Role of secondary electrons and metastable atoms in the electron-beam activation of argon-silane mixtures

The energy and spatial degradation of the primary beam electrons and the production of high-energy secondary electrons in ionizing collisions are analyzed by solving the Boltzmann integral equation for the electron distribution function. The effect of the primary and secondary electrons on the direct ionization of an Ar-SiH₄ mixture, the production of metastable argon atoms, and the dissociation of monosilane molecules is investigated over a wide range of the beam electron energies, argon pressures, and monosilane concentrations. The influence of metastable Ar* atoms on the dissociation of SiH₄ is studied by using the balance equation for metastable argon atoms and the equation for the ambipolar diffusion of ions and low-energy secondary (plasma) electrons in the beam plasma. It is shown that the main contribution to the activation of an Ar-SiH₄ mixture in an electron-beam plasma is provided by secondary electrons with energies higher than the excitation threshold for argon and the dissociation threshold for monosilane, whereas the contribution from metastable argon atoms, though potentially being comparable with that from secondary electrons, is less than in gas-discharge plasmas.     

Propagation of a surface microwave along the afterglow plasma column of a high-current pulsed discharge

It is demonstrated experimentally that the lifetime of the afterglow plasma of a high-current pulsed discharge in a dielectric tube filled with a mixture of argon with saturated mercury vapor is longer than 1 ms. Such a long lifetime, during which the electron density decreases from 10¹⁴ to 10¹² cm⁻³, is explained by the chemi-ionization of mercury vapor by long-lived metastable argon atoms. During this time, the afterglow plasma can serve as a microwave waveguide for a weakly damped low-noise E ₀-type axisymmetric surface mode, which allows one to use it for transmission of signals in the centimeter wavelength range.     

Influence of the chlorine concentration on the radiation efficiency of a XeCl exciplex lamp

The influence of the chlorine concentration on the radiation efficiency of coaxial exciplex lamps (excilamps) excited by a dielectric barrier discharge (DBD) in binary Xe-Cl₂ mixtures at pressures of 240–250 Torr is investigated experimentally and theoretically. The experiments were carried out at Cl₂ concentrations in the range of 0.01–1%. The DBD characteristics were calculated in the framework of a one-dimensional hydrodynamic model at Cl₂ concentrations in the range of 0.1–5%. It is found that the radiation intensities of the emission bands of Xe*₂(172 nm) and XeCl* (308 nm) are comparable when the chlorine concentration in the mixture is in the range of 0.01–0.1%. In this case, in the mixture, the radiation intensity of the Xe*₂ molecule rapidly decreases with increasing Cl₂ concentration and, at a chlorine concentration of ≥0.2%, the radiation of the B → X band of XeCl* molecules with a peak at 308 nm dominates in the discharge radiation. The radiation efficiency of this band reaches its maximum value at chlorine concentrations in the range of 0.4–0.5%. The calculated efficiencies of DBD radiation exceed those obtained experimentally. This is due to limitations of the one-dimensional model, which assumes the discharge to be uniform in the transverse direction, whereas the actual excilamp discharge is highly inhomogeneous. The influence of the chlorine concentration on the properties of the DBD plasma in binary Xe-Cl₂ mixtures is studied numerically. It is shown that an increase in the Cl₂ concentration in the mixture leads to the attachment of electrons to chlorine atoms and a decrease in the electron density and discharge conductivity. As a result, the electric field and the voltage drop across the discharge gap increase, which, in turn, leads to an increase in the average electron energy and the probability of dissociation of Cl₂ molecules and ionization of Xe atoms and Cl₂ molecules. The total energy deposited in the discharge rises with increasing chlorine concentration due to an increase in the power spent on the heating of positive and negative ions. The power dissipated by electrons decreases with increasing chlorine concentration in the working mixture. Recommendations on the choice of the chlorine content in the mixture for reducing the intensity of VUV radiation of the second continuum of the Xe*₂ excimer without a substantial decrease in the excilamp efficiency are formulated.     

