The role of the negatively charged (Si3−Si−F2)− complexes in interactions of fluorine with the (111) surface of silicon

We used the AM1 quantum chemical and cluster models to study the mechanism of formation of a SiF₂-like layer and dissociation of the Si−Si bond during the interaction of atomic fluorine with the (111) surface of silicon. It is shown that the negatively charged (Si₃−Si−F₂)⁻ complex with the five-coordinated centered silicon atom plays an important part in these processes. The above complex participates in the interaction of atomic fluorine with silicon to form a SiF₂-like layer and break the subsurface Si−Si bonds without penetration of fluorine atoms into the subsurface silicon layers. Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 14–21, January–February, 1996. Translated by I. Izvekova     

Homogeneous decomposition of formaldehyde induced by a carbon dioxide laser

The homogeneous pyrolysis of formaldehyde has been studied under laser heating. At temperatures of 950–1000 K H₂CO decomposition is shown to be a radical chain reaction. Methanol formation in formaldehyde pyrolysis is a homogeneous reaction.

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. , H₂CO 950–1000° . .

     

Ellipsometric study of the interaction of a germanium monocrystal with bromine

Using an ellipsometric technique, the dynamics of formation of adsorption layers in the process of interaction of bromine with a germanium monocrystal surface has been studied. It is shown that the reaction begins only when the second layer of bromine molecules is formed on the germanium surface. The ellipsometric data are in agreement with the results obtained by kinetic experiments.

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Эллипсометрическим методом исследована динамика адсорбционных заполнений во время химической реакции германия с бромом. Сделан вывод о том, что химическая реакция германия с бромом становится возможной только при адсорбции второго слоя молекул брома. Получено хорошее соответствие между результатами кинетических и эллипсометрических экспериментов.

     

“Inversion substitution” reactions with participation of molecular oxygen: ZH<Subscript>3</Subscript>I + O<Subscript>2</Subscript> → O<Subscript>2</Subscript>ZH<Subscript>3</Subscript> + I (Z = C,Si). The potential energy surface and rate constants

A new class of reactions of molecular oxygen O₂ + ZH₃I → O₂ZH₃ + I (Z = C, Si) proceeding by the mechanism of “inversion substitution” was investigated by quantum chemistry methods and the transition state theory (TST). The profiles of the potential energy surfaces (PES) along the reaction coordinate and the characteristics of transition states were calculated using the DFT approach with the B3LYP hybrid functional and the DZVP basis set. The characteristics of the transition states were then used for TST calculations of the rate constants for the direct and reverse “inversion substitution” reactions and their temperature dependences in the temperature interval 273–2000 K. The activation barriers to the substitution reactions under study were found to be substantially lower than the barriers to the abstraction reactions O₂ + ZH₃I → ZH₂I + HO₂ (by 16.3 kcal mol⁻¹ for Z = C and by 7.2 kcal mol⁻¹ for Z = Si). The results obtained show that the “inversion substitution” reactions dominate over the abstraction reactions in the interaction of molecular oxygen with carbon- and silicon-centered iodides as well as (probably) many other substrates. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1803–1807, September, 2008.     

Experimental study of homogeneous nucleation from the bismuth supersaturated vapor: evaluation of the surface tension of critical nucleus

The homogeneous nucleation of bismuth supersaturated vapor is studied in a laminar flow quartz tube nucleation chamber. The concentration, size, and morphology of outcoming aerosol particles are analyzed by a transmission electron microscope (TEM) and an automatic diffusion battery (ADB). The wall deposit morphology is studied by scanning electron microscopy. The rate of wall deposition is measured by the light absorption technique and direct weighting of the wall deposits. The confines of the nucleation region are determined in the "supersaturation cut-off" measurements inserting a metal grid into the nucleation zone and monitoring the outlet aerosol concentration response. Using the above experimental techniques, the nucleation rate, supersaturation, and nucleation temperature are measured. The surface tension of the critical nucleus and the radius of the surface of tension are determined from the measured nucleation parameters. To this aim an analytical formula for the nucleation rate is used, derived from author's previous papers based on the Gibbs formula for the work of formation of critical nucleus and the translation-rotation correction. A more accurate approach is also applied to determine the surface tension of critical drop from the experimentally measured bismuth mass flow, temperature profiles, ADB, and TEM data solving an inverse problem by numerical simulation. The simulation of the vapor to particles conversion is carried out in the framework of the explicit finite difference scheme accounting the nucleation, vapor to particles and vapor to wall deposition, and particle to wall deposition, coagulation. The nucleation rate is determined from simulations to be in the range of 10(9)-10(11) cm(-3) s(-1) for the supersaturation of Bi(2) dimers being 10(17)-10(7) and the nucleation temperature 330-570 K, respectively. The surface tension σ(S) of the bismuth critical nucleus is found to be in the range of 455-487 mN/m for the radius of the surface of tension from 0.36 to 0.48 nm. The function σ(S) changes weakly with the radius of critical nucleus. The value of σ(S) is from 14% to 24% higher than the surface tension of a flat surface.     

