Solid Atoms Research Articles

Long term stability of H atoms in HD-D2-He solids

We employed en electron spin resonance (ESR) technique for investigating the long term behavior of hydrogen and deuterium atoms in the HD-D2 impurity helium solids that were created by sending a gas mixture [H2]:[D2]:[He]= 1:4:100 through a radio-frequency electrical discharge into a volume of superfluid 4He at T = 1.5 K. H and D atoms were stabilized inside nanoclusters of impurity molecules. The exchange tunneling reactions D + H2 → HD + H and D + HD→D2 + H proceeded to eliminate D atoms and increase the concentration of H atoms in the HD-D2 impurity-helium solids. Local concentrations of H atoms inside the molecular nanoclusters of order 1020 cm−3 were achieved. The high concentration of H atoms was stable during ∼ 40 hours storage of the sample at T = 1.35 K. These solids are possible candidates for collective quantum phenomena of atomic hydrogen if the Bose-Einstein degeneracy regime can be attained.

The absorption and excitation spectroscopy of matrix-isolated atomic manganese: Sites of isolation in the solid rare gases

This study collects information from absorption and luminescence excitation spectra recorded for Mn atoms isolated in the solid rare gases Ar, Kr, and Xe and presents an analysis of the site occupancy, based on the polarizabilities of the rare gases and the observed spectral shifts. Two thermally stable sites of isolation exist for atomic Mn in solid Ar and Kr, while a single thermally stable site is present in Mn/Xe. Site occupancy assignments are based on the application of a polarizability model to the z (6)P(5/2)<--a (6)S(5/2); z (8)P(5/2)<--a (6)S(5/2), and y (6)P(5/2)<--a (6)S(5/2) electronic transitions of atomic Mn. From an analysis of the observed RG matrix-to-gas phase energy shifts for P<--S type transitions, this model allows the association of certain site types occupied by metal atoms in the rare gas solids. The required condition being a linear dependence of the matrix shifts with rare gas polarizability for those metal atoms "trapped" in a particular site type. Application of the polarizability model in conjunction with trends observed in site dominance, established a connection between the blue sites in Ar and Kr and the single site in Xe. Use of the known MgRG ground state bond lengths facilitated an identification of the sites of Mn atom isolation assuming the transference of the known MgRG bond lengths to the MnRG systems. Substitutional site occupancy of atomic Mn is assigned to the blue sites in Ar and Kr and the single site in Xe, while tetra-vacancy site occupancy is assigned to the red sites in Ar and Kr. Consistent with these assignments, Mn atoms in solid Ar show a preference for trapping in tetra-vacancy sites whereas in solid Kr, single substitutional sites are preferred and in Xe, this is the only site observed.

The atomic detail of an evaporating meniscus

Atomistic simulations of a simple Lennard-Jones fluid are used to investigate the very near-wall dynamics and thermodynamics of evaporating menisci. The specific configuration considered is a two-dimensional (in the mean) liquid drop centered on a cold spot on an atomically smooth solid wall with evaporating menisci extending from it onto hotter regions of the wall. In the four cases simulated, the interaction energy between the solid atoms, which make up the wall, and the fluid atoms, which are equilibrated in liquid and vapor phases, is varied by a factor of about 5. Results are interpreted in the context of a recently proposed continuum model [V. S. Ajaev and G. M. Homsy, “Steady vapor bubbles in rectangular microchannels,” J. Colloid. Interface Sci. 240, 259 (2001)], which is based on a low-capillary-number asymptotic analysis of the flow and heat equations. In this model, the nonlocal influence of the wall is modeled by a disjoining pressure, a common linearized nonequilibrium model is assumed for evaporation kinetics, and the interface curvature impacts thermodynamics through its effect on the local pressure. However, this model and others like it neglect both the atomic granularity of the fluid and any scale associated changes in its properties in the thinnest regions of the evaporating meniscus, which are the subject of this study. Quantitative agreement for meniscus shape and evaporative mass flux is found for a weakly wetting case, but the model must be modified in a straightforward way for more strongly wetting cases to account for a layer of nearly fixed fluid atoms on the wall. A finite solid-liquid interface thermal (Kapitza) resistance is found to be important, and the continuum model is reformulated accordingly. With an appropriate Kapitza resistance value the reformulation yields accurate predictions using the actual wall temperature as a boundary condition, rather than the fluid’s temperature at the wall.