Characteristics of a DC discharge with a water cathode in argon

The characteristics of a dc discharge excited between a metal anode and a water cathode in argon were studied experimentally. The dimensions of the positive column and the electric field in it were measured, and the vibrational temperature in the positive column was determined from the N₂ C ³Π_u → B ³Π_g (0–2) emission band. It is shown that the power deposited in the positive column is almost entirely spent on gas heating. The obtained dependence of the reduced electric field on the gas pressure and the ionization frequencies calculated by solving the Boltzmann equation indicate that electrons are lost diffusively, whereas ionization proceeds in a stepwise manner via the lower metastable states of argon atoms.     

Optical characteristics and parameters of the plasma of a barrier discharge excited in a mixture of mercury dibromide vapor with nitrogen and helium

Results are presented from experimental and theoretical studies of the optical characteristics and parameters of the plasma of an atmospheric-pressure barrier discharge excited in a HgBr₂: N₂: He mixture, which was used as the working medium of a small-size (with a radiation area of 8 cm²) exciplex gas-discharge radiation source. The mean radiation power of 87 mW was achieved at the radiation wavelength λ_max = 502 nm. The electron energy distribution function, the transport characteristics, the specific energy lost in the processes involving electrons, the electron temperature and density, and the rate constants of elastic and inelastic electron scattering by the components of the working mixture were calculated as functions of the reduced field E/N. The plasma of a discharge excited in a HgBr₂: N₂: He mixture can be used as the working medium of a small-size blue-green radiation source. Such a source can find application in biotechnology, photonics, and medicine and can also be used to manufacture gas-discharge display panels.     

New mechanism for the influence of Xe on the concentration of CO<Subscript>2</Subscript> molecules in self-sustained CO-laser discharges

The dependence of the CO₂ concentration on the discharge conditions and the mixture composition in a CO laser is studied experimentally. The experimental data are compared with the calculated results. A scheme of the reactions that govern the concentration of CO₂ molecules under the experimental conditions in question is constructed. It is shown that, in a gas-discharge plasma, an admixture of Xe in a mixture containing CO molecules gives rise to a new mechanism for the dissociation of CO₂ molecules by metastable xenon atoms. Under conditions close to the operating conditions of sealed-off CO lasers, the dissociation of CO₂ molecules in collisions with metastable. Xe(³P₂) atoms becomes the dominant dissociation mechanism in a He: CO mixture because it proceeds at a fast rate. This explains the observed decrease in the CO₂ concentration in a xenon-containing He: CO mixture.     

Purification and characterization of a dextranase from Sporothrix schenckii

A dextranase (EC 3.2.1.11) was purified and characterized from the IP-29 strain of Sporothrix schenckii, a dimorphic pathogenic fungus. Growing cells secreted the enzyme into a standard culture medium (20 °C) that supports the mycelial phase. Soluble bacterial dextrans substituted for glucose as substrate with a small decrease in cellular yield but a tenfold increase in the production of dextranase. This enzyme is a monomeric protein with a molecular mass of 79 kDa, a pH optimum of 5.0, and an action pattern against a soluble 170-kDa bacterial dextran that leads to a final mixture of glucose (38%), isomaltose (38%), and branched oligosaccharides (24%). In the presence of 200 mM sodium acetate buffer (pH 5.0), the K _m for soluble dextran was 0.067 ± 0.003% (w/v). Salts of Hg²⁺, (UO₂)²⁺, Pb²⁺, Cu²⁺, and Zn²⁺ inhibited by affecting both V _max and K _m. The enzyme was most stable between pH values of 4.50 and 4.75, where the half-life at 55 °C was 18 min and the energy of activation for heat denaturation was 99 kcal/mol. S. schenckii dextranase catalyzed the degradation of cross-linked dextran chains in Sephadex G-50 to G-200, and the latter was a good substrate for cell growth at 20 °C. Highly cross-linked grades (i.e., G-10 and G-25) were refractory to hydrolysis. Most strains of S. schenckii from Europe and North America tested positive for dextranase when grown at 20 °C. All of these isolates grew on glucose at 35 °C, a condition that is typically associated with the yeast phase, but they did not express dextranase and were incapable of using dextran as a carbon source at the higher temperature. Received: 29 December 1997 / Accepted: 4 March 1998     

