Associative ionization reactions involving excited atoms in nitrogen plasma

A model of kinetic processes in gas-discharge plasmas of pure nitrogen and its mixtures with nitrogen oxide and oxygen is presented. A distinctive feature of the model is that it includes associative ionization reactions involving N(² P) electronically excited atoms. Taking into account these processes allows one to explain both the anomalously slow decay of gas-discharge nitrogen plasma and the increase in the electron density in the region of the so-called pink afterglow in nitrogen. The possibility of substantially accelerating secondary ionization by adding NO molecules to a partially dissociated nitrogen is demonstrated. It is shown that such acceleration is caused by the associative ionization reaction N(² P) + O(³ P) → e + NO⁺. The calculated results agree well with available experimental data.     

Electron temperature in a decaying molecular nitrogen plasma in the presence of weak electric fields

The electron energy distribution function in an afterglow molecular nitrogen plasma is studied both experimentally and theoretically under the conditions of weak electric fields such that the electron gas is heated by superelastic collisions of electrons with vibrationally excited molecules. Based on the mean electron energy balance, it is established that, depending on the degree of plasma ionization and the vibrational temperature of nitrogen molecules, an afterglow plasma may evolve into two states, differing in electron temperature. This kind of bistability is found to stem from the difference in the main mechanisms for electron energy losses in the two stable states. The prediction that the shape of the electron energy distribution function should change in a jumplike manner when a weak electric field is imposed has been confirmed experimentally.     

Boron atom as an electron density relay

The electronic structure of tetrapyrrolidinodiborane [(C₄H₈N)₂B]₂ has been investigated by HeI and HeII UV photoelectron spectroscopy (UPS) and DFT/OVGF calculations. Our results indicate that the title molecule has very low first ionization energy and can be readily oxidized in solution. We estimated the magnitudes of alkyl substituent effects on nitrogen and boron atoms and the efficiency of boron atom to act as an electron density relay facilitating through-bond interactions between the nitrogen lone pairs.     

Measurements of the electron density and degree of plasma ionization in a plasma target based on a linear electric discharge in hydrogen

Laser interferometry methods were used to measure the density of free electrons and degree of plasma ionization in a hydrogen target intended for experiments on determining energy losses of heavy ion beams in an ionized matter. It is shown that the linear electron density can be varied in the range from 3.3 × 10¹⁷ to 1.3 × 10¹⁸ cm^?2 by varying the initial plasma parameters (the hydrogen pressure in the target and the discharge current). The error in measuring the linear electron density in the entire range of the varied plasma parameters was less than 1%. The maximum degree of plasma ionization achieved at the initial gas pressure of 1 mbar was 0.62 ± 0.05.     

Simulations of a surface glow discharge in a supersonic gas flow in the presence of external ionization

Results of two-dimensional hydrodynamic simulations of a surface glow discharge operating at pressures of 0.2–0.5 Torr in a nitrogen flow propagating with a velocity of 1000 m/s in the presence of external ionization are presented. The effect of the external ionization rate on discharge operation is analyzed. The current-voltage characteristics of the discharge are calculated for different intensities of external ionization in both the presence and absence of secondary electron emission from the cathode. The discharge structure and plasma parameters in the vicinity of the loaded electrode are considered. It is shown that, when the discharge operates at the expense of secondary emission from the cathode, the discharge current and cathode sheath configuration are insensitive to external ionization. It is also demonstrated that, even at a high rate of external ionization, the discharge operates due to secondary emission from the cathode.     

Dark phase effect in the evolution of the positive column of a glow discharge in nitrogen

The characteristics of the initial stage of the formation of the positive column of a glow discharge in nitrogen at reduced pressures are studied experimentally and numerically. A dip in the plasma emission intensity in the initial stage of the discharge (the so-called “dark phase”) is observed experimentally at the positive polarity of the high-voltage electrode (the cathode is grounded). The dark phase is preceded by an ionization wave (IW). When the anode is grounded, neither an IW nor a dip in the discharge emission intensity are observed. A theoretical model capable of describing the discharge development under the actual experimental conditions is constructed. It is shown that the dark phase effect may be caused by the high electron density (above the steady-state one) produced in the gas during the passage of the IW across the discharge gap. This mechanism of the dark phase formation differs from the mechanism proposed earlier to explain a similar effect in noble gases. Additional experiments carried out with pure argon, helium, and helium with a nitrogen admixture have shown that, in the case of a grounded cathode, gas breakdown is also accompanied by the passage of an IW, whereas in the case of a grounded anode, no IW is observed; however, the dark phase is present in both cases. It is shown using computer simulations that, in nitrogen (in contrast to noble gases), the mechanism resulting in the dark phase effect does not operate in the absence of an IW.     

