Rare Gas Mixtures Research Articles

Генерация в смесях инертных газов с фтором при накачке объемным диффузным разрядом

Генерация в смесях инертных газов с фтором при накачке объемным диффузным разрядом

New Insights from an Empirical Multi-Property Interatomic Potential and Predicted Collision-induced Light Scattering Spectra for Hg-rare Gas van der Waals Complexes

Abstract An empirical interatomic potentials for the interaction of Hg with inert gases are developed by simultaneously fitting the modified Tang–Toennies (MTT) potential function to spectroscopic data, thermo-physical and transport properties over a wide temperature range. The two-body anisotropic collision-induced light scattering spectra of mercury vapor with mixtures of rare gas has never been measured. The lineshape calculations through quantum mechanical methods have been used to predict a reasonable experimental spectra of the induced light scattering at T = 500 K using these models of potentials and a suitable models for the anisotropy. The results of calculations show that these models of the interactions and the induction mechanisms are the most accurate models reported to date for these mixtures.

Controlling VUV photon fluxes in low-pressure inductively coupled plasmas

Low-pressure (a few to hundreds of millitorrs) inductively coupled plasmas (ICPs), as typically used in microelectronics fabrication, often produce vacuum-ultraviolet (VUV) photon fluxes onto surfaces comparable to or exceeding the magnitude of ion fluxes. These VUV photon fluxes are desirable in applications such as sterilization of medical equipment but are unwanted in many materials fabrication processes due to damage to the devices by the high-energy photons. Under specific conditions, VUV fluxes may stimulate etching or synergistically combine with ion fluxes to modify polymeric materials. In this regard, it is desirable to control the magnitude of VUV fluxes or the ratio of VUV fluxes to those of other reactive species, such as ions, or to discretely control the VUV spectrum. In this paper, we discuss results from a computational investigation of VUV fluxes from low-pressure ICPs sustained in rare gas mixtures. The control of VUV fluxes through the use of pressure, pulsed power, and gas mixture is discussed. We found that the ratio, β, of VUV photon to ion fluxes onto surfaces generally increases with increasing pressure. When using pulsed plasmas, the instantaneous value of β can vary by a factor of 4 or more during the pulse cycle due to the VUV flux more closely following the pulsed power.

РОЗРАХУНОК СЕПАРАЦІЇ БАГАТОМПОНЕНТНИХ СУМІШЕЙ НА МЕМБРАНАХ З ПОЛИХ ВОЛОКОН

Calculation model presented in this work allows to determine key parameters of the multicomponent gas mixter separation process. Computing algorithms for the schemes of concurrent and countercurrent flow organization were formulated. The results of calculations for the cases of air separation and multicomponent mixture of rare gases were observed. Experimental data was compared with the results obtained from mathematical model.

Far-infrared collision-induced absorption in rare gas mixtures: Quantum and semi-classical calculations

We compare calculations of the translational collision-induced spectra and their integrated intensities of both He–Ar and Ne–Ar collisional complexes, using the quantum mechanical and a semiclassical formalism. Advanced potential energy and induced dipole functions are used for the calculations. The quantum method used is as described previously [L. Frommhold, Collision-induced Absorption in Gases (Cambridge University Press, 1993 and 2006)]. The semiclassical method is based on repeated classical atom-atom scattering calculations to simulate an ensemble average; subsequent Fourier transform then renders the binary absorption coefficient as a function of frequency. The problem of classical calculations is the violation of the principle of detailed balance, which may be introduced only artificially in classical calculations. Nevertheless, it is shown that the use of classical trajectories permits a fairly accurate reproduction of the experimental spectra, comparable to the quantum mechanical results at not too low temperatures and for collisional pairs of not too small reduced mass. Inexpensive classical calculations may thus be promising to compute spectra also of molecular pairs, or even of polyatomic collisional pairs with anisotropic intermolecular interactions, for which the quantum approach is still inefficient or impractical.

Evaluation of the grand-canonical partition function using expanded Wang-Landau simulations. III. Impact of combining rules on mixtures properties

Combining rules, such as the Lorentz-Berthelot rules, are routinely used to calculate the thermodynamic properties of mixtures using molecular simulations. Here we extend the expanded Wang-Landau simulation approach to determine the impact of the combining rules on the value of the partition function of binary systems, and, in turn, on the phase coexistence and thermodynamics of these mixtures. We study various types of mixtures, ranging from systems of rare gases to biologically and technologically relevant mixtures, such as water-urea and water-carbon dioxide. Comparing the simulation results to the experimental data on mixtures of rare gases allows us to rank the performance of combining rules. We find that the widely used Lorentz-Berthelot rules exhibit the largest deviations from the experimental data, both for the bulk and at coexistence, while the Kong and Waldman-Hagler provide much better alternatives. In particular, in the case of aqueous solutions of urea, we show that the use of the Lorentz-Berthelot rules has a strong impact on the Gibbs free energy of the solute, overshooting the value predicted by the Waldman-Hagler rules by 7%. This result emphasizes the importance of the combining rule for the determination of hydration free energies using molecular simulations.