Surface energy fluxes and control of evapotranspiration from a <Emphasis Type="Italic">Carex lasiocarpa</Emphasis> mire in the Sanjiang Plain,Northeast China

Data from four components of the radiation balance were used to investigate the surface energy budgets for a Carex lasiocarpa mire in the Sanjiang Plain, Northeast China, and the controlling factors of the evapotranspiration (ET) were discussed in detail. During the growing season 2006, the shortwave radiation (SW↓) reaching the mire surface added up to 2,854.3 MJ m⁻² and the net radiation (Rn) was 1,637.4 MJ m⁻² in total, with an average of 9.86 MJ m⁻² day⁻¹. G was the smallest flux at the water-atmosphere interface, with an average of about 0.91 MJ m⁻² day⁻¹, but showed high relative variability, even changing its sign. The latent and sensible heat fluxes (LE and H) amounted to 787.48 and 476.26 MJ m⁻², respectively, and the total sum of LE and H accounted for 77.18% of Rn. By conversion from LE, the average value of ET from the mire was 1.84 mm day⁻¹, amounting to 298.8 mm. The total ET was almost 60% of the total rainfall in the same period, proving that ET is the primary water consumer in the mire. The growth of C. lasiocarpa was related closely with surface resistance (r _s), and analysis of partial correlation indicated that r _s correlated negatively with leaf area index (LAI) when the interference of the available energy, Rn-G, was removed. There was a strong linkage between r _s and the evaporative fraction [LE/(LE + H)] as well as Bowen ratio (β). r _s was the key factor in controlling the variation of ET and regulating energy partitioning between LE and H. During the whole growing season, r _s and R _n−G were the two main factors coupled in ET processes. In spring, r _s dominated ET processes, and the increase in LAI led to a decrease in r _s, which in turn accelerated ET as vegetation developed until late August. After August, the available energy controlled the process of ET completely until ET reached an equilibrium in mid-October.     

Kinetics of the processes,plasma parameters,and output characteristics of a UV emitter operating on XeI molecules and iodine molecules and atoms

A kinetic model of the processes occurring in the plasma of a high-power low-pressure gas-discharge lamp is presented, and the output characteristics of the lamp are described. The lamp is excited by a longitudinal glow discharge and emits the I₂(D′-A′) 342-nm and XeI(B-X) 253-nm bands and the 206.2-nm spectral line of atomic iodine. When the emitter operates in a sealed-off mode on the p(He): p(Xe): p(I₂) = 400: 120: (100–200) Pa mixture, the fractions of the UV radiation power of iodine atoms, exciplex molecules of xenon iodide, and iodine molecules comprise 55, 10, and 35%, respectively. At the optimal partial pressure, the maximum total radiation power of the lamp reaches 37 W, the energy efficiency being about 15%.     

Ozone-stimulated emission due to atomic oxygen population inversions in an argon microwave plasma torch

It is shown that, in a microwave torch discharge in an argon jet injected into an oxygen atmosphere at normal pressure, quasi-resonant energy transfer from metastable argon atoms to molecules of oxygen and ozone generated in the torch shell and, then, to oxygen atoms produced via the dissociation of molecular oxygen and ozone leads to the inverse population of metastable levels of atomic oxygen. As a result, the excited atomic oxygen with population inversions becomes a gain medium for lasing at wavelengths of 844.6 and 777.3 nm (the 3³ P–3³ S and 3⁵ P–3⁵ S transitions). It is shown that an increase in the ozone density is accompanied by an increase in both the lasing efficiency at these wavelength and the emission intensity of the plasma-forming argon at a wavelength of 811.15 nm (the ² P ⁰4s–² P ⁰4p transition). When the torch operates unstably, the production of singlet oxygen suppresses ozone generation; as a result, the lasing effect at these wavelengths disappears.     