Identification of bis-quaternary ammonium compounds using mild ionization mass spectrometry

Ethonium, an antimicrobial chemotherapeutic agent, was investigated by mass spectrometry (MS) under various ionization conditions: electron impact, field ionization, field desorption (FD) and fast atom ionization. FDMS was found to be the most suitable procedure for ethonium identification. Relation of the ED mass spectra to the distance between the nitrogen atoms in bis-quaternary ammonium compounds is discussed. It was shown that the most intensive ions with m/z 499, 315 in the FD mass spectra corresponded to the ethonium specific fragmentation and their occurrence in the spectra could serve as a sufficient criterion useful in qualitative and quantitative assay of the drug in the sample.     

The track structure of alpha-particles from the point of view of ionization-cluster formation in "nanometric" volumes of nitrogen

Probability distributions of the size of ion clusters created in "nanometric" volumes of nitrogen by single alpha-particles of a gold-plated 241Am source, were measured and compared with those calculated by Monte Carlo methods in the same geometry. The diameter of the sensitive volumes had a mass per area of between 0.015 microgram/cm2 and 1.3 micrograms/cm2 which, for a material at unit density, corresponds to a nanometric target volume 0.15-13 nm in diameter. These nanometre sizes were simulated experimentally in a device called the Jet Counter. This consists of a pulse-operated valve which injects into an interaction chamber an expansion jet of molecular nitrogen gas, which is crossed by a narrow beam of alpha-particles. The resulting ions are counted and analyzed from the point of view of ionization cluster formation. The measured or calculated cluster size probabilities prove that the formation of ionization clusters along a "nanometre" track is governed by Poisson's law only in the case of very small target volumes, due to the contributions by secondary electrons. The present ionization cluster probabilities produced in "nanometric" volumes 0.15-13 nm in diameter, are the first ever determined experimentally and confirmed by Monte Carlo simulation.     

Effect of the electron drift in a transverse electric field on the width of spark channels in a multichannel sliding discharge

The effect of electron drift on the transverse size of the spark channels in a multichannel sliding discharge on a dielectric surface is considered in a semianalytic approximation. The strength of an electric field transverse to the channel axis is estimated by the mirror image method. The estimate obtained is used to analyze the differential equation for the density of electrons with allowance for their drift from the channel into the surrounding layers. It is shown that the channel expands to a certain steady-state radius at which an increase in the local electron density due to the ionization of atoms is balanced by its decrease due to the electron drift from the surface channel layer into the surrounding layers. Numerical estimates are carried out for the conditions of earlier experiments with discharges in He, Ne, Ar, and Xe. The analysis applies to the initial nanosecond stage of the spark development, when the hydrodynamic expansion of the channels is still insignificant.     

Shake-up and shake-off excitations with associated electron losses in X-ray studies of proteins

Photoionization of an atom by X-rays usually removes an inner shell electron from the atom, leaving behind a perturbed "hollow ion" whose relaxation may take different routes. In light elements, emission of an Auger electron is common. However, the energy and the total number of electrons released from the atom may be modulated by shake-up and shake-off effects. When the inner shell electron leaves, the outer shell electrons may find themselves in a state that is not an eigen-state of the atom in its surroundings. The resulting collective excitation is called shake-up. If this process also involves the release of low energy electrons from the outer shell, then the process is called shake-off. It is not clear how significant shake-up and shake-off contributions are to the overall ionization of biological materials like proteins. In particular, the interaction between the outgoing electron and the remaining system depends on the chemical environment of the atom, which can be studied by quantum chemical methods. Here we present calculations on model compounds to represent the most common chemical environments in proteins. The results show that the shake-up and shake-off processes affect approximately 20% of all emissions from nitrogen, 30% from carbon, 40% from oxygen, and 23% from sulfur. Triple and higher ionizations are rare for carbon, nitrogen, and oxygen, but are frequent for sulfur. The findings are relevant to the design of biological experiments at emerging X-ray free-electron lasers.     

