Monatomic Species Research Articles

On the effect of Knudsen-layer formation on studies of vaporization, sputtering, and desorption

We are here concerned with experiments in which time-of-flight (TOF) measurements are made with particles which are vaporized, sputtered, or desorbed due to a pulsed heat source. If the emitted particle number density is low enough, the particles will disperse collisionlessly. Provided the emission is truly thermal, the velocities will then be described by a “half-range” Maxwellian, i.e. a Maxwellian with only positive velocities normal to the target, for which it is well known that the surface temperature, T s, and the energy, Ê, defined by the peak position of the TOF spectrum are related by kT s = E ̂ /2 . More commonly the emitted particle density is high enough that near-surface collisions occur. For as few as 3 collisions per particle a Knudsen layer forms, i.e. there is a layer within a few mean free paths of the target surface in which the distribution function evolves to a “full-range” Maxwellian in a center-of-mass coordinate system. We show that for on-axis measurements the relation kT s = E ̂ /2 is replaced by kT s = E ̂ /η K , with η K ranging from 2.52 for a monatomic species to 3.28 for a species with many accessible internal degrees of freedom. Failure to recognize the formation of a Knudsen layer thus leads to a severely overestimated value for T s, at least for an-axis measurements.

Excess free energy in solutions of isotopic isomers. I. Monatomic species. II. Polyatomic species

The excess free energy Aex for solutions of isotopes is discussed. We consider the volume dependence of the free energy and integrate over the concentration weighted molar volume isotope effect, but under the constraint of constant g(r*). The result is significantly different than results from integration along the isothermal equilibrium path employed by previous authors. Both internal and external modes can make important contributions to Aex. This is exemplified by treating solutions of 36Ar/40Ar, C6H6/C6D6, H216O/H218O, and H2O/HDO.

Temperature Dependence of the Kinetics of Elementary Combustion Reactions of Refractory Species

Abstract The dependence of the rate coefficients of gas—phase oxidation reactions of monatomic and diatomic refractory species on temperature is discussed. Examples given pertain primarily to Al and B species reactions in the 300–1900 K range. These reactions show a wide variety of ln k(T) vs. T—1 behavior patterns. It is shown that, because of this variety, it is not yet possible to make reliable predictions regarding reactions for which no experimental data on temperature dependence are available. However, some guidance is given toward making preliminary estimates. Quantitative kinetic data on the reactions are given and the importance of their thermochemistry is discussed. The experimental method (HTFFR) used to obtain these data is briefly reviewed.

Kinetics of condensation in supersonic expansion (Ar)

The temperature and density in an overexpanded jet (Ar) were mapped by the method of characteristics. The kinetics of condensation in the supersaturated gas along the central streamline was analyzed on the basis of four models, subject to previously developed thermodynamic relations which must apply during the homogeneous condensation of any supersaturated vapor. Here we report on detailed dynamic analyses of the rates of cluster growth. Each association: An−1+A1⇄An was represented either by a single stabilization step, or by a sequence of two energy removing collisions, for which kinetic parameters based on Lennard-Jones (LJ) potentials were estimated. Coupled differential rate equations (up to 192) were solved, both for cases when the heat of condensation was neglected, and when it was partially or fully distributed throughout the fluid. The distribution of cluster sizes declines exponentially, according to N(n)=A exp(−an)+B exp(−bn), in agreement with generally observed distributions for monatomic species. For source pressures of 3 and 10 atm the condensation process becomes kinetically controlled within a distance downstream somewhat less than twice the aperture diameter. Our calculations indicated that after about three aperture lengths ‘‘freezing in’’ takes place if the heat of condensation is neglected. When the heat of condensation is distributed (partitioned in one of several plausible ways) freezing in is delayed beyond four aperture diameters. A modest fraction of the nascent clusters retain some of the heat of condensation and remain at a higher temperature than the ambient gas.