A B3LYP and MP2(full) theoretical investigation into explosive sensitivity upon the formation of the molecule-cation interaction between the nitro group of 3,4-dinitropyrazole and H+, Li+, Na+, Be2+ or Mg2+

The explosive sensitivity upon the formation of molecule-cation interaction between the nitro group of 3,4-dinitropyrazole (DNP) and H⁺, Li⁺, Na⁺, Be²⁺ or Mg²⁺ has been investigated using the B3LYP and MP2(full) methods with the 6-311++G** and 6-311++G(2df,2p) basis sets. The bond dissociation energy (BDE) of the C3–N7 trigger bond has also been discussed for the DNP monomer and the corresponding complex. The interaction between the oxygen atom of nitro group and H⁺ in DNP…H⁺ is partly covalent in nature. The molecule-cation interaction and bond dissociation energy of the C3–N7 trigger bond follow the order of DNP…Be²⁺ > DNP…Mg²⁺ > DNP…Li⁺ > DNP…Na⁺. Except for DNP…H⁺, the increment of the trigger bond dissociation energy in comparison with the DNP monomer correlates well with the molecule-cation interaction energy, natural charge of the nitro group, electron density ρ _BCP(C3–N7), delocalization energy E ⁽²⁾ and NBO charge transfer. The analyses of atoms in molecules (AIM), natural bond orbital (NBO) and electron density shifts have shown that the electron density of the nitro group shifts toward the C3–N7 trigger bond upon the formation of the molecule-cation interaction. Thus, the trigger bond is strengthened and the sensitivity of DNP is reduced.     

Similarity laws for cathode-directed streamers in gaps with an inhomogeneous field at elevated air pressures

Results are presented from experimental studies of cathode-directed streamers in the gap closure regime without a transition into spark breakdown. Spatiotemporal, electrodynamic, and spectroscopic characteristics of streamer discharges in air at different pressures were studied. Similarity laws for streamer discharges were formulated. These laws allow one to compare the discharge current characteristics and streamer propagation dynamics at different pressures. Substantial influence of gas photoionization on the deviations from the similarity laws was revealed. The existence of a pressure range in which the discharges develop in a similar way was demonstrated experimentally. In particular, for fixed values of the product pd and discharge voltage U, the average streamer velocity is also fixed. It is found that, although the similarity laws are violated in the interstreamer pause of the discharge, the average discharge current and the product of the pressure and the streamer repetition period remain the same at different pressures. The radiation spectra of the second positive system of nitrogen (the C³Π _u -B³Π _g transitions) in a wavelength range of 300–400 nm at air pressures of 1–3 atm were recorded. It is shown that, in the entire pressure range under study, the profiles of the observed radiation bands practically remain unchanged and the relative intensities of the spectral lines corresponding to the ³Π _u -B³Π _g transitions are preserved.     

Study of spectra of detached electrons produced in collisions of negative deuterium ions

Spectra of detached electrons produced in mutual collisions of D⁻ ions in the relative energy range of 1.8−6.1 eV were investigated. In addition to electrons corresponding to conversion of D⁻ ions into D⁰ atoms, peaks corresponding to the production of D₂ molecules and D₂⁻ molecular ions in the ground electronic state were revealed. The existence of D₂⁻ ions over a time longer than the period of molecular oscillations was confirmed experimentally for the first time.     