A nanoparticle in plasma

Charge and energy fluxes onto a nanoparticle under conditions typical of laboratory plasmas are investigated theoretically. Here, by a nanoparticle is meant a grain the size of which is much smaller than both the electron Larmor radius and Debye length and the thermionic emission from which is not limited by the space charge. Under conditions at which thermionic emission plays an important role, the electric potential and temperature T _p of a nanoparticle are determined by solving a self-consistent set of equations describing the balance of energy and charge fluxes onto the nanoparticle. It is shown that, when the degree of plasma ionization exceeds a critical level, the potential of the nanoparticle and the energy flux onto it increase with increasing nanoparticle temperature, so that, starting from a certain temperature, the nanoparticle potential becomes positive. The critical degree of ionization starting from which the potential of a nanoparticle is always positive is determined as a function of the plasma density and electron temperature. The nanoparticle temperature T _p corresponding to the equilibrium state of a positively charged nanoparticle is found as a function of the electron density for different electron temperatures.     

Thermal ionization of helium in the quantum mechanical representation of Feynman path integrals

The method of Feynman path integrals is used to calculate the degree of ionization in a thermodynamically equilibrium, dense helium plasma. A quantum statistical calculation which is based on the “first principles” and in which the permutation symmetry and electron spin variable are explicitly taken into account is carried out for a helium atom that is in thermal and mechanical contact with a plasma at a temperature from 30000 to 400000 K. Numerical integration over virtual paths is performed by the Monte Carlo method. When the temperature and density are varied, the amount of singly charged ions in a plasma varies nonmonotonically, reaching a maximum at a temperature of about 70 000 K and a pressure of 100 MPa. Radial profiles of the electron density around a helium nucleus are obtained, as well as contour lines of the degree of ionization in the p-T plane of the plasma states. The role of fluctuations is investigated. It is shown that the fluctuations of the local electron density near a helium ion have a radical effect on the values of the ionization-recombination equilibrium constants.     

Negative-polarity fast ionization wave in molecular gases: Electric field, electron density, and energy branching

The charge density per unit length, the longitudinal component of the electric field, and the electron density behind the front of a fast ionization wave initiated by a nanosecond negative voltage pulse in air, N₂, and H₂ in the 1-to 24-torr pressure range are reconstructed from the experimental data. It is shown that the electron density behind the wave front depends weakly on the sort of gas used and, at relatively high pressures (8–24 torr), is (2–3)×10¹² cm^?3. The energy deposited in the internal degrees of freedom is analyzed. It is shown that, for all gases used, most of the deposited energy (40–60%) is spent on the excitation of the electron degrees of freedom. The fraction of the energy deposited in the high-energy degrees of freedom (ionization and dissociation) monotonically decreases with increasing the pressure, whereas the fraction of the energy spent on the excitation of the low-energy degrees of freedom (rotational and vibrational) monotonically increases.     

Ionization instability and dusty plasma structurization

In the presence of ionization processes, a homogeneous equilibrium dust distribution often appears as a balance between plasma generation by ionization and plasma absorption by dust particles. It is shown that such equilibrium, often present in laboratory plasmas, is generally unstable against the formation of dust clumps separated by dust-free regions (dust voids). The driving force that separates an initially homogeneous dusty plasma into dust clumps and dust voids is the drag force produced by ions flowing out from the regions with reduced dust density. The lower the dust density, the lower the electron absorption by dust particles and the larger the ionization rate proportional to the electron density. An increase in the ion drag force leads to a further decrease in the dust density and, thus, drives the instability. In the nonlinear stage, the instability creates structures—dust clouds separated by dust voids. The dependence of the instability growth rate on the wavenumber (or, in other words, on the size of the dust-free and dust-containing regions) is investigated. It is shown that, for sufficiently small wavenumbers, a homogeneous distribution is always unstable. An analogy with a gravitational-like instability related to shadowing of the plasma flux by dust particles is pointed out. This effect, which is due to collective shadowing of the plasma flux, dominates the shadowing by individual dust particles discussed previously. Similar to the usual gravitational instability, perturbations with the largest scales are always unstable. Contrary to the usual gravitational instability, the largest growth rate corresponds not to the largest possible scale but to the size close to the mean free path of plasma particles colliding with dust particles. A special investigation is undertaken to determine the influence of the ion-neutral collisions on the growth rate of the instability.     

Experimental and theoretical studies of the spatial distribution of the ionization rate in a non-self-sustained discharge controlled by a fast electron beam

The spatial distribution of the current density of fast electrons and the ionization rate in a gap filled with atmospheric-pressure air under the conditions of a non-self-sustained discharge controlled by a fast electron beam were investigated. The experiments were carried out in a gas-discharge chamber with a grid electrode arranged in parallel to the exit window of the ionization source. Spatial variations in the current density of fast electrons resulting from the grid were measured. The propagation of the electron beam through the discharge system was simulated numerically by the Monte Carlo method in the so-called “effective collision” approximation. The calculated results agree well with the experimental data.     

