable conformers for this fragment with strain energies differing within the range 0–2.3 kcal/mol. We have shown that the most stable conformers have the following conformations for the groups of atoms on the bonds C5-C6, C6-O6, C13-O6, C10-C13: gt, g⁻, g⁺, g⁺ and gg, t, g⁺, g⁺ respectively; and on the bonds C2-O2, C11-O2, C7-C11: g⁺g⁻, g⁻, g⁺ respectively. For the conformers obtained, using the same method we calculated the frequencies and the potential energy distribution (PED) for their normal vibrations (NV). Comparative analysis showed high sensitivity of both the frequency and the PED of the normal vibrations to conformational changes within the indicated bulky substituents over the entire analyzed spectral region (800–1500 cm⁻¹ for the methylene groups). We introduce the concept of “conformational characteristicity” of both the frequency and the mode of vibration. We show that it is possible to analyze the conformations of the bulky substituents of the fragment of the 2,6-HEC molecule using its vibrational spectrum. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 42–53, January–February, 2006.     

Subthreshold and near threshold K+ meson photoproduction\newline on nuclei

The inclusive K⁺ meson production in photon–induced reactions in the near threshold and subthreshold energy regimes is analyzed with respect to the one–step (γN→K ⁺ Y, Y=Λ,Σ) incoherent production processes on the basis of an appropriate new folding model, which takes properly into account the struck target nucleon removal energy and internal momentum distribution (nucleon spectral function), extracted from recent quasielastic electron scattering experiments and from many–body calculations with realistic models of the NN interaction. Simple parametrizations for the total and differential cross sections of the K⁺ production in photon–nucleon collisions are presented. Comparison of the model calculations of the K⁺ differential cross sections for the reaction γ+C¹² in the threshold region with the existing experimental data is given, that displays the contributions to the K⁺ production at considered incident energies coming from the use of the single–particle part as well as high momentum and high removal energy part of the nucleon spectral function. Detailed predictions for the K⁺ total and differential cross sections from γH², γC¹² and γPb²⁰⁸ reactions at subthreshold and near threshold energies are provided. The influence of the uncertainties in the elementary K⁺ production cross sections on the K⁺ yield is explored. Received: 12 April 1999 / Revised version: 11 September 1999     

Thermoluminescence of corundum containing anion defects following ultraviolet laser and x irradiation

The thermoluminescence of single crystals of corundum containing anion defects following x-ray and laser excitation is investigated. Its features in the luminescence bands of F, F⁺, and Cr³⁺ centers are studied. Synchronous measurements of the thermoluminescence and thermally stimulated exoelectron emission are performed by the fractional glow technique following x-ray and laser excitation of the samples. It follows from the results obtained that several traps are active in the temperature range of the principal dosimetric peak (400–500 K). The spectral sensitivity curve contains maxima corresponding to absorption bands of F, F⁺, and Al _i ⁺ centers. A possible mechanism for the recombination luminescence of F centers is discussed. It is found that the material exhibits high sensitivity to small doses of ultraviolet laser radiation. Zh. Tekh. Fiz. 67, 72–76 (July 1997)     

Low–energy photodisintegration of 9Be and α+α+n→9Be +γ reactions at astrophysical conditions

A semi–microscopic model for the low–energy photodisintegration of the ⁹Be nucleus is constructed, and the experimental data are analyzed with its help. The older radioactive isotope data are supported by this analysis. The theoretical photodisintegration cross section is derived. The astrophysical rates for the reaction α+α+n→⁹Be+γ and the reverse photodisintegration of ⁹Be are calculated. The new reaction rate for α+α+n→⁹Be+γ is compared with previous estimations. Received: 19 January 1998     

Calibration of the spectral sensitivity of instruments for the near infrared region

We present an analysis of methods for calibration of the spectral sensitivity of instruments in the near IR region of the spectrum (0.90–2.05 μm), using as an example recording of the luminescence spectra of PbS semiconductor quantum dots using a diffraction monochromator and an InGaAs photodiode as the detector. We show that when high-sensitivity detectors are employed for calibration using the emission spectrum of an ideal black body, the problem of attenuation of the radiation flux is still important. Instead of neutral density glass and mesh light filters for attenuation of the radiation, we propose using UFS ultraviolet optical glasses (together with PS purple glasses), the maximum optical density of which is within the region of maximum spectral sensitivity of InGaAs photodiodes. We give examples of spectral calibration, taking into account instrumental characteristics and the effect of absorption by water vapor in the air, and also corrections of the luminescence spectra of quantum dots.     