川西亚高山森林土壤呼吸和微生物生物量碳氮对施氮的响应

随着全球大气氮沉降的明显增加,将有可能显著影响我国西部地区受氮限制的亚高山森林生态系统。土壤微生物是生态系统的重要组成部分,是土壤物质循环和能量流动的重要参与者。由于生态系统类型、土壤养分、氮沉降背景值等的差异,土壤呼吸和土壤生物量碳氮对施氮的响应存在许多不确定性。而施氮会不会促进亚高山森林生态系统中土壤呼吸和微生物对土壤碳氮的固定?基于此假设,选择了川西60年生的四川红杉(Larix mastersiana)亚高山针叶林为研究对象,通过4个水平的土壤施氮控制试验(CK:0 g m~(-2) a~(-1)、N1:2 g m~(-2)a~(-1)、N2:5 g m~(-2) a~(-1)、N3:10 g m~(-2)a~(-1)),监测了土壤呼吸及土壤微生物生物量碳氮在一个生长季的动态情况。结果表明:施氮对土壤呼吸各指标和土壤微生物碳氮都有极显著的影响,施氮能促进土壤全呼吸、自养呼吸、异养呼吸通量和土壤微生物生物量碳氮的增长,施氮使土壤呼吸通量提高了11%—15%,土壤微生物量碳提高了5%—9%,土壤微生物量氮提高了23%—34%。在中氮水平下(5 g m~(-2) a~(-1))对土壤呼吸的促进最显著。相关分析发现,土壤呼吸与微生物生物量碳氮和微生物代谢商极呈显著正相关,微生物量碳氮与土壤温度呈极显著的正相关,与土壤湿度呈极显著负相关。通过一般线性回归拟合土壤呼吸速率与土壤10 cm温湿度的关系,发现土壤呼吸速率与土壤温度呈极显著的正相关,与土壤湿度极显著负相关(P0.001),中氮水平下土壤温度敏感性系数Q_(10)值(7.10)明显高于对照(4.26)。     

Entropy budgets of deciduous plant leaves and a theorem of oscillating entropy production

From an energy budget of a deciduous plant leaf in moderate conditions, entropy fluxes into or out of the leaf due to solar radiation, infrared radiation, evaporation of water and heat conduction are calculated. Net entropy flow into the leaf is negative. On the assumption that the entropy in the leaf is in a steady state, the entropy production in the typical deciduous leaf in moderate conditions [the solar energy absorbed by both sides of the leaf isE _solar=0.0602 (J cm⁻² s⁻¹)] becomesS _prod=1.8×10⁻⁴ (J cm⁻² s⁻¹ K⁻¹). The positiveness of the entropy production shows that the Second Law of Thermodynamics certainly holds in the plant leaf. Entropy productions in other conditions are also calculated. The entropy production in the leafS _prod becomes a linear function of the solar energy absorbed by the leafE _solar:S _prod≈-(29.5E _solar)×10⁻⁴. A theorem is presented: the entropy production in plant leaves oscillates during the period of one day, paralleling the daily solar energy absorbed by leaves.     

An urn model study of variability within a compartment

Formulas are derived for the mean and variance of the number of radioactive atoms present in a compartment (or urn). Initally,n ₁ radioactive atoms andb stable atoms are placed in the urn; and subsequently,r stable atoms are added and an equal number,r, of a random mixture of stable and radioactive atoms is removed per unit time. The expected number of radioactive atoms,E(t), present at timet is, as expected,n ₁ e^−λt where λ=(r/Δt)/(b+r+n ₁). The relative variance, σ²(t)/n ₁ ² , vanishes to zero forr=1, atoms per unit time and for a large number ofn ₁ radioactive atoms; but for a large number of bothr andn ₁ atoms the relative variance is ∼e ^−λt , equal to the fractional retention, fort>1/λ. Thus in studies where radionuclides are injected into animals and a single compartment represents the data, if a large variance is observed it might be due to the fact that large numbers of atoms are transferred out in unit time. When a small variance is observed, this is probably due to the fact that few atoms are transferred in smaller units of time (such that λ is the same in both cases). Research sponsored by the Energy Research and Development Administration under contract with Union Carbide Corporation.     

