Status report on the folded tandem ion accelerator at BARC

The folded tandem ion accelerator (FOTIA) facility set up at BARC has become operational. At present, it is used for elemental analysis studies using the Rutherford backscattering technique. The beams of¹H, ⁷Li, ¹²C, ¹⁶O and ¹⁹F have been accelerated up to terminal voltages of about 3 MV and are available for experiments. The terminal voltage is stable within ±2 kV. In this paper, present status of the FOTIA and future plans are discussed.     

Second virial coefficient for a mixture of nonspherical molecules of arbitrary symmetry

The mixture and interaction second virial coefficients of a binary gas mixture of nonspherical molecules of arbitrary symmetry have been calculated for a set of unlike force parameters which is obtained from the force parameters for like interactions by using empirical combination rules. In the calculation molecular anisotropy of very general type has been accounted. The relative contribution of each branch of interactions has been evaluated as a function of temperature. The theoretical values of interaction second virial coefficient have been compared with the experimental data of N₂+A, N₂+He, N₂+H₂, N₂+O₂, N₂+C₂H₄, N₂+C₂H₆, CO₂+A, CO₂+H₂, CO₂+N₂, CO₂+O₂, CO₂+CO, CO₂+C₂H₄, CO₂+C₂H₆, O₂+A and H₂+CO. The agreement between theory and experiment is satisfactory for all the systems. Numerical estimations of the mixture second virial coefficients as a function of temperature and composition are given for the systems CO₂+A, CO₂+H₂, CO₂+N₂, CO₂+O₂ and CO₂+CO.     

Detection of CO using bulk ZnO Schottky rectifiers

Bulk Pt/ZnO Schottky rectifiers show gas sensitivities for CO of 4% and 8%, respectively, at 150 °C for 1 and 10% CO in N₂. The current or voltage changes are time dependent, with an activation energy of 40.7 kJmol^-1. Over a limited range of partial pressures of CO (P_CO) in the ambient gas, the on-state resistance R of the sensor at fixed bias voltage decreased according to R=(R_O+A(P_CO)^0.5)^-1, where A is a constant and R_O is the resistance in N₂. Since these devices are also sensitive to H₂ and C₂H₄, the ZnO rectifiers appear promising for a range of gas-sensing applications. PACS 81.05.Dz; 73.61.Ga; 72.80.Ey     

Selective effects of homogeneous and heterogeneous reactions in the system N21-CO

In the system N₂¹-CO, effects arising only from the reactions in the homogeneous phase have been isolated in a reactor with inert walls. After an induction period of 0.13 s, they involve a stationary rate of enhancement of N₂(B³Π_g) and N(⁴S) concentrations according to the reactions N₂(X¹Σ_g⁺)_v¹+CO→ CO(X¹Σ⁺)_v1 + N₂(X¹Σ_g⁺ and CO(X¹Σ⁺)_v1 + N₂(A³Σ_u⁺) → N (⁴S) + N(⁴S) + CO(X¹Σ⁺).In a reactor with active walls, both the above reactions in the homogeneous phase and heterogeneous reactions due to CO adsorbed on the walls are involved according to CO_ads + N₂(X¹Σ_g⁺)_{v1 ads or not} → CO(X¹Σ⁺)_v1 + N₂(X¹Σ_g⁺ and N(⁴S) + N(⁴S) + CO_ads → N₂(X¹Σ_g⁺) + CO(X¹Σ⁺)N(⁴S) + CO_ads → N₂(X¹Σ_g⁺) + CO(X¹Σ⁺). In this case, the rate of enhancement is not stationary. Furthermore, for cylindrical reactors with large diameters, the two types of reaction do not interact and their effects are additive.     

