Showing papers on "Argon published in 1984"

Structure of Charged Argon Clusters Formed in a Free Jet Expansion

Abstract: We describe measurements of the mass spectrum of charged argon clusters generated in a low-temperature free jet expansion. It contains detailed intensity variations which can be understood in terms of a simplified competing-lattice model of charged-cluster structure.

Pressure of Neon, Argon, and Xenon Bubbles in Aluminum

Abstract: Neon, argon, and xenon bubbles in aluminum have been investigated by means of electron-energy-loss spectroscopy and transmission electron microscopy. The density and pressure in the bubbles were determined from the pressure shift of the $^{1}S_{0}\ensuremath{-}^{1}P_{1}$ transition of the rare gases as well as from analysis of electron-diffraction patterns. The experiments yield overpressurized bubbles, containing liquid Ne and solid Ar and Xe at room temperature. In the case of Xe, epitaxial growth of the rare gas in the Al matrix was observed.

Temperature determinations in a free-burning arc. I. Experimental techniques and results in argon

Abstract: Spectroscopic techniques have been used to measure the temperature distribution in free-burning arcs in argon at 1 atm pressure. The experiment provides evidence for departures from LTE in the arc, and demonstrates deficiencies in the theory describing continuum emission from high temperature plasmas. This paper describes the apparatus and data reduction procedures and compares the measurements with recent theoretical calculations.

Characteristics of high-frequency and direct-current argon discharges at low pressures: a comparative analysis

Abstract: A theoretical analysis of the characteristics of high-frequency and direct-current argon discharges at low pressures is carried out and a comparison of the basic properties of these discharges is presented. This analysis uses recent calculations of electron energy distributions and collision rate coefficients in argon under the action of uniform AC and DC electric fields together with a steady-state discharge model expressing the balance between collisional ionisation of the gas and the loss of electrons to the wall. The electric field strength, E, required for the steady-state operation of planar and cylindrical discharges is calculated as a function of the gas density N, the angular frequency omega , and the diffusion length for the discharge tube Lambda , assuming predominant direct ionisation.

Extended x-ray-absorption fine-structure study of Ag particles isolated in solid argon

Abstract: Extended x-ray-absorption fine structure was used to determine the nearest-neighbor distance for silver particles isolated in solid argon. The silver particles were produced in the gas phase with the use of the gas-aggregation technique. In this technique the metal atoms are evaporated in an argon atmosphere in the pressure range 0.1-10 Torr; a cooling of the metal atoms takes place through collision with cold gas atoms. The metal aggregates are then transported by the gas stream through an aperture into a liquid-helium cryopump, where most of the gas is condensed. The metal flow rate is monitored by a quartz oscillator. The size of the metal clusters can be selected by changing the argon pressure, density of the metal atom vapors, and geometric configuration of the cell. The size distribution was determined by introducing a sample holder into the molecular beam, sampling an atomic quantity of approximately ${10}^{15}$ atoms/${\mathrm{cm}}^{2}$, and evaluating the corresponding electron micrographs. Particles with average sizes between 25 and 130 \AA{} were studied. A careful analysis of the data was performed with a thin silver foil at 78 K as a standard. No remarkable effects were observed in the near-edge region of the $K$ edge, but there was a noticeable contraction of the nearest-neighbor distance. This contraction can be explained if we assume the presence of surface stresses. There is good agreement between the observed contraction and the calculated values.

Improved ion containment using a ring-cusp ion thruster

Abstract: A 30-centimeter diameter ring-cusp ion thruster is described which operates at inert gas ion beam currents up to about 7 ampere, with significant improvements in discharge chamber performance over conventional divergent-field thrusters. The thruster has strong boundary ring-cusp magnetic fields, a diverging field on the cathode region, and a nearly field-free volume upstream of the ion extraction system. Minimum ion beam production costs of 90 to 100 watts per beam ampere (W/A) were obtained for argon, krypton and xenon. Propellant efficiencies in excess of 0.90 were achieved at 100 to 120 W/A for the three inert gases. The ion beam charge-state was documented with a collimating mass spectrometer probe to allow evaluation of overall thruster efficiencies.

A LEED and RHEED Investigation of Cu on Au(111) in the Underpotential Region

Abstract: The structure of Cu adsorbates, deposited electrochemically up to one monolayer onto Au(111) at underpotentials, has been investigated using LEED and RHEED after transfer of the gold electrode from the electrochemical cell into an UHV chamber. Before electrochemical deposition, the gold surface was thoroughly cleaned in the UHV-chamber by argon bombardment and subsequent annealing. The Cu adsorbate is then found to yield a (√3 × √3)R30° superstructure at medium coverages, which is in clear contrast to the growth behaviour for Cu evaporated in UHV onto a bare Au(111) surface, where pseudo-morphic growth is observed from the very beginning.