Spectral interferences from polyatomic barium ions in inductively coupled plasma mass spectrometry

For the Element2 (a relatively new high-sensitivity magnetic sector ICP MS), we have studied spectral interferences from barium arising as a result of polyatomic barium ions, which have a considerable effect on the intensity of lines for rare-earth isotopes. We have shown that the parameters characterizing the interferences (the ratios of the signals from oxygen-containing, chlorine-containing, and fluorine-containing BaX⁺ ions to the Ba⁺ signal, and the apparent rare-earth concentrations C_{app j} resulting when overlap from polyatomic Ba occurs) are higher for the Element2 than for a quadrupole instrument (PQ2⁺). However, it is possible to solve many spectral overlap problems when using the Element2 by using the medium-resolution (MR) or high-resolution (HR) modes. We have estimated the resolutions needed to completely separate the masses of the polyatomic barium ions and the rare-earth isotopes in the spectrum. For cases when it is impossible to use the high-resolution mode due to the ∼100-fold decrease in sensitivity, we recommend preliminary separation of Ba from the rare earths. In this work, we have used extraction chromatography to determine low rare-earth concentration levels in Lake Baikal water. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 813–818, November–December, 2006.     

Assessment of the possibility of determination of the electrophysical characteristics of CdxHg1–xTe p+–n-structures using differential hall measurements

The process of reconstuction of the distribution profile of hole concentration in the p ⁺–n structure by the method of differential Hall measurements upon implantation of ions As⁺ (Е = 190 keV, D = 3.10¹⁴ cm^-2, j = 0.025 μA/cm²) into epitaxial films Cd _x Hg_1–x Te for x ~ 0.2, with the initial electron concentration and mobility n = 10¹⁴ cm^-3 and μ = 2∙10⁵ cm²∙V^–1∙s^–1 is numerically simulated. The dependences of degree of reconstruction of the hole-concentration distribution profile on the depth of a shunting n-layer and magnitude of the magnetic field, at which the electrophysical parameters of the p ⁺–n structure are measured, are calculated. The dependence of the limiting magnetic field determining the magnetic-field range for measurements on the n-layer depth is found. It is shown that in calculations one should use the conduction values measured at the same magnetic fields as the Hall coefficients for determination of the holeconcentration distribution profile using the Petritz model.     

Subthreshold K + meson production in proton–nucleus reactions and nucleon spectral function

The inclusive K ⁺ meson production in proton–nucleus collisions in the near threshold and subthreshold energy regimes is analyzed with respect to the one–step (pN→K ⁺ YN, Y=Λ,Σ) and two–step (pN→NNπ, NN2π; πN→K ⁺ Y) incoherent production processes on the basis of an appropriate new folding model, which takes properly into account the struck target nucleon removal energy and momentum distribution (nucleon spectral function), extracted from recent quasielastic electron scattering experiments and from many–body calculations with realistic models of the NN interaction. Comparison of the model calculations of the K ⁺ total and double differential cross sections for the reaction p+C ¹² with the existing experimental data is given, illustrating both the relative role of the primary and secondary production channels at considered incident energies and those features of the cross sections which are sensitive to the high momentum and high removal energy part of the nucleon spectral function that is governed by nucleon–nucleon short–range and tensor correlations. It is found that the in–medium modifications of the available for pion and kaon production invariant energies squared due to the respective optical potentials are needed to account for considered experimental data. Received: 2 April 1997 / Revised version: 7 August 1997     

Rydberg transitions for positron-hydrogen collisions

e⁺ + H(ns) ↦e⁺ + H(n^′s) transitions for arbitrary n and n^′ have been studied using the distorted-wave formalism in the momentum space [Ghoshal and Mandal, Phys. Rev. A 72, 032714 (2005)]. The distorted-wave scattering amplitudes have been written in a simple closed analytical form. A detailed study has been made on differential and total cross sections in the energy range 20–300 eV. Resonance-like behaviour of the differential cross section has been observed in the the region of lower scattering angles for high Rydberg transitions. To the best of our knowledge the distorted-wave results for differential and total cross sections for such arbitrary transitions are reported for the first time in the literature.     