Computational investigation of the performance of ZIF-8 with encapsulated ionic liquids towards CO2 capture*

The effect of Ionic Liquid (IL) encapsulation in Metal Organic Frameworks (MOFs) is extensively studied towards the enhancement of the MOFs as CO₂-selective materials. The influence of the IL anion-cation pair type is investigated through the combination of two different cations, namely 1-butyl-3-methylimidazolium [bmim⁺] and 1-octyl-3-methylimidazolium [omim⁺] and three distinct anions, namely bis-trifluoromethylsulfonyl-imide [Tf₂N^?], tricyanomethanide [TCM^?], and tertracyanoborate [B(CN)₄^?], that can be encapsulated in ZIF-8, resulting in a series of ZIF hybrids (IL@ZIF-8). The study investigates the impact of the anion and the cation on the separation of CO₂ from mixtures with CH₄ and N₂. Monte Carlo simulations of adsorption of the three gases in both the pristine ZIF-8 and in ILs@ZIF-8 reveal that CO₂ capacity increases dramatically for the case of ILs@ZIF-8. Moreover, analysis of the simulations and additional density functional theory calculations show that CO₂/CH₄ (related to natural gas purification) and CO₂/N₂ (related to post-combustion CO₂ capture) mixture selectivity is affected by the distribution, composition and type of the IL pair. Moreover, the sorbent selection parameter, S, and the regenerability factor, R, are used to evaluate the performance of all IL@ZIF-8 analogues along with other known CO₂-selective materials.     

DC discharge experiment in an Ar/N2/CO2 ternary mixture: A laboratory simulation of the Martian ionosphere's plasma environment

A low-pressure DC plasma discharge sustained in a 1.6%Ar–2.7%N₂–95.3%CO₂ ternary mixture is studied. This plasma was generated in a total pressure range from 1.0 to 4.0 Torr, a power of 6.3 W and a 12 l/min flow rate of gases. The electron temperature was found to be 8.41 eV and the ion density, in the order of 10⁹ cm⁻³. The species observed in the plasma mixture were CO₂, CO₂⁺, CN, CO, CO⁺, O₂, O₂⁺, N₂, N₂⁺, NO, C⁺, Ar and Ar⁺. At the pressure range in the present study, the species observed do not change their intensity due to an increase in the pressure and they separate in two groups according to their emission intensity: the band of the first group (CO₂, CO₂⁺ and CN) is approximately a factor of 3 more intense than that of the second group (CO, CO⁺, O₂, O₂⁺, N₂, N₂⁺, NO, C⁺, Ar and Ar⁺). The behavior of the emission intensities may be correlated to the constant ion density and electron temperature measured. Also, we observed the same constant behavior in the ratios of the neutral and positive species intensities to that of the N₂ intensity, as a function of pressure. This may suggest that the different rate coefficients and cross sections of elastic collision, excitation and de-excitation of electronic or vibrational levels, inelastic and superelastic collisions of electrons with the gas phase and products, neutral–neutral interactions, resonant charged transfer processes, recombination, to mention some, to produce these species change in the same proportion, as a function of the pressure to keep the relative ratios of the species almost constant.     

介质阻挡均匀大气压氮气放电特性研究

基于一维流体力学模型，对介质阻挡均匀大气压氮气放电特性进行了数值计算研究.模型中考虑了氮气中主要的电离、激发过程，所包含的粒子种类为e，N₂，N⁺₂，N⁺₄，N₂(a¹∑^-_u)，N₂(A³∑⁺_u).模拟结果显示，氮中的放电具有低气压下汤生放电的特性.放电电流幅度较小，放电过程中气体电压变化缓慢，电子密度远低于离子密度，而且最大值出现在阳极，电子不能在放电间隙中被俘获，不存在中性等离子体区，气体中的电场趋于线性变化.亚稳态N₂(A³∑⁺_u)和N₂(a¹∑⁺_u)在整个放电空间都具有非常高的密度，比电子密度高三个量级以上，亚稳态密度的最大值出现在阳极，这样的分布决定了放电的空间结构.放电所需的种子电子主要由亚稳态之间潘宁电离提供，这种机理使放电的电离水平较低，导致氮气中的放电只能是汤生放电.随着放电参数的变化，多电流峰放电也可在氮气中获得. 关键词： 大气压均匀放电 介质阻挡放电 数值模拟 氮气     

H+ and He2 + induced W L X‐rays intensity ratios: part I,Si(Li) data and EDS high resolution insight

