Showing papers on "Argon published in 1971"

Liquid argon: Monte carlo and molecular dynamics calculations

Abstract: Thermodynamic properties of liquid argon are calculated by Monte Carlo and molecular dynamics techniques, using accurate pair-potential functions determined from the properties of solid and gaseous argon, together with the Axilrod-Teller three-body interaction. Satisfactory techniques for evaluating three-body contributions to thermodynamic properties without excessive requirements of computer time are described. Quantum corrections are included. Agreement with experiment is excellent: the best pair and triplet potentials give an excellent description of the properties of solid, gaseous and liquid argon.

Orientational correlations in molecular liquids by neutron scattering. Carbon tetrachloride and germanium tetrabromide

Abstract: Neutron diffraction structure factors for liquid carbon tetrachloride and germanium tetrabromide have been measured and are analysed to determine the molecular centre positional correlation function allowing for molecular angular correlation. The analysis shows quite unequivocally that appreciable molecular orientational correlations must exist in these liquids but that it may only extend to nearest neighbours. This is thought to cause the pair distribution function for the molecular liquids to be markedly different from that for atomic liquids such as argon and the difference is discussed.

Perturbation Theory for Nonuniform Fluids: Surface Tension

Abstract: The Barker—Henderson perturbation theory for uniform fluids is extended to include nonuniform fluids. The surface tension for a Lennard‐Jones fluid is calculated by minimizing the free energy in the surface layer. Comparison with the corresponding experimentally obtained values of surface tension for argon shows that excellent agreement is obtained in the case of a Lennard‐Jones 12–6 fluid with values of the potential parameters determined from the second virial coefficient.

The intermolecular pair potential of argon

Abstract: The absorption spectrum of diatomic argon in the 780–1080 A region has been re-analysed using the Rydberg-Klein-Rees method. The results of this study together with an analysis of the equilibrium and transport properties of gaseous argon have been used to establish a potential energy function which is consistent with all the available data. The dissociation energy of the argon dimer is estimated at some 7 per cent greater than that indicated in previous analyses of the spectroscopic data.

The elastic scattering of electrons by nitrogen, neon, phosphorus and argon atoms

Abstract: Results are presented for the elastic scattering of electrons by nitrogen and phosphorus atoms in the energy range 0-1 ryd, and by neon and argon atoms in the energy range 0-4 ryd. Calculations have been carried out in a single state approximation but include both exchange and polarization effects. The polarization potential has been determined using the polarized orbital method of Temkin. Results are in good agreement with experiment for neon and argon but it is found that the single state approximation is not sufficient for a correct representation of the nitrogen 3P resonance.

Ultraviolet Absorption Spectra of Transition Metal Atoms in Rare-Gas Matrices.

Abstract: Absorption spectra from 2200 to 4000 A have been obtained of chromium, manganese, iron, cobalt, copper, nickel, tin, and palladium atoms trapped in argon matrices at 4.2°K and of iron and copper in krypton and xenon matrices at 4.2 and 20°K. Observed transitions were found to correlate with gas phase transitions, under the assumption of a matrix and atomic configuration dependent shift of the atomic transitions. Energy shifts of the transitions were inversely proportional to matrix atom size. Configurations with an odd number of 3d electrons were shifted less than those with an even number. Within a given configuration, the transitions at higher energies were shifted more than those at lower energies. A Lennard‐Jones potential was unsuccessful in generating the observed energy shifts caused by the interaction between the trapped atom and the matrix. Atom diffusion and resultant aggregation within the matrix both during the condensation of the solid from the gas phase and during warming was found to be a s...