Mass spectra of various deuterium-labelled forms of bis-O-trimethylsilyl-N-acetylsphinganine

The mass spectrum of the trimethylsilyl derivative of N-acetylsphinganine (dihydrosphingosine) has been studied in detail. Origins, structures and mechanisms of formation of the principal ions are supported by deuterium labelling and exact masses. The ions formed on electron impact ionization at 70 eV can be divided into two main categories with respect to electron abstraction from one of the oxygen atoms or the nitrogen atom on the long-chain carbon backbone.     

One-dimensional semiempirical model of plasma in an accelerator with closed electron drifts

The plasma structure in an accelerator with closed electron drifts is investigated experimentally and numerically based on the measured data on the angular and energy ion distribution in a plasma jet. The mathematical model is constructed in a one-dimensional steady-state approximation and is aimed at calculating spatial distributions of the electric potential and plasma density in the region of the most intense ionization of neutral atoms. A comparison of the numerically calculated model potential distributions with the results from direct probe measurements shows that the proposed approach provides an online analysis of the plasma structure in the ionization-acceleration zone.     

Theoretical study of the pre- and post-translational effects of adenine and thymine tautomers and methyl derivatives

The study of pre-translational effects (ionization, tautomerization) and post-translational effects (methylation) of adenine and thymine has only recently been the focus of some studies. These effects can potentially help regulate gene expression as well as potentially disrupt normal gene function. Because of this wide array of roles, greater insight into these effects in deoxyribonucleic acids (DNA) are paramount. There has been considerable research of each phenomenon (tautomerization, methylation and ionization) individually. In this work, we attempt to shed light upon the pre-translational effects and post translational effects of adenine and thymine by investigating the electron affinities (EAs) and ionization potentials (IPs) of the major and minor tautomers and their methyl derivatives. We performed all calculations using the density functional theory (DFT) B3LYP functional accompanied with 6-311G(d,p), 6-311+G(d,p) and 6-311++G(df,pd) basis sets. Our results reveal that the thymine tautomer has a higher EA and IP than the adenine tautomers. The higher EA suggests that an electron that attaches to the AT base pair would predominately attach to the thymine instead of adenine. The higher IP would suggest that an electron that is removed from the AT base pair would be predominately removed from the adenine within the base pair. Understanding how tautomerization, ionization and methylation differences change effects, discourages, or promotes one another is lacking. In this work, we begin the steps of integrating these effects with one another, to gain a greater understanding of molecular changes in DNA bases.     

Negative air ions as a source of superoxide

The physico-chemical characteristics and possible formation mechanisms of negative air ions are considered. It was found that the products of oxygen and nitrogen negative ionization reduce ferricytochromec and nitroblue tetrazolium, and that these reactions were inhibited by superoxide dismutase. The interaction of negatively ionized oxygen with water led to hydrogen peroxide accumulation, which was inhibited by tetranitromethane or catalase. Nitrogen ionization under these conditions caused the formation of the hydrated electron e _aq ^— and the superoxide anion O ₂ ^— . The data obtained indicate that the biological activity of negative air ions may be dependent on superoxide. The generation of reactive oxygen ions in the gas phase and also at a gas/water interface is described. A scheme for superoxide production under oxygen and nitrogen ionization is proposed.     

Electronic structure theory based study of proline interacting with gold nano clusters

Interaction between metal nanoparticles and biomolecules is important from the view point of developing and designing biosensors. Studies on proline tagged with gold nanoclusters are reported here using density functional theory (DFT) calculations for its structural, electronic and bonding properties. Geometries of the complexes are optimized using the PBE1PBE functional and mixed basis set, i. e., 6-311++G for the amino acid and SDD for the gold clusters. Equilibrium configurations are analyzed in terms of interaction energies, molecular orbitals and charge density. The complexes associated with cluster composed of an odd number of Au atoms show higher stability. Marked decrease in the HOMO-LUMO gaps is observed on complexation. Major components of interaction between the two moieties are: the anchoring N-Au and O-Au bond; and the non covalent interactions between Au and N-H or O-H bonds. The electron affinities and vertical ionization potentials for all complexes are calculated. They show an increased value of electron affinity and ionization potential on complexation. Natural bond orbital (NBO) analysis reveals a charge transfer between the donor (proline) and acceptor (gold cluster). The results indicate that the nature of interaction between the two moieties is partially covalent. Our results will be useful for further experimental studies and may be important for future applications.