Recently, the authors have presented experimental evidences that for some energy windows, proton‐induced W L shell X‐rays intensity ratios of transitions to L₁ subshell depend on the ion beam energy and on the chemical species even after known matrix effects are subtracted. These results, which put in question the assumption of the invariance of the relative intensity of X‐ray transitions to the same atomic subshells, are further exploited in this work, where more data for three different W compounds (W, Li₂WO₄ and P₂O₅.24WO₃.xH₂O) are presented followed by a detailed study using an ultra‐pure (99.995%) W thick foil, used to avoid any possible target contamination interference on the results. Samples were irradiated by H⁺ beams in various conditions in the energy range between 0.25 and 2.38 MeV and by He^{2 +} beams having energies between 3.5 and 5.0 MeV. Spectra were collected using the Si(Li) detector at CTN 2.5 MV Van de Graaff standard Proton‐Induced X‐ray Emission (PIXE) set‐up as well as using the Energy Dispersive Spectrometry (EDS) high resolution X‐ray microcalorimeter spectrometer (XMS) at CTN 3.0 MV Tandetron accelerator high resolution high energy (HRHE) PIXE end station. Results were normalized to the theoretical intensity ratios and plotted as function of the ratio of collision characteristic times allowing the comparison of H⁺ and He^{2 +} results. W L X‐rays intensity ratio variations are presented and compared with theoretical expected results. Radiative Auger emission transitions observed in an EDS high resolution XMS spectra are shown to probably play a crucial role in the highly unexpected results obtained for intensity ratios of transitions to the same subshell. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical study of the effect of gas flow in low pressure inductively coupled Ar/N<Subscript>2</Subscript> plasmas

The effect of gas flow in low pressure inductively coupled Ar/N₂ plasmas operating at the rf frequency of 13.56 MHz and the total gas pressure of 20 mTorr is studied at the gas flows of 5–700 sccm by coupling the plasma simulation with the calculation of flow dynamics. The gas temperature is 300 K and input power is 300 W. The Ar fractions are varied from 0% to 95%. The species taken into account include electrons, Ar atoms and their excited levels, N₂ molecules and their seven different excited levels, N atoms, and Ar⁺, N⁺, N₂ ⁺, N₄ ⁺ ions. 51 chemical reactions are considered. It is found that the electron densities increase and electron temperatures decrease with a rise in gas flow rate for the different Ar fractions. The densities of all the plasma species for the different Ar fractions and gas flow rates are obtained. The collisional power losses in plasma discharges are presented and the effect of gas flow is investigated.     

Scaling and performance of CO2 lasers at supra-atmospheric pressure

The performance of a compact uv photo-preionized TE laser is studied in the pressure range 1–5 bar. As the pressure is increased, the laser pulse shape is little altered, but both the peak power and the total output pulse energy increase significantly with pressure, even for constant input electrical energy. For various gas mixtures and excitation source capacitors the measurements suggest approximate output energy scaling with the product of the source charge per unit electrode area [C.m^–2] and the molecular partial pressure [CO₂+N₂+CO]. This is explained in terms of the pressure-dependent discharge impedance. An input-energy-related discharge instability limits the optimum laser pressure to 1.5–2.5 bar, and we show that, at constant input energy, the instability boundary depends on the molecular partial pressure alone. The pre-ionization photo-electron yield varies negligibly with pressure, but the discharge tolerance to added oxygen decreases asp ^–3 top ^–4, dependent on gas mixture. Nevertheless sealed operation for >10⁵ shots has been obtained with a 5% CO₂5% CO3% N₂2% H₂85% He gas mixture at a total pressure of 5 bar.     