Nonlinear diffusion in excited Si crystals

We discuss the properties of the nonlinear diffusion equation for the case of diffusion in excited systems. The diffusion coefficient is directly proportional to the concentration of impurities and depends on the time in a special way. For the description of the excited systems, we used a special temperature function, which defined the time dependent diffusion coefficient and the Boltzmann's distribution of the excited vacancies or impurity atoms in solids. This model was used for approximation of very fast diffusion of metastable vacancies irradiated by soft X-rays in an excited Si crystal.

Elsepa—Dirac partial-wave calculation of elastic scattering of electrons and positrons by atoms, positive ions and molecules

elsepa—Dirac partial-wave calculation of elastic scattering of electrons and positrons by atoms, positive ions and molecules

Effect of microcontent Nb in solution on the strength of low carbon steels

Two low carbon steels were hot rolled by thermo-mechanical process (TMCP), and one contains niobium (0.013%), and another does not contain niobium. By the comparison of tested results, it can be found that the yield strength of Nb steel is 55 MPa higher than that of Nb free steel on an average. In addition, the ferrite grain of Nb steel is finer than that of Nb free steel under the same finishing temperature, cooling rate, and final temperature. However, there are no Nb(C,N) particles to precipitate from austenite and ferrite by the observation of transmission electron microscopy (TEM). It can be concluded by theoretical analysis that both the increase of strength and the refinement of ferrite grain of Nb steel result from solid solution Nb atoms.

Influence of Gas–Solid Kinetic Energy Exchange Processes on Gas Effusion from Slitpores in Non-equilibrium Molecular Dynamics Simulations

The boundary-driven type non-equilibrium molecular dynamics (BD-NEMD) simulations have been carried out to clarify the influence of solid flexibility on gas effusion through a slitpore, by using a rigid solid model (R-model) and a flexible solid model (F-model). The LJ potential particles with parameters of argon are chosen for both the effusing gas molecules and the solid atoms. It is found that the R-model combined with a velocity scaling technique provides a reasonable effusion flux, probably a maximum in magnitude, compared with the fluxes calculated from the F-model without a fictitious thermostat (a velocity scaling) for effusing molecules. In the F-model, temperature lowering has been observed at the pore exit, being enhanced with an increase in the strength of springs that unite the solid atoms. This observation indicates that the molecules escaping from the pore exit take the excess kinetic energy for evaporation from solid atoms or surrounding molecules, which results in the low exit temperature, and that the rate of gas–solid kinetic energy exchange decreases with increasing spring strength (i.e. decreasing solid flexibility). It is suggested that the effusion flux is influenced by two factors: the solid flexibility and the molar potential energy in the pore.

Crossroads electronic structure of MnS, MnSe, and MnTe

AbstractBy using the LDA + U method, we have investigated the electronic structure of MnBVI (BVI = S, Se, Te)–which are end‐point materials for wide gap semiconductors, A1−xIIMnxBVI (AII = Zn, Cd, Hg)–using parameters calculated by the so callled solid atom method. All these MnBVI compoounds have semiconducting electronic structure in the antiferromagnetic phase. The character of each energy gap is on the cossroads between charge transfer type insulators and band insulators. The LDA + U method yields enhanced energy gaps and magnetic moments, as compared to those of the LDA calculation in agreement with experimental values. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Acoustic waves in discrete media, similarities at meso and nano scales

In this paper the similarities between the acoustic behavior of a discrete medium of random arrangement of solid spheres, at mesoscopic scales, and thermal vibrations of lattice ions in crystalline solid, at nano scales, are investigated. Results for the ultrasonic waves in random media show that such effects as cutoff frequency, wave dispersion, energy distribution in vibration modes, wave attenuation by scattering and absorption, observed in discrete media, have close resemblance to the similar phenomena observed at atomic scales, such as phonons or lattice thermal vibration of atoms in the crystalline solids. For example, the cutoff frequency in lattice vibration is related to the interatomic spacing, similarly, the cutoff frequency at mesoscopic scales depends on grain separation or grain size. These similarities are also observed in wave scattering and attenuation and their dependence on wave number, kR (k=2π/λ and R is particle size). In this paper experimental measurements with data analysis on cutoff frequency, wave attenuation, and wave dispersion related to the above mentioned similarities will be presented, and the extension of these findings to the behavior of acoustic waves in other discrete media will be discussed.