Gas‐Breakdown Dependence on Beam Size and Pulse Duration with 10.6‐μ Wavelength Radiation

Abstract: Gas‐breakdown threshold measurements with 10.6‐μ wavelength radiation have been made in air, argon, and helium. The threshold was determined with 50‐ and 200‐nsec‐duration pulses and was found to depend on the peak intensity of the pulse, showing that the breakdown process is a balance between the rate of energy absorption and some rate of energy loss. The threshold also decreased as the focal spot size was increased from 10−2 to 10−1 cm, showing that the loss process is reduced for larger beam sizes. The threshold varied inversely with gas pressure from 0.1 to 4 atm, which is evidence that the loss mechanism is not electron diffusion but must be some other process which exhibits a beam‐size dependence and is apparently weakly dependent on gas pressure.

Laser beam induced breakdown in helium and argon

Abstract: Measurements of the laser radiation power density Pth required to cause ionization growth and breakdown in the pressure ranges 1800 less-than-or-eq, slant p less-than-or-eq, slant 16 000 torr and 200 less-than-or-eq, slant p less-than-or-eq, slant 20 000 torr have been made in helium and argon, respectively. Three Q-switched lasers (ruby, Nd, and dye) were used to investigate the dependence of Pth upon pressure, radiation frequency ω and flash duration 2τ. Values of Pth lie between 1011 W cm−2 at low pressures and 1010 W cm−2 at high pressures. The experimental results are compared with expressions based upon an extrapolation of classical microwave breakdown theory to optical frequencies. Reasonably good agreement between measured and calculated threshold intensities is obtained. The interaction between the laser radiation and the gas proceeds via inelastic electron-atom collisions and photoionization. Three pressure regimes may be specified: a low pressure regime in which the threshold intensity and ionization growth rate are governed principally by diffusion losses, an intermediate regime in which the laser flash duration is the dominant factor and a high pressure regime where electron-ion recombination controls the rate of ionization growth and the onset of breakdown. The ionization rate is shown to be proportional to the laser beam intensity and, in the intermediate regime, the threshold intensity Pth is proportional to ω2/τ at constant pressure. A value for the electron-atom momentum transfer collision frequency of about 3-4×109p and of about 5 eV for the electron mean energy is derived for argon at the onset of breakdown.

ESR and Optical Spectroscopy of the AlO Molecule at 4°K; Observation of an Al Complex and Its Interaction with Krypton

Abstract: The AlO molecule in its 2Σ ground state has been trapped in neon, argon, and krypton matrices at 4°K, and its ESR and optical spectra (infrared, visible, and uv) measured. In neon its magnetic parameters are g‖ = 2.0015(3), g ⊥ = 2.0004(3), A‖( 27Al) = 872(1) MHz, and A ⊥ ( 27Al) = 713(1) MHz. Δg⊥ is in accord with the magnitude and sign of the spin‐doubling constant, γ0, determined in the gas phase. An infrared band at 924 cm−1 in neon (917 cm−1 in argon) is assigned to AlO: the large shift from the gas value of 965 cm−1 indicates a highly ionic molecule. Besides the strong B 2Σ←X 2Σ absorption bands near 4500 A, a progression of weak bands have been observed between 5500 and 11 000 cm−1 and assigned to the A 2πi←X 2Σ transition of AlO. ESR spectra of a weakly bonded Al–X molecule (where X may be Al2O) are observed in concentrated matrices; its magnetic properties are found to be very dependent upon the matrix gas used. In krypton matrices, 83Kr hfs is also observed but disappears (reversibly) above about 35°K, suggesting the formation of a metastable Kr–AlX complex.

Experimental Electron-Transfer Cross Sections for Collisions of Oxygen Ions in Argon, Nitrogen, and Helium at Energies of 7-40 MeV