A Simplified Numerical Study of the Kr/Cl2 Plasma Chemistry in Dielectric Barrier Discharge

In this paper, the generation of excimers and exciplexe radiation in mixtures of rare gas with halogen by homogeneous dielectric barrier discharge (DBD) is investigated. The typical characteristics of an excilamp based on KrCl* exciplexe molecules and the kinetic processes for the formation and the decay of this molecules in the Kr/Cl2 mixture are studied. The computer model developed is based on the Kr/Cl2 mixture chemistry, the equivalent electric circuit and the Boltzmann equations. The importance in the kinetic processes of some species such as the metastable state of Krypton (Kr*(3P0,2)) and the negative ion of chloride (Cl−) is considered. The results illustrate the time variations of charged species (ne, Kr+, Cl−, Cl+, Cl+2, Kr+2), excited atoms and molecules (Kr*(3P0,2), Kr*(3P1), Cl*, Cl*2), the excimers (Kr*2, KrCl*(B), KrCl*(C), Kr2Cl*) and the UV photon concentrations (in 222 nm, 235 nm, 258 nm and 325 nm range). The effects of chlorine concentration and the total gas pressure in the Kr-Cl2 discharge on the electric parameters and radiation emissions are investigated.

Formation and Reaction of Oxidants in Water Ice Produced from the Deposition of RF-Discharged Rare Gas and Water Mixtures

Thin-film water ice samples are doped with reactive species, such as OH and HO(2) and both singlet and triplet O atoms, which are produced in a discharge, in order to trace the reactions that form oxidants such as H(2)O(2) and O(2), in relation to the icy surfaces of satellites in the outer solar system. The present experiments show that, while reactions of OH formed in the discharge produce H(2)O(2), the majority of H(2)O(2) is formed via other pathways, with reactions of singlet O atoms being the most likely candidate. Strong evidence comes from thermal processing of the ices and using the matrix-isolation technique to quantify the OH abundance in the discharge, which is far below the required amount to account for the relatively high H(2)O(2) abundances measured. Reactions of HO(2) are also of possible significance but appear to be less dominant in the formation of H(2)O(2) than singlet O.

Enhanced technique for measuring collisional quenching rate coefficients in rare-gas mixtures

We set forth an improved technique for the investigation of collisional quenching in high-pressure rare-gas mixtures, which relies on the absorption probing measurements of the decay rates of the excited states of these gases in the afterglow of a fast electron beam discharge. We determined more precisely the rate coefficients of the plasmochemical reactions Xe* + Xe + Ar → Xe2* and Xe*+Ar → products + Ar for the metastable (6s[3/2]20) and resonance (6s[3/2]10) levels of atomic Xe investigated in our earlier work. The rate coefficients for the reactions Xe(6s[3/2]20) + 2Ar → ArXe* + Ar((7.2±1.4)×10-36 cm6 s-1) and Xe(6s[3/2]10) + 2Ar → ArXe*((5.3±2.4)×10-36 cm6 s-1) were measured for the first time.

UV and VUV Excilamps with High Peak Power

Design and output parameters of pulse sources of spontaneous UV and VUV radiation based on the non-equilibrium discharge plasma of high-pressures gases are presented in this paper. Emission characteristics of plasma of diffuse discharge preionized by runaway electrons and barrier discharge in rare gases and mixtures of rare gases with halogens have been investigated in gases at pressure up to 15 atm at excitation power up to 100 MW/cm3. The radiant power from 4π solid angle up to 1.2 MW with pulse duration of - 8 ns on Xe2* dimers were obtained at 12 atm pressure xenon. A high-voltage (up to 150 kV) short pulsed excitation mode was tested on dielectric barrier discharge KrCl- and XeBr-excilamps.

