Fluorescence of N,N-di(2-carboxyethyl)-<Emphasis Type="Italic">p</Emphasis>-anisidine in solution and crystalline states

The fluorescence properties of N,N-di(2-carboxyethyl)-p-anisidine (I) in solvents of various nature and in the crystalline state have been studied at room temperature (273 K) and at the boiling point of liquid nitrogen (77 K). Fluorescence in aqueous solutions of I with protonated (λ _ex /λ _fl ^max = 225/290 nm) and unprotonated (λ _ex /λ _fl ^max = 270/380 nm) amino nitrogen has been detected. On going from aqueous solutions to nonaqueous, the fluorescence band of unprotonated I experiences a blue shift and its intensity rises. The fluorescence intensity of the band in aprotic polar solvents is higher than that in protic solvents. A linear dependence of the fluorescence intensity of deprotonated I on Cu(II) concentration (ranging from 1.0 to 5.0 mg/dm³) in aqueous solution has been found. The fluorescence intensity of I in aqueous solutions at 77 K and pH 1–6 has been shown to increase in the presence of Zn(II) (1–170 mg/dm³) and Cd(II) (2–330 mg/dm³) although a similar dependence is not observed at 293 K.     

CARS study of the vibrational kinetics of nitrogen molecules in the burning and afterglow stages of a pulsed discharge

The vibrational kinetics of the nitrogen molecule in the ground state X ¹Σ _g ⁺ in the burning and afterglow stages of a pulsed discharge are investigated by coherent anti-Stokes Raman spectroscopy (CARS). The total cross section for vibrational excitation of the nitrogen molecule by electron impact to the first eight vibrational levels is determined. The rate constant for the associative ionization reaction involving nitrogen atoms in the metastable states ² P and ² D is estimated. It is found that the best agreement between the calculated and measured populations of the nitrogen molecules in the ground state X ¹Σ _g ⁺ in the afterglow stage of a pulsed discharge is obtained when the rate constant for VV exchange K ₀₁ ¹⁰ has the value predicted by the quantum-classical Billing-Fisher model. Zh. Tekh. Fiz. 67, 34–42 (May 1997)     

Theoretical analysis of the impurity distribution in single-crystal silicon

Results are presented for the total energies calculated for oxygen and carbon impurities in silicon at T=0 K. The equilibrium positions of these point defects are determined at low (10⁻³–10⁻² at. %) concentrations. Fiz. Tverd. Tela (St. Petersburg) 39, 1384–1385 (August 1996)     

Formation and stabilization of F centers following direct generation of self-trapped excitons in KCl crystals

The spectrum of luminescent F centers generated in high-purity KCl crystals by 7–10.2-eV photons has been measured at 230 K. The pulsed annealing of these centers (250–550 K), as well as the dependence of the efficiency of stable F-center generation on irradiation temperature (80–500 K) has been studied. The efficiencies of F ⁻ and Cl ₃ ⁻ -center generation are maximum under direct optical creation of self-trapped excitons in the region of the Urbach intrinsicabsorption tail. Besides the exciton decay with formation of F centers and mobile H centers, a high-temperature exciton decay channel which involves creation of cation defects stabilizing the H centers has been revealed. Fiz. Tverd. Tela (St. Petersburg) 41, 433–441 (March 1999)     

Theoretical analysis of impurity distributions in crystalline silicon

Calculations of the total energy of oxygen and carbon impurities in silicon at T=0 K are presented. The equilibrium position of point defects is determined for low (10⁻³–10⁻² at. %) concentrations. Fiz. Tverd. Tela (St. Petersburg) 39, 2001–2002 (November 1997)     

Properties of shock-compressed liquid krypton at pressures of up to 90 GPa

The following quantities of shock-compressed liquid krypton are measured behind a plane shock front at pressures up to 90 GPa: compressibility up to densities of 7 g/cm³, brightness (color) temperatures of 6000–24000 K, and electrical conductivities of 40–60000 (Ω·m)⁻¹. X-t diagram methods are used to estimate sound speeds of up to 5.5 km/s at pressures of 30–75 GPa. The optical absorption coefficients in the violet and red (30–300 cm⁻¹) are measured at pressures of 20–90 GPa from the rise in brightness of the shock front luminosity. The optical reflection coefficient of the shock front (∼13%) at a pressure of 76.1 GPa is measured for the first time. Zh. éksp. Teor. Fiz. 116, 551–562 (August 1999)     

Conductivity of ionic crystals when they are irradiated by picosecond electron beams

This paper discusses the pulsed electron conductivity σ of KCl, KBr, and NaCl crystals when they are excited by an electron beam (0.2 MeV, 50 ps) with current densities in the interval j=(30–10⁴) A/cm². It is shown that the lifetime of the electrons in the conduction band is τ≪100 ps. To explain the experimental σ(j) dependences, a model is proposed that includes electron capture by structural defects and stable radiation defects at low excitation densities and electron capture predominantly by unstable radiation defects generated by the excitation pulse at high excitation densities. Fiz. Tverd. Tela (St. Petersburg) 41, 1200–1203 (July 1999)     