Quantum yield and mechanism of singlet oxygen generation via UV photoexcitation of O2-O2 and N2-O2 encounter complexes

The mechanism and spectral dependence of the quantum yield of singlet oxygen O(2)(a (1)Δ(g)) photogenerated by UV radiation in gaseous oxygen at elevated pressure (32-130 bar) have been experimentally investigated within the 238-285 nm spectral region overlapping the range of the Wulf bands in the absorption spectrum of oxygen. The dominant channel of singlet oxygen generation with measured quantum yield up to about 2 is attributed to the one-quantum absorption by the encounter complexes O(2)-O(2). This absorption gives rise to oxygen in the Herzberg III state O(2)(A' (3)Δ(u)), which is assumed to be responsible for singlet oxygen production in the relaxation process O(2)(A' (3)Δ(u), υ) + O(2)(X (3)Σ(g)(-)) → O(2)({a (1)Δ(g)}, {b (1)Σ(g)(+)}) + O(2)({a (1)Δ(g), υ = 0}, {b (1)Σ(g)(+), υ = 0}) with further collisional relaxation of b to a state. This mechanism is deduced from the analysis of the avoiding crossing locations on the potential energy surface of colliding O(2)-O(2) pair. The observed drop of the O(2)(a (1)Δ(g)) yield near spectral threshold for O(2) dissociation is explained by the competition between above relaxation and reaction giving rise to O(3) + O (O + O + O(2)) supposed in literature. The quantum yield of O(2)(a (1)Δ(g)) formation from encounter complex N(2)-O(2) measured at λ = 266 nm was found to be the same as that for O(2)-O(2).     

Anomalous behavior near <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> and synchronization of Andreev reflection in two-dimensional arrays of SNS junctions

The temperature dependences of resistance and the current-voltage characteristics of two-dimensional arrays of superconductor-normal metal-superconductor (SNS) junctions have been measured at low temperatures. It has been found that, in two-dimensional arrays of SNS junctions the following occur: (i) a change in the energy spectrum within an interval of the order of the Thouless energy is observed even when the thermal spread far exceeds the Thouless energy for a single SNS junction; (ii) the manifestation of the subharmonic gap structure with high harmonic numbers is possible even when the energy relaxation length is smaller than that required for the realization of a multiple Andreev reflection in a single SNS junction. These results point to the synchronization of a great number of SNS junctions. A possible mechanism that may be responsible for the features observed in the behavior of two-dimensional arrays of SNS junctions is discussed.     

Ultrasonic degradation of metal complexes with organic substances present in mineralized water,brines, and sodium chloride solutions

The applicability of ultrasound over a frequency range from 18 kHz to 2 MHz to the degradation of metal complexes with organic compounds present in water, brines, and sodium chloride solutions was studied. It was found that ultrasound over a frequency range from 18 to 100 or 200 kHz can be used for the degradation of complexes in sodium chloride solutions and brines or seawater, respectively. In this case, the best results were obtained over the frequency range 80–100 or 80–200 kHz, respectively.     

Ultrasonic Intensification of Sample Preparation for the Spectrophotometric Determination of Arsenic in Foodstuffs

A rapid procedure has been developed for the spectrophotometric determination of arsenic in various foodstuffs. The stimulating effect of ultrasound at the sample preparation stage has been found. The best parameters of ultrasound (frequencies from 22 to 40 kHz, intensity no less than 3.43 W/cm², treatment time 3 min) for the ultrasonic stimulation of digestion of some foodstuffs have been found.