Nitrogen‐15 Determination of Nonrandomly Distributed Dinitrogen in Air

AbstractDinitrogen gas produced by denitrification of 15N‐enriched fertilizers does not achieve a random nitrogen (N) isotope distribution when mixed with atmospheric N2. Thus, such mixtures cannot be analyzed by the conventional mass spectrometric measurement of the 28 and 29 ion current ratio. A method is proposed which uses a high voltage arc passed between a pair of tungsten electrodes sealed in a glass vessel to fix some of the N2 as NOx species, and thereby redistribute the N isotopes of the N2 into an NOx pool. The NOx species are absorbed and oxidized by acidic permanganate, reduced to NH+4, and distilled by routine procedures. Since the are process redistributes the N isotopes randomly by ultimately converting the N to a monatomic species (NH+4), accurate isotope ratio analysis of the distilled samples by the conventional dual‐collector mass spectrometer method is possible. The method is precise, accurate, simple, inexpensive, and avoids many of the problems associated with direct 15N analysis of gas samples with nonrandom N isotopic distributions. This description of the basic methodology may enable direct measurement of denitrification losses from highly labeled fertilizer sources while its application to estimation of total denitrification losses requires additional methodologies not described in this paper.

Translational energies gained by gases during collisional ionization with high-velocity neutral particles

A series of experiments are described in which a variety of monatomic and diatomic neutral species having kiloelectron volt translational energies enter an electrically floated collision gas cell where they ionize the target species. The target species are extracted and energy analysed to determine the translational energies gained by them in the collisions. The experimental results have been compared with those predicted by inelastic scattering theory and good agreement has been obtained.

Electrical Transport Studies of the Hydrogen-Related Compensating Donor in B-Doped Silicon Diodes

ABSTRACTTransport of the hydrogen-related, acceptor-compensating defect has been observed in reverse-bias annealed Al-gate Schottky and n+-P diodes from hydrogenated, B-doped p-Si. Secondary ion mass spectroscopy (SIMS) profiling (deuterium substitution) confirmed field-aided migration. Significant differences in field transport (and thermal diffusion) between diodes from Hand D-treated p-Si(B) qualitatively indicates a monatomic species. The effect is interpreted as field drift of a positively charged species, possibly H+, with a donor charge state in the upper-half band gap, in conflict with long-held theory predicting very deep level activity. Acceptor compensation is unstable under minority (electron) carrier injection by forward bias or illumination at 25°C, supporting the acceptor-protonic trap pair model (A-H+) of Sah, Pan and Hsu [J. Appl. Phys. 57, 5148 (1985)].

Vibrational Spectra of Species Adsorbed on Surfaces: Forms of Vibrations and Selection Rules for Regular Arrays of Adsorbed Species

In this paper we describe systematically the forms, symmetries, and selection rules for the normal modes of vibration of a number of examples of surface-adsorbed species. The chosen cases are discussed for adsorption both on isolated sites and in regular arrays of surface sites such as are occupied at higher coverage. For this purpose the Site Group and Factor Group (Correlation Field) approach to the vibrations of regular 3-dimensional lattices is adapted to the 2-dimensional systems. For simplicity the examples chosen are mainly monatomic and diatomic species adsorbed in various ways on specific sites of (111), (100), or (110) planes of metals with face-centered cubic lattices. The selection rules discussed refer to activities in infrared, Raman, or electron energy loss spectroscopy (dipolar and impact scattering mechanisms) and distinguish between adsorption on insulating and conducting (metal) surfaces. The examples chosen for regular arrays all refer to cases where experimental results (mainly from electron energy loss spectroscopy) are available in the literature, where they have been discussed mainly at the site group level.

Departures from CLTE composition in a nitrogen arc at atmospheric pressure. II. Interpretation of departures from CLTE

For pt.I see ibid., vol.16, 829-40 (1983). Departures from chemical equilibrium and from Boltzmann's distribution for excited level populations expected with the assumption of CLTE in a nitrogen plasma at atmospheric pressure are explained by means of a collisional-radiative model taking into account both monatomic and diatomic species, in which transport processes are treated by a parametric approach.

Changes in tokamak plasma properties during impurity injection

Certain applications of laser blow-off impurity injection, such as ion temperature measurement and spectral line identification, may require the ratio of injected ions-to-plasma electron density to exceed 0.001. To achieve these concentrations we have relied on injection of micron-sized clusters as well as monatomic species. In these cases the injected impurity can no longer be regarded as a nonperturbing trace sourced at the plasma edge. We therefore have studied the variation of plasma parameters in PLT and PDX as a function of the amount of injected impurities, the species mix, and the type of impurity. We find that for ohmically-heated plasmas, impurity injection can increase ?e more than 4×1012 cm−3 without causing disruptions. During Ge-injection experiments where Δne/ne ∠0.05, Te initially rises ∠5% on axis and decreases ∠20% at the edge. The plasma current drops 1–3 % and the loop voltage and radiated power both double. Varying the amount of injected impurities changes the peaking and decay times of both high and low impurity charge states by 10–50 %. Analysis of this in terms of altered temperature profiles, impurity penetration profiles and impurity transport is presented. These studies suggest criteria for performing nonperturbing transport experiments.