Build-up of F and M color centers in KCl single crystals under combined electron and proton irradiation

The F and M color-center build-up kinetics in KCl crystals under combined irradiation with electrons of energy 15 and 100 keV and 100-keV protons have been studied in the flux range of 10¹³–10¹⁵ cm⁻² and at a flux density of 3×10¹¹ cm⁻² s⁻¹. It is shown that consecutive irradiation with electrons and protons produces results not obtainable under electron or proton irradiation alone. Fiz. Tverd. Tela (St. Petersburg) 40, 2015–2018 (November 1998)     

Measurement of the 45Ca β spectrum in search of deviations from the theoretical shape

The ⁴⁵Ca β spectrum has been measured with a Si(Li) detector. New limits on the mixing parameter (|U _eH |²≤0.005) for a neutrino with mass in the interval 75–100 keV are obtained. The possibility of a monochromatic peak appearing at the end of the β spectrum is analyzed. Fiz. Tverd. Tela (St. Petersburg) 66, No. 2, 81–84 (25 July 1997)     

Creation of excitons and defects in a CsI crystal during pulsed electron irradiation

Data presented on the influence of the temperature in the range 80–650 K on the spectral kinetics of the luminescence and transient absorption of unactivated CsI crystals under irradiation by pulsed electron beams (〈E〉=0.25 MeV, t _1/2=15 ns, j=20 A/cm²). The structure of the short-wavelength part of the transient absorption spectra at T=80–350 K exhibits features, suggesting that the nuclear subsystem of self-trapped excitons (STE’s) transforms repeatedly during their lifetime until their radiative annihilation at T⩾80 K, alternately occupying di-and trihalide ionic configurations. It is established that a temperature-induced increase in the yield of radiation defects, as well as F and H color centers, and quenching of the UV luminescence in CsI occur in the same temperature region (above 350 K) and are characterized by identical thermal activation energies (∼0.22 eV). It is postulated that the STE’s in a CsI crystal can have a trihalide ionic core with either an on-center or off-center configuration; the high-temperature luminescence of CsI crystals is associated with the radiative annihilation of an off-center STE with the structure (I⁻(I⁰I⁻ e ⁻))*. Fiz. Tverd. Tela (St. Petersburg) 40, 640–644 (April 1998)     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

EPR spectrum of donors in 6H SiC in a broad temperature range

An EPR study of donors in 6H SiC crystals with an uncompensated donor concentration (N _D−N _A) of 2×10¹⁸ to 1×10¹⁶ cm⁻³ performed in the temperature range 4.2 to 160 K at frequencies of 9 and 140 GHz showed that 6H n-SiC samples have two donor states in the gap. One of them originates from nitrogen occupying three inequivalent lattice sites with ionization energies of 150 and 80 meV, and the second is connected with a structural defect lying deeper in the gap than nitrogen. The temperature dependences of donor EPR line intensities have been found to deviate from the Curie law. The observed EPR line-intensity peaks of donors are produced in a temperature-driven successive redistribution of donor electrons between the donor levels. The temperature dependences of EPR line intensities obtained from samples with low donor concentrations were used to determine the valley-orbit splitting of nitrogen in cubic sites. Fiz. Tverd. Tela (St. Petersburg) 40, 1824–1828 (October 1998)     

Field evaporation of a Hf-Mo alloy

The field evaporation of a Hf-Mo alloy (15 wt. % Hf) is investigated using a time-of-flight atom probe. A moderately heated tip detects an impurity of Hf and Mo oxides on the surface. Thermofield microprotrusions grown at T=1440–1850 K in an electric field (retarding to electrons) of intensity E=(3.2–5)×10⁷ V/cm are analyzed at room temperature and above. Zh. Tekh. Fiz. 68, 69–73 (March 1998)     

Dielectric relaxation in Bi12SiO20 crystals

Anomalies of the dielectric properties of undoped and aluminum-and gallium-doped crystals of Bi₁₂SiO₂₀ are investigated in the frequency and temperature range ν=10²–10⁸ Hz and T=300–800 K. They are shown to be due to Debye relaxation processes and determined by the relaxor parameters. The mechanism of electron thermal polarization is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1223–1229 (July 1997)     

Topological Berry phase and semiclassical quantization of cyclotron orbits for two dimensional electrons in coupled band models

The semiclassical quantization of cyclotron orbits for two-dimensional Bloch electrons in a coupled two band model with a particle-hole symmetric spectrum is considered. As concrete examples, we study graphene (both mono and bilayer) and boron nitride. The main focus is on wave effects – such as Berry phase and Maslov index – occurring at order (^h/_2p)\hbar in the semiclassical quantization and producing non-trivial shifts in the resulting Landau levels. Specifically, we show that the index shift appearing in the Landau levels is related to a topological part of the Berry phase – which is basically a winding number of the direction of the pseudo-spin 1/2 associated to the coupled bands – acquired by an electron during a cyclotron orbit and not to the complete Berry phase, as commonly stated. As a consequence, the Landau levels of a coupled band insulator are shifted as compared to a usual band insulator. We also study in detail the Berry curvature in the whole Brillouin zone on a specific example (boron nitride) and show that its computation requires care in defining the “k-dependent Hamiltonian” H(k), where k is the Bloch wavevector.     