Parametric study of the flow and temperature fields in an inductively coupled r.f. plasma torch

Abstract: A theoretical investigation of the effect of different parameters on the flow and the temperature fields in a radiofrequency inductively coupled plasma is carried out. The parameters studied are: central injection gas flow rate, total gas flow rate, input power, and the type of plasma gas. The results obtained for argon and nitrogen plasmas at atmospheric pressure indicate that the flow and the temperature fields in the coil region, as well as the heat flux to the wall of the plasma confinement tube, are considerably altered by the changes in the torch operating conditions.

Emission spectroscopy study of N2-H2 glow discharge for metal surface nitriding

Abstract: A N2-H2 glow discharge for metallic surface nitriding at low pressure (1-3 Torr) and low current density (1-6 mA cm-2) has been studied by emission spectroscopy. The N2, N2+, N, N+, H, NH and metal atom excited states have been detected. The rotational temperature of N2+(B2 Sigma u+) states in the negative glow near the cathode surface has been found to be in quasi-equilibrium with the cathode temperature. By using argon as a gas impurity (2%), it has been established that the N2(C3 Pi u) states are excited in the negative glow near the cathode fall by direct electron impact. The NH radicals appear to issue near the cathode surface as do metal atoms.

Reaction dynamics and the cage effect in microclusters of bromine-argon (Br2Arn)

Abstract: We present the results of a molecular dynamics trajectory study of the photodissociation/recombination reaction of Br2 in argon clusters. Initial conditions of the clusters correspond to the cold, collision-free regime obtainable in a supersonic molecular beam. We compare our results to gas- and liquid-phase studies.

The scattering of low-energy electrons by argon atoms

Abstract: Phaseshifts, differential, total and momentum transfer cross sections are calculated using an R-matrix approach for the elastic scattering of electrons by argon atoms in the impact energy range 0-19 eV. The coupled-state calculation is based upon a single-configuration atomic ground-state wavefunction coupled to a 1P pseudostate. A critical assessment of earlier theoretical and experimental data is made and the conclusion is reached that the present results are the most satisfactory over the entire energy range considered.

Non-LTE diagnostic of an argon arc plasma

Abstract: The plasma of an argon arc has been investigated by applying the partial LTE model (overpopulation of the ground state) and accounting for different kinetic temperatures of electrons and atoms. The plasma analysis is performed without making use of atomic constants (e.g. transition probabilities) from the literature. The experimental results show that LTE is obtained in the 4mm arc at atmospheric pressure for electron densities above 6×10 16 cm -3 . Below this value, increasing under-population of the excited levels with decreasing arc current can be observed. This deviation from LTE may explain some not understood results from previously published spectroscopic observations of argon plasmas.

Etching of diamond with argon and oxygen ion beams

Abstract: The angular dependences of the sputter yield of (100) single crystal diamond under argon and oxygen ion beams were investigated. For argon ion beams a large increase in the sputter yield was observed as the angle of incidence was increased from the normal, with a maximum occurring at approximately 60 deg from the normal. The magnitude of the increase and the angle at which the maximum was observed were dependent on the ion energy and species. The shape of the yield versus angle of incidence curve indicated the presence of a highly damaged surface layer. Large deviations from this curve were observed where ion incidence was along channeling directions. Studies with oxygen ion beams showed almost no angular dependence for 500 eV ions while at an energy of 1000 eV the angular dependence was similar to that for argon ions. At normal incidence the yield for 500 eV oxygen ions was seven times larger than that for 500 eV argon ions. For 1000 eV oxygen and argon ions the corresponding ratio is only 2.5. Implications for mechanisms of inert and reactive ion etching are discussed. Application of the data to ion beam shaping of diamond tips for ultrahigh pressure research is discussed.

Spatial concentrations of silicon atoms by laser‐induced fluorescence in a silane glow discharge

Abstract: A capacitively coupled, rf glow discharge of silane in argon was studied to determine the spatial concentration of silicon atoms. Laser‐induced fluorescence was used to determine the ground state concentration profiles. The fluorescence profiles clearly show the sharp boundaries of the sheath regions. These profiles were much more sensitive to plasma chemistry changes than profiles obtained from plasma emission. Experiments with nitrogen addition demonstrated significant changes in the silicon atom profiles near the sheath boundary.

Sputtering and luminescence in electronically excited solid argon

Abstract: We have measured both ultraviolet luminescence and ejection of atomic argon from solid argon films electronically excited by megaelectronvolt light ions. The two phenomena reflect (respectively) the radiative and the nonradiative parts of the energy release during electronic recombination and deexcitation. The measurements can be correlated by a model of diffusion of ionic excitons followed by formation and decay of self-trapped excimers.