The absorption spectrum of radiation-colored SrCl2-Ce crystals

We investigate the spectra of the x-ray radiation-induced absorption of SrCl₂−Ce crystals over the spectral range 345–830 nm and their temperature transformations in the interval from 77 to 450 K. We found that radiative color centers are characterized by a complex spectrum of induced absorption that contains wide bands of photochromic PC (750, 519, 378 nm) and PC⁺ (620, 446, 340 nm) centers and quasi-linear bands of Ce²⁺ centers. The most significant thermal transformations of radiative color centers occur in the vicinity of the thermostimulated luminescence peak of 394 K, at which the holes of the PC⁺ centers recombine with the electrons of the Ce²⁺ centers. Ivan Franko L’vov State University, 8, Kirilla i Mefodiya St., L’vov 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 545–547, July–August, 1997.     

Charge transfer mechanism in high-purity silicon-basedn +-π-p + structures

Results of experimental investigations into current-voltage characteristics of n⁺-π-p⁺ structures based on high-purity silicon doped with boron are presented. It is shown that the I–V characteristic at high injection levels is described by Stafeev's theory, when the thickness d of the π-region is smaller than or close to three ambipolar diffusion lengths L_a. For a current density J above (3–50) A/cm² at varying temperatures, the I–V characteristic obeys a relation usually occurring in the case where charge-carrier recombination in the n⁺ and p⁺-regions dominates that found in the π-region. The effect of mutual electron-hole scattering on the behavior of the I–V characteristic is evident at J>630 A/cm³. For d/L_a=9, the I–V characteristic at high injection levels is treated by the theory of double carrier injection into a semiconductor with consideration for diffusion corrections. In the temperature range below 200–276 K, the I–V curve for all samples studied exhibits a linear dependence on J followed by a portion corresponding to a maximum occupancy of recombination levels by injected electrons. Here the Fe or Au donor levels presumably act as recombination levels. The electron and hole capture areas (cross sections) by recombination centers are roughly estimated. Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 35–45, July 2000.     

Electronic and electronic-vibrational phenomena in the new organic conductor ϰ-(ET)2[Hg(SCN)2Cl] with a metal-insulator transition

The reflection spectra and optical conductivity spectra of the new organic conductor ϰ-(ET)₂[Hg(SCN)₂Cl] with a metal-insulator transition in the spectral regions 700–5500 and 9000–40 000 cm⁻¹ have been studied in polarized light at 300 K. A comparisonis made between the spectra obtained and the corresponding spectra of related isostructural conductors based on the ET molecule, and also the properties of the crystal structure of the investigated compounds. An electronic transition between the ET molecules of the dimer (ET) ₂ ⁺ in the spectral region 700–5500 cm⁻¹ has been identified, as have the features of the electronic-vibrational structure arising as a consequence of the interaction of this transition with the completely symmetric intramolecular vibrations of the ET molecule. It is found that the conductor with the stronger dimer interaction between the ET molecules has the higher the transition temperature. Fiz. Tverd. Tela (St. Petersburg) 39, 1313–1319 (August 1997)     

Electric-field-enhanced metal-induced crystallization of hydrogenated amorphous silicon at room temperature