Abstract: In the energy range 7-40 MeV, electron-capture and electron-loss cross sections have been determined for incident oxygen ions of charges +2 to +8 passing through argon, nitrogen, and helium. Thin-target conditions have been used in all cases. The analysis technique used to extract the cross sections allows relative cross sections to be obtained without consideration of the measurement of the gas target thickness. Multiple-transfer cross sections can thereby be determined reliably in the presence of cross sections three orders of magnitude greater. Double, triple, and quadruple transfers were observed. The electron-loss cross sections appear to pass through maxima when the velocity of the ion is in the vicinity of the velocity of the electron to be lost. Single-electron-capture cross sections have magnitudes as large as ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 40 MeV in argon, and depend on velocity approximately as ${V}^{\ensuremath{-}n}$, where $n$ lies between 3 and 6 for the heavy gases. The importance of capture from inner shells of the heavy gases is inferred by comparison with helium, in which $n$ is 8 or 9 and the cross sections are relatively small. The possibility of capture into the $K$ shell of the oxygen ion is suggested by the systematic differences observed in capture by +7 and +8 oxygen ions from capture by lower-charged ions.

Ionization of atoms and positive ions by electron and proton impact

Abstract: Earlier work on cross sections for the removal by electron and proton impact of a 2p or 3p electron from all neutral atomic systems in the sequences boron to neon and aluminium to argon is corrected. The new results are in much better agreement with experiment and the work has now been extended to obtain cross sections for certain isoelectronic positive ions.

Physical and Analytical Aspects of a Microwave Excited Plasma

Abstract: A microwave generator operated at about 100 W was used to maintain a narrow plasma (6 cm long × 0.3 mm diam) in flowing argon at atmospheric pressure. A tapered rectangular cavity was used to couple microwave power to the plasma discharge. Approximate temperature and electron density measurements of the plasma were made with and without water being nebulized; the temperature measurements indicated a lack of thermodynamic equilibrium. The plasma discharge was examined as a source of excitation in atomic emission spectrometry. An aerosol (produced by a system consisting of a pneumatic sprayer, a heated chamber, and a special condenser) was introduced into a plasma having an excitation and ionization temperature of about 5000 K and an electron density of about 1015 cm−3 at a position 0.3 mm from the center of the plasma. Limits of detection were measured for cadmium, gallium, indium, mercury, and zinc. Appreciable signals were also observed for calcium, lead, strontium, and boron, but no limits of detection were measured.

Nucleation and growth of rare-gas crystals

Abstract: An in situ electron microscope study has been made of the nucleation and growth of crystals of Ar, Kr and Xe grown by vapour deposition on to cooled graphite and amorphous carbon substrates. In particular, the saturation density of nuclei has been measured as a function of temperature and gas arrival rate. An important feature of this experiment is that the relevant parameters for these systems, such as surface diffusion and cluster energies, are known theoretically. Consequently, it has been possible to attempt an absolute check on nucleation theory. The theory is found to hold within a factor less than 3 for Kr and Xe, at all temperatures and arrival rates studied. Despite good agreement with the theory using the parameters for nucleation taking place on the bare graphite surface it is almost certain that growth of three-dimensional crystals does not occur until a strongly bound monolayer of adsorbed gas atoms approximately covers the substrate. The possible ways in which this feature may be understood are explored. In the case of argon the measured saturation densities (though less accurate) seem to be significantly below the theoretical values, and possible reasons for this are given.

Experimental investigation of transport properties of low-temperature plasma by means of electric arc

Abstract: The results of experimental investigations into the transport properties of low-temperature plasma carried out since 1960 in the Institute for High Temperatures of the USSR Academy of Sciences are summed up. Serving as the source of plasma was a well-stabilized dc arc. Properties of argon, nitrogen, air, and carbon dioxide plasma were studied at atmospheric pressure and at temperatures ranging from 7000° to 16 000°K. Data are presented on electric and thermal conductivity, total radiation of the aforementioned gases, as well as the results of investigations of argon viscosity up to 13 000°K.