In situ auger electron spectroscopy applied to the study of chemisorption and diffusion during reactive implantation of titanium into iron

Auger electron spectroscopy (AES) has been used to monitor the composition of Fe and FeTi surfaces, in situ, during Ti, Fe and Ar ion implantation in the presence of CO, C₂H₂, CH₄, N₂ and NH₃ gases at pressures of approximately 10^-6 Torr. Implantation-induced subsurface carburization was observed for Ti⁺ implantation into Fe in CO and C₂H₂ and for Ar⁺ implantation into an FeTi alloy in CO. In these cases, Ti carbide-like bonding of carbon at the surface preceded the subsurface carburization. Carburization was not observed when CH₄ gas was present during Ti implantation of Fe nor was it observed when CO gas was present during Fe self-implantation. Subsurface nitridation, similar to but not exactly analogous to the carburization, occurred during Ti⁺ implantation of Fe with NH₃ gas present. Nitridation did not occur with N₂ gas present even though some nitrogen was adsorbed. The influences of surface chemical reactions are considered, along with other chemical and physical arguments, in proposing an interpretation of these results.     

[Co(CO2)n]+团簇的红外解离光谱实验和理论研究

本文利用红外光解离光谱研究了一价钴阳离子与二氧化碳之间的相互作用. 通过密度泛函理论计算得到[Co(CO₂)_n]⁺团簇的几何结构,并且模拟了它们的振动光谱与实验数值进行比较. 研究结果表明,在[Co(CO₂)_n]⁺(n=2∽6)团簇中,钴阳离子通过电四极矩静电作用以端点结合的方式与二氧化碳中的氧原子结合在一起. 团簇的红外光谱都集中在二氧化碳反对称伸缩的波数附近,并且随着团簇尺寸的变化出现蓝移,最后把[Co(CO₂)_n]⁺的红外光解离光谱与稀有气体贴附的[Co(CO₂)_n]⁺-Ar的红外光解离光谱进行了比较.     

Infrared multiple-photon dissociation of ion-molecules in a beam

First experiments on the infrared multiple-photon dissociation with ion molecules in beams have been carried out. A mass selected beam of SF⁺₅ parent ions is crossed with a focused pulsed CO₂ laser beam. Using mass spectroscopy for the detection, fragmentation into SF⁺₄ and SF⁺₃ has been observed. The relative yield and power dependence have been measured. Features of the MPD of ions in beams are discussed.     

On the nature of ion bombardment-induced phase transformations in films of transition metals

Abstract

Electron diffraction studies have been made of polycrystalline Ni films irradiated with well separated beams of ions of different nature, namely ions of inert (He⁺, Ne⁺, Ar⁺, Kr⁺, Xe⁺) and reactive (N⁺ and O⁺) gases. The Ni films were prepared under vacuum conditions (P? 3·10^?6Pa during evaporation) preventing an appreciable contamination of the films with impurities. The samples were irradiated at T? 300 K with ion beams of energies from 10 to 100 keV in the dose range between 5·10¹⁶ cm^?2 and the value leading to sample destruction.

Irradiation with noble gas ions revealed no phase transitions in the Ni films. A similar result was obtained in irradiation of Fe and Cr films with He⁺ ions. The bombardment of Ni films with reactive gas ions does cause changes in the lattice structure of the samples under study, depending on the nature of the bombarding ions. The N⁺ ion bombardment gives rise to the hcp phase with the lattice parameters typical of the Ni₃N compound, and the O⁺ ion bombardment results in the fcc phase with the NiO-type parameter.

The conclusion is drawn on the chemical origin of the phase transformations in the Ni films under ion bombardment. The necessity of revising the concept about the polymorphous nature of phase transformations induced in the films of transition metals by ion bombardment is substantiated.     

Negative molecular ion formation and work function changes: Experimental results for CO and CO2 scattering from nickel (+potassium) surfaces

The scattering of CO⁺ and CO⁺₂ at grazing incidence from Ni(111)+K and clean Ni(111), Ni(110) surfaces produces CO, CO₂ and dissociated species. The observation of negative species O⁻ and CO⁻₂ is strongly dependent on the K coverage or work function of the surface. The dissociation of CO⁺ (CO) is weakly changed by the presence of K, whereas in the CO⁺₂ (CO₂) case dissociation via CO⁻₂ → CO + O⁻ is strongly increasing with K coverage.     