A novel simple method of crystallization of hydrogenated amorphous silicon (a-Si:H) thin films is described. Namely, we studied a metal-induced crystallization enhanced by a dc electric field in sandwich p⁺–i–n⁺structures. The samples were fabricated from wide-bandgap a-Si:H with high hydrogen content (13–51 at. % H). Macroscopic islands of a-Si:H (up to ∼1 mm in diameter) in the region between upper (CrNi) and lower (ITO) contacts crystallize instantaneously when a sufficiently high dc electric field (≳10⁵ V cm^-1) is applied. The crystallization sets in at room temperature and ambient atmosphere and is spatially selective. A proposed microscopic mechanism of such an easy macroscopic crystallization consists in easy diffusion of Ni and/or Ni silicides (representing nucleation sites) through a dense network of voids in hydrogen-rich a-Si:H. Received: 30 November 2000 / Accepted: 3 May 2001 / Published online: 27 June 2001     

Radiation-induced conduction in quantum-confined p +-n silicon junctions

Electron-beam diagnostics are used to study the radiation-induced conduction of supershallow p ⁺-n silicon junctions obtained by nonequilibrium boron diffusion. Current-voltage (I–V) characteristics of radiation-induced conduction of a both forward-and reverse-biased p ⁺-n junction are demonstrated for the first time, which has been made possible by the presence of self-organized transverse quantum wells inside a supershallow p ⁺ diffusion profile. The variation of the dark-current I–V characteristics with electron irradiation dose shows that formation of self-organized longitudinal quantum wells inside supershallow p ⁺ diffusion profiles favors an increase of the breakdown voltage in p ⁺-n silicon junctions. Fiz. Tverd. Tela (St. Petersburg) 41, 1871–1874 (October 1999)     

Pulse characteristics of Hg0.8Cd0.2Te n +-p junctions

The pulse characteristics of Hg_0.8Cd_0.2Te n ⁺-p junctions are investigated. It is shown that the shape of the voltage pulse appearing in a junction on passage of a forward (reverse) current is determined by the recombination (generation) of nonequilibrium electrons in the hole region. An increase in the current pulse causes the appearance of an electric field, which draws electrons into the interior of the base region, and leads to variation of their lifetime because of the complex structure of the n ⁺-p junction. Zh. Tekh. Fiz. 67, 130–133 (July 1997     

Line strength and collisional broadening studies of hydrogen sulphide in the 1.58 μm region using diode laser spectroscopy

High resolution diode laser spectroscopy has been applied to the detection of hydrogen sulphide at ppm levels utilizing different transitions within the region of the ν ₁+ν ₂+ν ₃ and 2ν ₁+ν ₂ combination bands around 1.58 μm. Suitable lines in this spectral region have been identified, and absolute absorption cross sections have been determined through single-pass absorption spectroscopy and confirmed in the Doppler linewidth regime using cavity enhanced absorption spectroscopy (CEAS). The desire for a sensitive system potentially applicable to H₂S sensing at atmospheric pressure has led to an investigation on suitable transitions using wavelength modulation spectroscopy (WMS). The set-up sensitivity has been calculated as 1.73×10⁻⁸ cm⁻¹ s^1/2, and probing the strongest line at 1576.29 nm a minimum detectable concentration of 700 ppb under atmospheric conditions has been achieved. Furthermore, pressure broadening coefficients for a variety of buffer gasses have been measured and correlated to the intermolecular potentials governing the collision process; the H₂S–H₂S dimer well depth is estimated to be 7.06±0.09 kJ mol⁻¹.     

Defect profiles in graded band-gap layers of <Emphasis Type="Italic">P</Emphasis>-HgCdTe heteroepitaxial structures under ion-beam etching

The results of investigations into spatial distribution of donors in p-HgCdTe graded band-gap layers with various composition profiles in the near-surface layer upon ion-beam etching are reported. It is found that the depth of the resulting n⁺-layer is weakly dependent on the conditions of ion-beam etching and composition of material of the surface and is about 0.5–1 μm. The electron density in the n⁺-layer on the surface is observed to increase with time of beam etching. It is shown that the conditions of ion-beam etching and the composition of the near-surface region significantly affect only the depth of the foregoing layer with low electron density. Analysis of experimental data shows that the process of ion-bean etching in p-HgCdTe heteroepitaxial structures with a composition gradient in the near-surface region is different from etching in HgCdTe homogeneous epitaxial structures and crystals. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 51–56, September, 2008.