Theoretical and Experimental Investigation of Pressure and Flow in Induction Plasmas

Abstract: An investigation of flow in and around an induction plasma is made as a continuation of an earlier theoretical study of magnetic pressure in an induction plasma. The existence of the predicted flow out from both ends of the plasma is confirmed by flow visualization and stagnation pressure measurements in and around an ambient‐pressure vortex‐free argon plasma in a 28‐mm tube. Flow visualization and gas velocities are obtained by photographing injected solid particles. For a 3‐kW argon discharge, the velocity at the upstream end of the plasma was 6±0.3 m/sec, and velocity varies linearly with discharge power over the range investigated (up to 4.7 kW). A model of the plasma is devised to enable the maximum velocity of magnetically pumped flow to be calculated from the static magnetic pressure. Pressures in the range of +0.04 in. water, caused by expansion thrust of hot gases from the torch, were measured behind the plasma. The thrust pressure is calculated and is in reasonable agreement with measurements. A...

Secondary electron ejection from metal surfaces by metastable atoms. ii. measurements of secondary emission coefficients using a gas cell method.

Abstract: For pt. I see ibid., vol.4, 1683 (1971). Secondary electron emission coefficients for thermal energy He(21S), He(23S), Ne(3P0.2) and Ar(3P0.2) metastable atoms incident on a gold surface have been measured using an improved gas cell absorption method. The coefficients for the same metastable species incident on chemically cleaned stainless steel and copper surfaces and on atomically clean cadmium and tungsten surfaces have been determined by calibration against the gold surface. The results indicate that rather larger coefficients than those used hitherto should be used for gold surfaces. The implications of the results presented for the measurement of metastable fluxes are examined.

Physical adsorption of gases on graphite

Abstract: Isotherms for adsorption of nitrogen, carbon monoxide, oxygen, argon, krypton and nitric oxide on graphitized and ungraphitized samples of carbon black (Sterling FT) have been measured at ca. 78 K. The adsorption isotherms for krypton and nitric oxide on the graphitized sample are sharply stepwise, indicating two-dimensional condensation. The isotherms for nitrogen, carbon monoxide, oxygen and argon have been used to plot the amount adsorbed on the ungraphitized sample against amount adsorbed on the graphitized sample at the same pressure. This provides a method for comparing multilayer adsorption of different gases on the two surfaces. Discontinuities in the “comparison plot” correspond to second-layer completion, and it is suggested that the influence of the surface extends beyond the second adsorbate layer. Anomalous behaviour of argon is attributed to a more open packed structure on the ungraphitized surface.

Charge state dependence of X-ray and Auger electron emission spectra for rare-gas atoms. I. The argon atom

Abstract: A non-relativistic Hartree-Fock self-consistent procedure has been used to investigate some properties of the argon atom and its ions. The electronic binding energies of the various sub-shells have been calculated for both single and double hole states. The energy of the X-ray photon and Auger electron associated with the filling of a vacancy in the L shell has been calculated. The incident- ion energy-dependent line shifts observed in the X-ray and Auger- electron emission spectra obtained from Ar+-Ar collision studies are explained in terms of the filling of the L23 single vacancy in argon ions with different numbers of valence electrons.

New observations of the spectra of argon X to XV and of isoelectronic emission lines in silicon VII to X, phosphorus X, sulphur IX to XII and chlorine X to XIV

Abstract: This paper reports the classification of spectral lines of chlorine IX to XIV and of argon X to XV emitted from the plasma formed in a theta pinch. The spectra were recorded photographically using a fast shutter to isolate the period of interest. Additional wavelengths of isoelectronic lines in silicon, phosphorus and sulphur are also presented, these lines were emitted from laser produced plasmas. The transitions identified are 2pn-2pn-1, 3s, 4s, 3d, 4d, and 2s22pn-2s2pn+1 and 2s2pn-2pn+1. The wavelengths of the 2s22pn-2s2pn+1 emission lines enable the calculation of ground term intervals in the solar abundant elements silicon, sulphur and argon. The measured interval in argon XIV adds confirmation to the identification of the coronal forbidden line at 4412 AA.