Blue shifts <Emphasis Type="Italic">vs</Emphasis> red shifts in σ-hole bonding

σ-Hole bonding is a noncovalent interaction between a region of positive electrostatic potential on the outer surface of a Group V, VI, or VII covalently-bonded atom (a σ-hole) and a region of negative potential on another molecule, e.g., a lone pair of a Lewis base. We have investigated computationally the occurrence of increased vibration frequencies (blue shifts) and bond shortening vs decreased frequencies (red shifts) and bond lengthening for the covalent bonds to the atoms having the σ-holes (the σ-hole donors). Both are possible, depending upon the properties of the donor and the acceptor. Our results are consistent with models that were developed earlier by Hermansson and by Qian and Krimm in relation to blue vs red shifting in hydrogen bond formation. These models invoke the derivatives of the permanent and the induced dipole moments of the donor molecule. Figure Computed electrostatic potential on the molecular surface of Cl-NO₂. Color ranges, in kcal mol⁻¹, are: red, greater than 25; yellow, between 10 and 25; green, between 0 and 10; blue, between −4 and 0; purple, more negative than −4. The chlorine is facing the viewer, to the right. Note the yellow region of positive potential on the outer side of the chlorine, along the extension of the N–Cl bond. The blue region shows the sides of the chlorine to have negative potentials. The calculations were at the B3PW91/6–31G(d,p) level.     

Investigation of the mechanism for rapid heating of nitrogen and air in gas discharges

A rapid heating of nitrogen-oxygen mixtures excited by gas discharges is investigated numerically with allowance for the following main processes: the reactions of predissociation of highly excited electronic states of oxygen molecules (which are populated via electron impact or via the quenching of the excited states of N₂ molecules), the reactions of quenching of the excited atoms O(¹ D) by nitrogen molecules, the VT relaxation reactions, etc. The calculated results adequately describe available experimental data on the dynamics of air heating in gas-discharge plasmas. It is shown that, over a broad range of values of the reduced electric field E/N, gas heating is maintained by a fixed fraction of the discharge power that is expended on the excitation of the electronic degrees of freedom of molecules (for discharges in air, η_E?28%). The lower the oxygen content of the mixture, the smaller the quantity η_E. The question of a rapid heating of nitrogen with a small admixture of oxygen is discussed.     

Nitrogen use efficiency of sugarcane in relation to its BNF potential and population of endophytic diazotrophs at different N levels

The nitrogen fixing bacterial endophytes Gluconacetobacter diazotrophicus and Herbaspirillum spp. have been proposed to benefit sugarcane (Saccaharum spp. hybrids) growth. Variable populations of these endophytes exist depending upon ontogenic and climatic variations as well. This study investigates the effect of variable chemical nitrogen application in soil on the population of endophytic diazotrophs, acetylene reduction ability of excised roots, plant N-nutrient use efficiency and probable interactions among different parameters in eight commercial sugarcane varieties of subtropical India. Recovery efficiency (RE), agronomic efficiency (AE), partial factor productivity (PFP) and physiologic efficiency (PE) indicators were used for accounting N-nutrient use efficiency. The population of G. diazotrophicus was more at N₇₅ compared to N₀ and N₁₅₀, whereas Herbaspirillum population increased from N₀ to N₁₅₀. ARA was positively correlated with Gluconacetobacter population in rhizosphere and root, whereas it had poor correlation with Herbaspirillum population. Positive correlation of RE and AE with ARA of roots, Gluconacetobacter and Herbaspirillum populations in roots and stems indicate their positive contribution in total nitrogen uptake by the plant per kg of N applied. Average PFP was 808.9 at N₇₅ compared to 408.7 at N₁₅₀ indicating that N was utilized efficiently at low N input status in sugarcane. Strong positive correlations of AE₇₅ (agronomic efficiency from 75 kg N ha⁻¹ to 150 kg N ha⁻¹) with N-uptake (r ² = 0.615), cane yield (r ² = 0.758) and PFP (r ² = 0.758) and other parameters compared to AE (agronomic efficiency from 0 kg N ha⁻¹ to 75 kg N ha⁻¹ or 150 kg N ha⁻¹) correlations with N-uptake (r ² = 0.111), cane yield (r ² = 0.368) and PFP (r ² = 0.190) indicated that the AE of sugarcane was strongly directed towards producing more cane yield per unit of N fertilizer once the sugarcane plant has established using initial dose of nitrogen and thus AE₇₅ seems to be a more appropriate indicator for accounting N-nutrient use efficiency in sugarcane.