Absorption spectra of e-beam-excited Ne, Ar, and Kr, pure and in binary mixtures

A technique using the broadband emission of a laser plume as probe radiation is applied to record UV-visible (190-510 nm) absorption spectra of Ne, Ar, and Kr, pure and in binary mixtures under moderate e-beam excitation up to 1 MW/cm(3). In all the rare gases and mixtures, the absorption spectra show continuum related to Rg(2) (+) homonuclear ions [peaking at λ∼285, 295, and 320 nm in Ne, Ar, and Kr(Ar/Kr), respectively] and a number of atomic lines related mainly to Rg(∗)(ms) levels, where m is the lowest principal quantum number of the valence electron. In argon, a continuum related to Ar(2) (∗) (λ∼325 nm) is also recorded. There are also trains of narrow bands corresponding to Rg(2) (∗)(npπ (3)Π(g))←Rg(2) (∗)(msσ (3)Σ(u) (+)) transitions. All the spectral features mentioned above were reported in literature but have never been observed simultaneously. Although charge transfer to a homonuclear ion of the heavier additive is commonly believed to dominate in binary rare-gas mixtures, it is found in this study that in Ne/Kr mixture, the charge is finally transferred from the buffer gas Ne(2) (+) ion not to Kr(2) (+) but to heteronuclear NeKr(+) ion.

Ethanol Monomers and Dimers Revisited: A Raman Study of Conformational Preferences and Argon Nanocoating Effects

The gauche-trans conformational distribution in ethanol can be determined from the OH stretching Raman spectrum of seeded supersonic jet expansions, which thus provides a sensitive conformational thermometer. Depending on the rare gas mixture, one, two or four ethanol dimer conformations are abundant. Their conformational assignment is facilitated by the observation of hydrogen bond acceptor modes, which have similar Raman cross sections but much inferior infrared intensities than donor modes. Ethanol monomers and dimers can be progressively Ar-coated, and the resulting spectra may be compared with those in a bulk argon matrix. The low frequency range of torsional transitions provides some evidence for conformation-changing transitions in Raman jet spectra.

Spectroscopic studies of molecular ions and their dissociation dynamics by the threshold photoelectron-photoion coincidence

In a recently built threshold photoelectron-photoion coincidence spectrometer, the photoelectrons and photoions were velocity-focused simultaneously to effectively improve the collection efficiency and the energy resolution. With this spectrometer we measured the threshold photoelectron spectra (TPES) and the mass-selected threshold photoelectron-photoion coincidence mass spectra (TPEPICO-MS) of NO molecules, as well as Xe/Ar/Ne rare gas mixtures. The ionization potential of NO was accurately determined, and the vibrationally resolved TPES spectra of the X1Σ+, c3Π and B1Π states of NO+ were obtained. Subsequently, the dissociation dynamics of the c3Π state of NO+ were studied by measuring the TPEPICO-MS and fitting the time-of-flight profile of the N+ fragment. The overall kinetic energy released from dissociation was determined to be 0.717 eV.

Moments of hyper-Rayleigh spectra of selected rare gas mixtures

In this work we have analyzed spectral moments characterizing properties of the collisionally hyper-Rayleigh scattered light. This is a supplementary study undertaken in order to complete the series of our previously published papers on the collisional hyper-Rayleigh scattering spectral profiles. In order to evaluate the moments we have extended the theory so that it could embrace the (hyper)polarizabilities of higher rank. Using the expressions developed on the grounds of the theoretical principles and applying appropriate computational methods with ab initio hyperpolarizability values as an input, we have obtained desirable moment values for three diatomic noble gas systems: HeNe, HeAr, and NeAr, at several temperature points. The semiclassical and the quantum treatments have been taken into account, and the moments were calculated both from the sum rule method as well as from the spectral profiles. The results were compared and discussed.

Interaction induced absorption of rare gas mixtures physisorbed on nanotubes and fullerenes – Computer simulation study

We have performed a molecular dynamics simulation of the exohedral complexes of rare gas mixtures physisorbed on the surface of nanotubes and fullerenes. We identified liquid-like phases and calculated interaction induced dipolar absorption spectra in the studied systems.

Electron diffraction study of two-component clusters Ar–Kr: Features of the nucleation, growth mechanisms, and structural states

The structure and growth processes of clusters formed in adiabatically expanding supersonic jets of gaseous mixtures of rare gases are investigated for the first time. Ar–Kr mixtures containing 0.1, 0.5, 2.5, and 20% Kr atoms are studied at temperatures and pressures of the gaseous mixtures at the nozzle entrance T0=100 and 120K and P0=0.15–2.5atm. The experiments give a number of results that shed light on the picture of cluster formation in Ar–Kr gaseous mixtures. It is established that the mechanisms of cluster formation depend substantially on the krypton impurity concentration and the temperature of the gaseous mixture at the nozzle entrance. The evolution of the character of the cluster formation, from homogeneous to heterogeneous, is traced. The intensification of rare-gas cluster growth by an impurity is revealed for the first time, and the concentration dependence of the transition from a process of formation of solid solutions to the formation of clusters containing only impurity krypton is traced.