Flame stabilization by plasma jets

Abstract: The possibility of increasing flame reaction rates, stability and hence the throughput of chemical energy achievable by the addition of a small proportion of electrical power is stuided. The power is added to a subsidiary stream of different gases by a magnetically rotated plasma jet. Rates of rotation of the order 10 5 rev/min contribute to uniform heating and mixing with the very much larger main stream flow (up to blow-out) of methane + air mixtures. The products are sampled by a traversing micro-probe and analysed. Quite small additions of electrical power (e. g. 10% of the chemical energy flux—equivalent to an increase of approx. 116 °C in final temperature) produce large increases in throughput— almost 700 % with N 2 plus argon as the carrier gas. This compares with about 50 % predicted for a perfectly stirred system on the basis of measured global kinetics. Even the effect of argon alone, as the carrier gas, cannot be accounted for by such predictions. Radicals known to be important in flame propagation, such as OH, H and O were deliberately produced by including H 2 O, O 2 and CH 4 in the carrier stream . These were an improvement over argon alone but none appreciably exceeded N 2 in effectiveness. The conclusion is that a limited amount of electrical power used to stabilize a large throughput of flame reactants is most effective if employed to generate energetic and long-lived molecular fragments by imparting it in high concentration to a species of large dissociation energy which is capable of producing, subsequently, radicals important in flame propagation. The practical implications may be important, e. g. for stabilizing large throughputs in jet propulsion.

A Spectroscopic Source with Improved Analytical Properties and Remote Sampling Capability

Abstract: A direct current capillary arc operating in argon at atmospheric pressure has been combined with a device which produces fine particles directly from the surface of a solid metal sample. The aerosol generator is a simple device in which the sample acts as the cathode of a dc arc discharge. Rapid movement of the cathode spot produces uniform sampling over a well-defined area. A flowing gas stream transports the aerosol particles to the capillary arc discharge. Reproducibility and analytical curves are shown for ten elements in low alloy, stainless, and tool steel samples, for total analytical times of 10 sec.

Detection of sputtered atoms by atomic absorption spectroscopy

Abstract: Atomic absorption spectroscopy has been used to detect the atoms sputtered from nickel and iron cathodes in glow discharges of argon. The observed spatial distribution of sputtered atoms differs from that predicted by diffusion theory suggesting that some sputtered material is ejected from the cathode as molecules or groups of atoms. These are undetectable by atomic absorption until dissociated in the cathode dark space of the discharge. A relationship for calculating the deposition rate of sputtered atoms from the spectroscopic measurements has been obtained, but uncertainties in some quantities prevent an accurate calculation in this case. However, an upper limit to the deposition rate of atoms is lower than the gravimetric deposition rate measured under the conditions studied. Thus, atomic absorption spectroscopy, which detects only individual atoms, cannot be used to measure total sputtering deposition rates while there is evidence that some sputtered material may be ejected as molecules.

Photon emission from sputtered particles during ion bombardment

Abstract: The emission of photons when the surfaces of metals, semiconductors, ionic crystals, and glasses are bombarded by 5 to 8 ke V argon ions has been studied by spectroscopic analysis. The conditions for the resonance transfer of an electron between a metal and an ionized or excited atom at small distances from the surface are examined and applied to the sputtered particles of the various materials. It is established that these conditions govern the light emission from the low-energy sputtered particles of metals and are also to some extent applicable to semiconductors. For insulators, however, the radiation appears to come only from excited sputtered atoms.

Device for degassing a liquid

Abstract: The invention relates to a device for degassing liquids, while applying a cyclone, in particular for removing an inert gas such as argon from liquid sodium, comprising a cylindrical container provided with a tangential supply duct for the liquid to be degassed, a central discharge for the gas at the upper side of the container, a cyclone mantle and a discharge duct for the degassed liquid, wherein at the inner side of the container wall a horizontal partition is provided that lies under the tangential supply duct of the liquid, at which partition a cylindrical cyclone mantle is attached that is concentrical with this container, which cyclone mantle is open at the upper and lower sides and whose upper edge lies over the supply duct and whose lower edge is situated near the bottom of the container, while at a slight distance under the partition the discharge duct for the degassed liquid is connected to the space between the cyclone inner mantle and the container wall.