Study of spatial characteristics of the incomplete surface discharge in atmospheric-pressure air

Spectra of plasma of the incomplete surface discharge in atmospheric-pressure air were measured. Bands of the systems 2⁺, 1⁺, and 1⁻ of nitrogen were identified. It was shown that periodic excitation at the C ³Π _u nitrogen level results in local equilibrium characterized by a vibrational temperature of 2250 K. For the 1⁺ band system of nitrogen, the structure of rotational transitions was resolved; it was found that the distribution in the 1⁺ band system is significantly nonequilibrium due to the A ³Σ _u ⁺ level metastability. Transverse and longitudinal sections of the plasma band were scanned. It was found that the transverse distribution of the emission intensity has a maximum at a distance of 1 mm from the electrode edge followed by an exponential decay. The plasma band width was ∼5 mm. The longitudinal discharge structure consisted of a system of microchannels (with a characteristic diameter of 0.4 mm) with diffuse overlapping. The average channel density was 10–15 cm⁻¹. It was shown that the spatial distribution of incomplete discharge plasma is characterized by high stability against wide-range variations of discharge electrical characteristics. For example, as the excitation voltage varies within 2–6 kV, the plasma band width changed by no more than 1 mm, and the vibrational temperature varied within 10–12%.     

Rotational measurements on (4–6), (5–7) bands of CN (B2Σ+ − X2Σ+) transition

Rotational analysis of (4–6), (5–7) bands of CN (B ² Σ ⁺ −X ² Σ ⁺) transition extending to highJ and emitted from the chemiluminescent reaction involving pink afterglow of nitrogen is presented.     

Relaxation and shift of the precession frequency of the spin of a negative muon in n-type silicon

The residual polarization of negative muons in n-type silicon with impurity density (1.6±0.2) · 10¹³ cm⁻³ is investigated as a function of temperature in the range 10–300 K. The measurements are performed in an external magnetic field of 0.08 T oriented transversely to the spin of the muons. Relaxation of the muon spin and a shift of the precession frequency are observed at temperatures below 30 K. The relaxation rate at 30 K equals 0.25±0.08 μs⁻¹. The shift of the precession frequency at 20 K equals 7 · 10⁻³. Both the relaxation rate and the shift of the precession frequency increase as the temperature decreases. At temperatures below 30 K the relaxation rate is described well by the relation Λ=bT ^−q , where q=2.8±0.2. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 539–543 (10 April 1996)     

Temperature dependence of electric resistance and magnetoresistance of pressed nanocomposites of multilayer nanotubes with the structure of nested cones

Bulk samples of carbon multilayer nanotubes with the structure of nested cones (fishbone structure) suitable for transport measurements, were prepared by compressing under high pressure (∼25 kbar) a nanotube precursor synthesized through thermal decomposition of polyethylene catalyzed by nickel. The structure of the initial nanotube material was studied using high-resolution transmission electron microscopy. In the low-temperature range (4.2–100 K) the electric resistance of the samples changes according to the law ln R ∝ (T ₀/T)^1/3, where T ₀∼7 K. The measured magnetoresistance is quadratic in the magnetic field and linear in the reciprocal temperature. The measurements have been interpreted in terms of two-dimensional variable-range hopping conductivity. It is suggested that the space between the inside and outside walls of nanotubes acts as a two-dimensional conducting medium. Estimates suggest a high value of the density of electron states at the Fermi level of about 5×10²¹ eV⁻¹ cm⁻³. Zh. éksp. Teor. Fiz. 113, 2221–2228 (June 1998)     

Changes in the optical properties of conducting polydiacetylene THD brought on by doping

Changes in the optical properties of conducting polydiacetylene THD (poly-1,1,6,6-tetraphenylhexadiindiamine) brought on by doping are investigated for the first time. Spectral dependences of the extinction coefficients were studied in the range 400–25 000 cm⁻¹ both for the undoped polymer (σ<10⁻⁹ S/cm) and at various doping levels (up to σ∼5×10⁻³ S/cm). The results obtained attest to the appearance of high carrier concentrations in polydiacetylene THD with conductivities σ⩾10⁻⁴ S/cm. The relatively low observed macroscopic conductivity is explained by the complex hierarchy of structural formations that are intrinsic to polymers. The results obtained are compared with the corresponding data for conducting polyacetylene. Fiz. Tverd. Tela (St. Petersburg) 40, 1162–1166 (June 1998)