Precision X-ray wavelength measurements in helium-like argon recoil ions

Abstract: The authors report precise wavelength measurements of the 1s2 1S0-1s2p3P1,2,1P1 transitions in Ar16+ produced by collisions of 5.9 MeV amu-1 U66+ ions with an argon gas target. By use of this 'recoil source', the precision is not limited by Doppler shifts while the influence of spectator electrons is minimised by observation of their relative importance as a function of gas pressure. The accuracy obtained is at the 12 p.p.m. level dominated by the X-ray calibration standard. The measurement is thus sensitive to quantum-electrodynamic (QED) and electron correlation effects.

Effects of metallic vapor on the properties of an argon arc plasma

Abstract: During thermal plasma processing of materials, vapor generated from injected particulate matter will enter the plasma. Even traces of metallic vapors may have a strong effect on the properties and the behavior of the plasma and on the associated heat flux to the injected particles. In this paper a model system is considered in which an argon plasma at atmospheric pressure is “contaminated” by small amounts of copper vapor. By using the Chapman-Enskog approximation for a multicomponent gas mixture the transport properties are calculated for such a contaminated argon plasma. The results show that there is a drastic effect on the electrical properties. For temperatures below 104 K, the electrical conductivity, for example, increases by more than an order of magnitude if metallic vapor is present. The presence of metallic contaminants is also somewhat felt by the reactional thermal conductivity. In contrast, there is no effect on the heavy-particle properties as long as the percentage of the contaminants remains small.

Absorption of infrared radiation in silicon

Abstract: The absorption of 9–11 μm radiation by thin wafers of lightly doped, n‐type Si has been measured at several lattice temperatures from 300 to 800 K. The temperature dependence of the absorption coefficient at λ=10.6 μm is extracted from the data and compared with previous measurements and also with recent theoretical models. A novel processing technique is described in which coupling of the CO2 laser radiation to the Si lattice is significantly enhanced by the simultaneous absorption of radiation from an argon laser.

The structure of fluorene (C13H10) and the fluorine–argon van der Waals complex from a high-resolution near ultraviolet spectrum

Abstract: The high-resolution fluorescence excitation spectrum of the fluorene molecule and its van der Waals complex with argon has been studied. Rotationally resolved spectra were obtained by combining a well-collimated supersonic molecular beam with a single frequency tunable ultraviolet (uv) source. The S1 ← S0 vibronic transitions in fluorene and fluorine–argon were observed. The rotational constants of both molecules were determined and from these the structure of the fluorine–argon complex has been deduced. The argon atom is located above the middle five-membered ring. The coordinates of the position vector of the argon atom in the complex in the centre-of-mass system of fluorene are r = (3.46 ± 0.03) A and . A model calculation of the fluorine–Ar structure was found to be in quite good agreement with the experimental result. The newly introduced inertial defect of the complex ΔIc = −7(3) amu A2 provided an experimental measure of the zero-point motion of argon in the complex.

Local thermodynamic equilibrium in free-burning arcs in argon

Abstract: Measurements of normalized radial emission coefficients in a free‐burning arc in argon are presented as a function of position in the arc. The variation of these emission coefficients with pressure indicates departures from local thermodynamic equilibrium at 1‐atm pressure.

Sputter deposition of platinum films in argon/oxygen and neon/oxygen discharges

Abstract: We have investigated the resistivity, crystallography, and chemistry of films sputter deposited from a platinum target in argon and neon discharges containing small amounts of oxygen. The results presented here indicate that the oxidation behavior of platinum is strongly dependent upon the type of rare gas used for the deposition. A comparison of the dependence of platinum‐oxygen bond formation on cathode voltage and deposition rate suggests that oxide formation in the films is controlled by a reaction which occurs at the target surface and is enhanced when neon carrier gas is used. A large increase in resistivity above that of bulk platinum metal is always correlated with platinum‐oxygen bond formation in the film.

The enhanced sputtering yield of graphite at elevated temperatures: The energy of the released carbon atoms

Abstract: The velocity distributions of carbon atoms released by argon ion bombardment of graphite have been measured using a time-of-flight method. As expected, at room temperature we observe a velocity distribution of the sputtered carbon atoms due to a collision cascade mechanism. At elevated temperatures, the velocity distributions cannot be interpreted by a collision cascade mechanism even though a low surface binding energy is assumed. They can be explained by thermal desorption with a Maxwell-Boltzmann distribution near the bulk temperature.

Fluorescence and absorption in electron beam excited argon

Abstract: Argon under pressures ranging from 0.5-8 bar was excited by an intense relativistic electron beam. The fluorescence of the Ar2* excimer radiation at 126 nm and of the Ar*(4p) to Ar*(4s) transitions in the visible as well as the transient absorption between 200 and 600 nm have been studied. The analysis of the results leads to a kinetic model which accounts for all observed features. For high excitation densities Ar2*(1u, Omu -) is determined to be the main absorber in the near UV. Cross sections for photoabsorption in Ar2*(1u,Ou-) are given. The so-called 'background' absorption is identified to be caused mainly by photoionisation of Ar*(4p) and photodissociation of vibrationally excited Ar2+.