Tracer diffusion in polyatomic liquids

New tracer diffusion data are reported. These plus other recently published work provide a wide range of solute (monatomic and polyatomic species) size and solvent molecular type. A hard sphere model, making use of diffusion data from computer simulations on hard sphere fluid, predicts fairly well the diffusivities for the solutes in nonpolar solvents of compact molecules. The model fails for solvents of normal alcohols (except methanol) and of noncompact molecules. Stokes’ law is re-examined and found to be invalid except for solute molecules large compared to solvent molecules. The oft-quoted validity of Stokes’ law for self-diffusion is shown to be an accident of conditions under which the law is usually applied. Further verified is a recently published correlation formula relating tracer diffusivity, solvent viscosity, and particle size.

The oxidation of isobutene to p‐xylene over bismuth oxide‐based catalysts

AbstractStudies have been carried out of the optimisation of a bismuth oxide based catalyst for the oxidation of isobutene to p‐xylene. Using a Bi2O3‐P2O5 (Bi:P = 2) catalyst located sequentially before a 3% Pt/Al2O3 catalyst, up to 60% selectivity can be observed at 50% conversion. The kinetics of the oxidation of isobutene over the Bi2O3‐P2O5 catalyst have been measured for all major products. The results support a mechanism involving allylic oxidation of isobutene. Dimethylhexadiene and p‐xylene are suggested to originate from reaction between two adsorbed isobutene molecules and monatomic oxygen species adsorbed at different sites. A major side product, methacrolein, appears to originate from reaction between two adsorbed isobutene molecules and a diatomic oxygen species.

The glass transition of atomic glasses

The nature of the glass transition in liquids composed of monatomic species is discussed using a hard sphere model. Recent molecular dynamics calculations by Woodcock [J. Chem. Soc. Faraday II 72, 1667 (1976)] indicate that a hard sphere fluid undergoes a glass transition when it is compressed to a high enough density. This calculation provides one of several independent methods of estimating the packing fraction at the glass transition, ηg, for hard spheres. The estimated value of ηg for hard spheres, 0.533 ±0.014, is substantially lower than 0.637, the packing fraction of dense random packed hard spheres. The effective ηg for a number of atomic glass forming liquids with continuous interatomic potentials also are estimated, and are found to lie in or near the range of estimated values of ηg for hard spheres. The liquids considered are a Lennard-Jones fluid for which the liquid–glass transition has been studied by a molecular dynamics calculation [A. Rahman, M. Mandell, and J. McTague, J. Chem. Phys. 64, 1564 (1976)], and several real glass forming metal–metalloid alloys. The similarity of ηg for the hard sphere fluid, for the Lennard-Jones fluid, and for metal alloys suggest that it is the short ranged repulsive forces acting between atoms which are responsible for the glass transition. These results also suggest that in general, ηg=0.53±0.02 for atomic liquids. This provides a criterion for predicting the glass transition temperatures for materials which have not yet been observed in the glassy state.

The viscosity and diffusion coefficients of the binary mixtures of xenon with the other noble gases

This paper presents new experimental data for the viscosity of binary mixtures of xenon with the remaining monatomic gases, helium, neon, argon, and krypton. The measurements have been performed in a high precision oscillating-disk viscometer at atmospheric pressure and within the temperature range 25–500°C. The data have an estimated uncertainty of ±0.1% at 25°C increasing to ±0.3% at 500°C. The collision integrals for the interactions of xenon with the other monatomic species conform to the extended law of corresponding states formulated by Kestin, Ro and Wakeham. For each binary interaction the scaling parameters σ ij and ∈ ij have been obtained. The ensemble of experimental results can be correlated by means of the appropriate kinetic theory expressions reinforced by the extended law of corresponding states. The deviations do not exceed 0.5%. The binary diffusion coefficients were calculated from the measured mixture viscosity and compared with the available experimental results. The standard deviation was estimated as ±2% which is within the mutual uncertainty of the two sets.