Design and operation characteristics of a high power transverse flow pulser sustained cw CO2 laser

A transverse flow, transverse discharge cw CO₂ laser in which de discharge is sustained by employing high repetition rate high voltage pulses has been developed. Pulser sustained discharge through electrodes of innovative design provided uniform excitation at electrical input power densities more than 10 W/cc. Laser output power more than 2.5 kW was obtained in a laser gas mixture consisting of 0.5 mbar of CO₂, 16 mbar of N₂ and 38.5 mbar of He. Design details and operational characteristics of this laser are presented.     

Elucidation of mechanisms of ion—molecule reactions by ICR

The mechanisms of some positive and negative ion—molecule reactions have been determined by a quantitative ion cyclotron double-resonance technique with isotopically labelled reactants. Four types of mechanism were elucidated: charge transfer, ion transfer, atom abstraction and complex formation. Thus, charge transfer is shown to be the only major reaction channel in the systems CO⁺ + CO₂, SO⁻ + SO₂, and O⁻ + NO₂, while in the system Cl₂⁻ + Cl₂ CO ion transfer is the main mechanism, and complex formation with oxygen-atom scrambling is the major channel in O⁻ + N₂O.The experimental results also point out cases where the reaction proceeds by more than one mechanism as, for example, in the reaction system H₂O⁺ + H₂O.     

Diagnostics of a Compact Discharge-Pumped XeCl Laser with BCl3 Halogen Donor

The results of a comprehensive study of a compact UV-preionized XeCl laser are presented. The subjects of this study were: discharge voltage and current measurements, dye laser probing of the active medium, and the mass spectrometry of gas mixture degradation products. It is shown that the gas lifetime was significantly improved when the laser was operated with BCl₃ as a halogen donor instead of commonly used HCl. By the dye laser absorption and gain probing, the temporal and spatial dependences of the densities for several plasma components, Ne*, Xe*, Xe⁺*, Cl^–, XeCl* and of ground state boron atoms were measured. Some aspects of plasma kinetics for uniform and constricted phases of the discharge are discussed. By the mass spectrometry of gas mixture degradation products on long-term operation of the laser device several gaseous (N₂, O₂, CO₂, H₂O, C₂H₄) and solid (NiCl₂, H₃BO₃) products were detected in the laser chamber. NH₄Cl was determined to be a stable fraction of the deposits on optics surfaces. The reasons for the improvement of gas lifetime for BCl₃-containing gas mixtures are discussed.     

Stark broadening of some C(I) and N(I) vacuum ultraviolet lines

Stark widths of 11 C(I) lines and 3 N(I) multiplets were measured in the wavelength range 1310 Å ⩽ λ ⩽ 2583 Å. Gas mixtures (Ar+CO₂, Ar+N₂ and Ar+CO₂+N₂) were used for production of plasmas in a wall-stabilized arc under atmospheric pressure. Results were provided by five experiments (T varies from 13,370 to 13,950 K and N_e from 1.13 x 10¹⁷ cm^-3 to 1.35 x 10¹⁷ cm^-3. Stark shifts were measured for 2 C(I) lines and the ion-broadening parameter A was evaluated for 7 C(I) lines using different methods. The Stark widths and shifts are generally in reasonable agreement with other experimental or theoretical values, while the estimated A values are much higher than those predicted by Griem's theory.     

Numerical study on characteristics of nitrogen discharge at high pressure with induced plasma

Based on the fluid theory of plasma, a model is built to study the characteristics of nitrogen discharge at high pressure with induced argon plasma. In the model, the spices such as electron, N₂⁺, N₄⁺, Ar⁺, and two metastable states (N₂ (A³∑_u⁺), N₂ (a¹∑_u^-)) are taken into account. The model includes particle's continuity equations, electron's energy balance equation, and Poisson equation. The model is solved with a finite difference method. The numerical results are obtained and used to investigate the effect of time taken to add nitrogen gas and initially-induced argon plasma pressure. It is found that lower speeds of adding the nitrogen gas and varying the gas pressure can induce higher plasma density, and inversely lower electron temperature. At high-pressure discharge, the electron density increases when the proportion of nitrogen component is below 40%, while the electron density will keep constant as the nitrogen component further increases. It is also shown that with the increase of initially-induced argon plasma pressure, the density of charged particles increases, and the electron temperature as well as the electric field decrease.