Average Atomic Energy Research Articles

A Monte Carlo Study of the Argon Fluid Around a Benzene Molecule

Abstract Monte Carlo simulations have been carried out for argon fluid containing one benzene molecule at the supercritical region. The purpose in this study is to examine the effect of plate-like molecule on the structure of neighboring fluid composed of simple spherical molecules of the system. In the first neighbor shell of argon from the center of benzene molecule, the average potential energy of argon atoms is confirmed to have a large density dependence. This potential energy is relatively large in the high density region. It is found that the spatial distribution of argon fluid is significantly affected by the molecular shape of benzene and it has little direct connection with the attractive interaction between benzene and argon.

Energetic beam-induced surface segregation during deposition

The energetic beam-induced surface segregation of metal (Cu thin film)/metal (Ag substrate) structure irradiated by energetic atom and ion beams has been observed. It has been found that Ag was segregated onto the surface of Cu film (about 15 nm in thickness deposited under UHV) during the ion beam deposition and irradiation of the deposited film afterwards. The energy of the Ar + ion beam used both for sputter-deposition and irradiation ranged over several keV. All the measurements were made with a PHI-590 scanning Auger microprobe at room temperature at a base pressure of about 6 × 10 −8 Pa. During sputter-deposition the Ag enrichment is greater the higher the energy of the sputtering ion beam used. This kinetic energy influence of deposited atom on the degree of segragation is interesting as the average energy of atoms sputtered with an ion beam energy of 2–4 keV is several eV and their energy difference should be in the range 10 −1-1 eV. The deposited film shows greatest enrichment of Ag when it is irradiated by an Ar + ion beam with keV energy.

Neutron and PIMC Determination of the Longitudinal Momentum Distribution of HCP, BCC and Normal Liquid 4 Het

Abstract Deep inelastic neutron scattering has been used to measure the neutron Compton profile (NCP) of a series of condensed 4He samples at densities from 28.8 atoms/nm3 (essentially the minimum possible density in the solid phase) up to 39.8 atoms/nm3 using a chopper spectrometer at the Argonne National Laboratory Intense Pulsed Neutron Source. At the lowest density, the NCP was measured along an isochore through the hep, bcc, and normal liquid phases. Average atomic kinetic energies are extracted from each of the data sets and are compared to both published and new path integral Monte-Carlo (PIMC) calculations as well as other theoretical predictions. In this preliminary analysis of the data, account is taken of the effects of instrumental resolution, multiple scattering, and final-state interactions. Both our measurements and the PIMC theory show that there are only small differences in the kinetic energy and longitudinal momentum distribution of isochoric helium samples, regardless of their phase or crystal structure.

Amorphization of c‐Si by the sputter deposition of Au studied by x‐ray photoelectron diffraction

It is well known that Si atoms can be dislodged from a silicon matrix and diffuse through a thin Au overlayer. In the course of studying this phenomenon, we have found x‐ray photoelectron diffraction (XPD) to be a particularly powerful investigative tool: XPD, a relatively new surface analytical technique, is very sensitive to the surface quality of crystalline materials. We have avoided surface contamination by depositing very thin (<5 nm) Au films onto a (100) Si surface, both by sputtering and by thermal evaporation within the surface instrument. XPD reveals that the c‐Si surface is amorphized by the former process and much less damaged by the latter, a fact we attribute to the higher average kinetic energy of incident Au atoms during sputtering. This interpretation is supported by related studies; for example, by the reaction of adventitious carbon to form Si–C bonds on c‐Si during sputter cleaning and sputter deposition. The study illustrates the important role of even relatively low particle energies and fluxes in ion‐assisted processing.

Sputtering from a liquid Ga-In eutectic alloy

Angular distributions of Ga and In atoms sputtered from a liquid Ga-In eutectic alloy target have been measured for 3, 25, and 50 keV Ar + bombardment. Although the bulk composition of the alloy is 16.5 at.% In, thermodynamic surface segregation results in a surface monolayer of > 94 at.% In. A comparison of the measured sputtered-flux composition with the alloy surface composition profile indicates that the fraction of sputtered atoms originating in the first atomic layer, F 1, is ≈ 0.87 at 25 and 50 keV, and increases to 0.94 at 3 keV. Measurements of the secondary-ion flux for 25–250 keV Ar + bombardment indicate that F 1 is independent of projectile energy up to 250 keV, in agreement with the predictions of the collision-cascade model. The increase observed at 3 keV may be the result of a decrease in the average energy of recoil atoms participating in the collision cascades.

Core-electron binding energies in free and supported metal clusters

Core-electron binding energy shifts in free and supported clusters are discussed using the Born-Haber formalism. For grounded clusters this approach shows that the shift reflects the decrease in the average atomic cohesive energy of the cluster relative to that of the bulk metal. This shift is closely related to the surface-atom core level shift. For free clusters there is a second term reflecting the unit charge left on the cluster by the emission of the photoelectron. In small clusters this term results in the suppression of conduction electron screening. Clusters supported on amorphous carbon remain charged in the final-state, and are similar to free clusters, but have smaller shifts because the substrate reduces the energy of the final state by forming an image charge. The shift in monolayer islands on metallic substrates is determined largely by the adsorption enthalpy of the adatoms.