Quantitative investigation of xenon consumption and recovery during low pressure ac discharges in rare gas mixtures

Xe gas in low pressure discharge tubes (20 mm diameter, 800 mm length, commercial hollow electrodes at 650 mm distance, 50 Hz, 60 mA) was quantitatively determined using quadrupole mass spectrometry (QMS). The temporal Xe gas evolution over 5 to 20 h of gas discharge revealed the consumption and recovery of Xe gas to be dependent on the prevailing gas composition. The net consumption in the standard Xe/He(2.0/98) gas mixture was much faster than that in Xe/Kr/He(1.9/93/5). Xe recovery during discharges in ‘preloaded’ tubes refilled with pure gas revealed an increasing efficiency in the sequence He < Ne < Ar < Kr, being most efficient with the first refilling. The temporal evolution of Xe gas was simulated by a kinetic model based on two equilibria between Xe gas and Xe incorporated in the glass tube walls (1) and the electrodes (2). It is concluded that the experimental setup, the elaborated measurement procedures and the kinetic model provide a good basis for further quantitative investigations. The net Xe gas consumption during discharge, which is presently limiting the tube lifetime, may be reduced particularly by improving the discharge gas composition and the electrode materials.

Planar excilamp on rare gas chlorides pumped by a transverse self-sustained discharge

The design and parameters of a UV—VUV spontaneous radiation source — an excilamp operating on chlorides of rare gases ArCl*, KrCl* and XeCl* in the wavelength range 175—308 nm are presented. The Ne—Xe(Kr, Ar)—HCl mixtures were excited by a high-pressure self-sustained discharge with spark preionisation. It is shown that upon pumping mixtures of rare gases and halogens by a transverse discharge, the intensities of the B—X emission band of molecules ArCl*, KrCl* and XeCl* are comparable and up to 90% of the emission energy of excilamps can be concentrated in the UV region. The peak UV power density at 222 and 308 nm on the output window of the excilamp was ∼2 kW cm-2 for the pulse energy up to ∼ 3 mJ. The output emission energy of the excilamp at 175 nm achieved ∼0.6 mJ and the peak power density was ∼0.4 kW cm-2.

Thermodynamic Stability of Fluid−Fluid Phase Separation in Binary Athermal Mixtures: The Role of Nonadditivity

We studied the thermodynamic stability of fluid-fluid phase separation in binary nonadditive mixtures of hard-spheres for moderate size ratios. We are interested in elucidating the role played by small amounts of nonadditivity in determining the stability of fluid-fluid phase separation with respect to the fluid-solid phase transition. The demixing curves are built in the framework of the modified-hypernetted chain and of the Rogers-Young integral equation theories through the calculation of the Gibbs free energy. We also evaluated fluid-fluid phase equilibria within a first-order thermodynamic perturbation theory applied to an effective one-component potential obtained by integrating out the degrees of freedom of the small spheres. A qualitative agreement emerges between the two different approaches. We also addressed the determination of the freezing line by applying the first-order thermodynamic perturbation theory to the effective interaction between large spheres. Our results suggest that for intermediate size ratios a modest amount of nonadditivity, smaller than earlier thought, can be sufficient to drive the fluid-fluid critical point into the thermodinamically stable region of the phase diagram. These findings could be significant for rare-gas mixtures in extreme pressure and temperature conditions, where nonadditivity is expected to be rather small.

Low-pressure mercury-free plasma light sources: experimental and theoretical perspectives

The replacement of mercury in conventional fluorescent lamps by other components is highly desirable for environmental reasons. This paper gives a short review of new types of mercury-free plasma light sources operating at low pressure. In particular, the features of cylindrical glow discharges in rare-gas mixtures including xenon are discussed, focusing on the generation of the 147 nm resonance radiation of xenon and its transition into visible light by new phosphors with sufficient efficiency. Laser absorption and vacuum ultraviolet emission spectroscopy are applied for several rare-gas mixtures to reveal the contributions of the different gas components and their excited states to the power balance and radiation output. The experimental research is assisted by theoretical studies applying self-consistent hybrid models of the cylindrical column plasma. The good agreement between experimental and theoretical results obtained at selected discharge conditions makes it possible to predict optimum discharge parameters by means of extensive model calculations. It is demonstrated that about half of the efficacy of a mercury fluorescent lamp can be reached if the rare-gas discharge is operated at pressures below 100 Pa.