High-temperature oxidation of SiC in a glow-discharge oxygen plasma

The oxidation kinetics and structure of the oxide scales formed on high-density SiC were studied in molecular oxygen at 740 Torr and in a glow-discharge oxygen plasma at 0.1 Torr at temperatures of 1000, 1100, and 1200°C. The monatomic oxygen formed by the glow discharge markedly increased the reaction rate and the vaporization of some of the oxidation products. The marked differences in kinetics suggest that the rate-controlling step during oxidation in molecular oxygen is the dissociation of adsorbed diatomic oxygen to the monatomic species. Films formed in molecular oxygen were mostly amorphous SiO2 with small inclusions of SiC and graphite, whereas films formed in dissociated oxygen were primarily amorphous SiO2 containing SiO, S2O3, and the coesite form of SiO2.

Translational hydrodynamics for binary mixtures: Memory from internal molecular relaxation

We consider a binary fluid mixture made up of a monatomic species and a two level molecular species and investigate the decay of correlations in the hydrodynamic variables. By restricting our considerations to the translationally hydrodynamic situations, we deduce a set of generalised hydrodynamic equations in which the frequency, ω, and wavenumber, k, dependent memory arises solely from the internal molecular relaxation. From these generalised hydrodynamic equations, we identify various ( k, ω) dependent transport coefficients and the relaxation time which depends on k. We also compute the spectrum of (polarised) light scattered from such a binary mixture. We illustrate the dependence on k, ω and concentration in the problem by a numerical calculation of the generalised transport coefficients, the Rayleigh-Brillouin spectrum and the dispersion of various modes in this spectrum for the case of thermal relaxation in a parahydrogen-helium mixture.

Use of Monte Carlo in calculating the thermodynamic properties of water clathrates

AbstractEquilibrium dissociation pressures for monovariant three‐phase (icesolid clathrate‐gas) systems were predicted for water clathrates with Ar, Kr, Xe, N2, O2, CO2, and CH4 as the guest molecules. A Monte Carlo approach of statistically sampling the states available to the guest molecule inside the host water cage was used to evaluate the configurational properties including the free volume integral. The Lennard‐Jones 6–12 potential adequately described the guest‐host interaction for monatomic species, for example, Ar, Kr, and Xe, while the nonspherically symmetric Kihara potential with a fixed hard‐core geometry was required for polyatomic guests. The simulated motion of the guest within the cell was consistent with the geometrical features of the host lattice.

Thermal conductivities of binary mixtures of organic vapours and inert diluents

Thermal conductivities of the six sets of binary mixtures of n-pentane, diethyl ether, n-hexane and diethyl peroxide (5- and 6-membered chain species) with one another, and of their sixteen sets of binary mixtures with helium, neon or argon (monatomic species), or nitrogen (diatomic species), have been measured at 50 and 100°C. Marked similarities between all the polyatomic molecules (chain species) are shown by their individual thermal conductivities and by their behaviour in mixtures with monatomic and diatomic molecules.

Effect of Adsorbate–Adsorbate Interactions on the Net Charge of an Alkali Atom on a Metal Substrate

A previous calculation of the effective charge of an isolated alkali atom on a metal substrate is generalized, in the low-coverage limit, to include interactions between adsorbed atoms. The long-range Coulomb field traditionally invoked to explain the decrease in net charge with increasing coverage is incorporated into our self-consistent procedure. The complex nature of the dependence of this decrease on coverage is emphasized. Short-range interactions due to the overlap of adsorbate atomic orbitals are included by allowing for both monatomic and diatomic adsorbate species. The effect of the long-range Coulomb and overlap interactions is illustrated by numerical results for K adsorbates on a (100) W surface at various coverages. Several factors combine to cause a similar net charge per nucleus on the two species.

Vaporization of Dimolybdenum Carbide

The vaporization behavior of Mo2C was investigated over the temperature range 2125° to 2550°K by Langmuir experiments utilizing an induction heated, ultrahigh vacuum thermobalance. A congruently vaporizing composition of MoC0.49 was found at temperatures from 2250° to 2300°C. The total evaporation rate increases and the heat of evaporation decreases as the carbon-to-metal ratio of the sample decreases from 0.49 to 0.44. A mass-spectrometric examination of the evaporate from MoC0.49 did not reveal any MoxCy molecules. Therefore, evaporation by decomposition is assumed. If monatomic gaseous species and unit evaporation coefficients are also assumed, the pressures over MoC0.49 are logPMo(atm)=7.112±0.139−(3.420±0.033) (104/T),logPC(atm)=6.351±0.139−(3.420±0.033) (104/T). The errors quoted are standard deviations. The heat of formation at 298°K of MoC0.49 is −4.9 kcal/mole from third-law calculations, in good agreement with the −5.5 kcal measured calorimetrically.