Electrical properties of glass and liquid semiconductor system (As 2Te 3) x(Tl 2Se) 1− x

We have measured the electrical conductivity and thermoelectric power of the pseudobinary system (As 2Te 3) x (Tl 2Se) 1− x both in the glass and liquid states. The effect of the addition of excess Tl has also been examined. It has been found that the activation energy of electronic conduction is closely related to the average atomization energy of these materials.

STRUCTURAL AND ELECTRICAL EFFECTS OF DOPANT SEGREGATION TO SILICON GRAIN BOUNDARIES

High resolution STEM analysis of grain boundary segregation of arsenic in CVD grown polysilicon has shown that there is substantial equilibrium segregation after annealing at temperatures between 700 and 1000 C. The variation of extent of segregation with temperature allows calculation of the average binding energy of arsenic atoms to silicon grain boundaries (0.65eV/atom) and the average saturation concentration in the boundaries (12at%). 2 possible segregation sites are discussed with emphasis on the electrical effects of segregation to them.

Experimental investigation on field evaporation of singly and doubly charged rhodium

Field ion mass spectrometry and field ion appearance spectroscopy are used to analyze field evaporation processes of singly and doubly charged rhodium ions. Field strengths were ranged between 17 and 41 V/nm at tip temperatures between 600 and 100 K. The appearance energies of doubly charged species were found to increase with increasing field strengths from 29.5 ± 0.4 to 32.0 ± 0.2 eV, those of the singly charged species from 11.2 ± 0.1 to 12.4 ± 0.1 eV. Activation energies as determined from measured temperature dependences of the ion counting rates are equal for both kinds of ionic species and decrease from 1.5 to 0.05 eV with increasing field strengths. The observed field dependences cannot be predicted from the image hump model of field evaporation. Evaporation of Rh 1+is better understood using the charge exchange model. Generation of Rh 2+ is described by the postionization of Rh 1+ within the frame of a simple potential energy model from which the average interaction energy of rhodium surface atoms with their nearest neighbours may be derived.

A kinetic theory of nucleation in dilute solid solutions; Application to the precipitation of carbon and nitrogen in α-iron

A kinetic theory of nucleation and growth of precipitate particles in solid solutions is derived. This theory, which is applicable at low solute concentration and rather low solute-solute binding energy, is obtained by application of equilibrium conditions to the reversible chemical rate equations of cluster formation. The equilibrium approximation permits sufficient cancellation of terms in the equations so that the remaining equations are integrable. Computer solutions for a variety of parameters of the kinetic equations were used to check the range of validity of the approximation necessary to obtain the analytic expression. The analytic expression describes the experimentally observed time dependence of the precipitation of carbon and nitrogen in α-iron. The analytic theory also predicts an average binding energy of precipitate atoms in a cluster which is in reasonable agreement with experimentally derived carbon-carbon and nitrogen-nitrogen binding energies in iron. The size of the fundamental precipitate nucleus, as well as the distribution of sizes of precipitate particles, is also predicted, but no experiments exist to check these predictions.

Energy Distribution of Sputtered Cu Atoms

Energy distributions have been obtained for atoms sputtered from Cu (100), (110), (111), and polycrystalline surfaces bombarded by 80- to 1200-eV Hg and noble gas ions. Energies of sputtered atoms are found to depend markedly on the angle of ejection. For a (110) surface bombarded by 1000-eV Kr ions, average energies of ejected atoms are 9.5 eV in the [110] direction (normal to the surface) and 14.2 eV in the [011] direction (60° to the surface normal). Ejection energies decrease for lighter ions in the order Kr+, Ar+, Ne+, and He+. For a (110) surface bombarded by 600-eV He ions, the average energy of atoms ejected in the [110] direction is 5.5 eV. Dependence on bombarding ion energy is very weak above 1000 eV where average energies of sputtered atoms are found to be in the range 5–15 eV. Ejection energies decrease at lower bombarding ion energies and decrease rapidly below 200 eV. For atoms ejected in the [110] direction from a (110) surface under Kr ion bombardment, the average energy decreases from 5.5 eV at an ion energy of 200 eV to 2.6 eV at an ion energy of 80 eV.

Calorimetric measurement of the average energy of sputtered metal-atoms

A method was recently developed for determinlng the average kinetic energy of sputtered particles by directly converting this energy into heat. The bombarding ions were created by a magnetically constricted beam of electrons in a gas of low pressure. Results of preliminary measurements indicate that the average energy of gold atoms sputtered under krypton ion bombardment increases linearly from 10 ev per atom at 400 ev to 20 ev per atom at 1 kev krypton ion energy, after corrections for the heat of sublimation. (W.D.M.)