Molecular Dynamics Study of the Effect of Atomic Roughness on the Slip Length at the Fluid−Solid Boundary during Shear Flow

A systematic study into the effect of solid roughness on the slip boundary condition during shear flow is presented. Atomic roughness is modeled by varying the size and spacing between solid atoms at constant packing fraction while the interaction parameters and the thermodynamic state of the fluid are kept constant. It is shown that the fluid structure as manifest in the amplitude of the density oscillations increases with increasing smoothness of the surfaces. The fluid-solid slip length is shown to exhibit nonmonotonic behavior as the solid structure is varied from smooth to rough. Slip occurs for both smooth and rough surfaces, and stick occurs only for surfaces commensurate with the fluid.

Positronium in Solids: Computer Simulation of Pick-Off and Self-Annihilation

Positronium (Ps) is simulated using Path Integral Monte Carlo (PIMC). This method can reproduce the results of previous simple theories in which a single quantum particle is used to represent Ps within an idealized pore. In addition, the calculations treat the e{sup -} and e{sup +} of Ps exactly and realistically model interactions with solid atoms, thereby correcting and extending the simpler theory. They study the pick-off lifetime of o-Ps and the internal contact density, {kappa}, which controls the self-annihilation behavior, for Ps in model voids (spherical pores), defects in a solid (argon), and microporous solids (zeolites).

ESR and X-ray Investigations of Deuterium Atoms and Molecules in Impurity-Helium Solids

Impurity-helium solids created by injecting deuterium atoms and molecules into superfluid 4He have been studied via electron spin resonance (ESR) and x-ray diffraction methods. We measured the g-factor, the hyperfine constant and the spin-lattice relaxation time of D atoms in D-D2-He solids. These measurements show that D atoms are mainly stabilized in D2 clusters. Using an x-ray method we found the size of D2 clusters to be ∼90A in diameter and the densities of D2 molecules in the samples to be of order 2.5⋅1021 cm−3 . The highest average concentration of D atoms achieved in D-D2-He solids was ∼1.5⋅1018 cm−3 . The local concentrations of D atoms within D2 clusters is found to be large (∼2⋅1019 cm−3).

Stabilization of High Concentrations of Nitrogen Atoms in Impurity-Helium Solids

Impurity-helium (Im-He) solids created by injecting gaseous helium with an admixture of nitrogen atoms and molecules into superfluid 4He have been studied via electron spin resonance (ESR). We have studied the efficiency of stabilization of N atoms in Im-He samples prepared from nitrogen-helium gas mixtures with different fractions of nitrogen varying from 0.25% to 4%. Some of the observed ESR spectra of N atoms in the Im-He samples are very broad. The highest local concentration of N atoms determined from dipole-dipole broadening of the ESR line is ∼8×1020 cm−3. The highest average concentrations of N atoms in N-N2-He solids were much lower (of order 1019 cm−3). The samples with high concentrations of N atoms were stable in liquid helium, and remained stable even after draining liquid helium from the sample at T≤3.5 K.

Nonlinear Diffusion in Excited HgCdTe and Si Crystals

We discuss the properties of the nonlinear diffusion equation for the case of diffusion in excited systems. The diffusion coefficient is directly proportional to the concentration of impurities and depends on time in a special way. For the description of the excited systems, we used a special temperature function, which defined the time dependent diffusion coefficient and the Boltzmann distribution of the excited vacancies or impurity atoms in solids. This model was used for the approximation of indium concentration profiles in HgCdTe of a rapid diffusion component and very fast diffusion of metastable vacancies irradiated by soft X-rays in an excited Si crystal.

Solid atomization technology and process of molten metal and alloy

A novel solid atomization technology has been developed with a high-velocity gas jet stream containing solid salt particles as the atomization medium. Metal and alloy powders with a finer particle size, a narrower size distribution and a higher cooling rate than those of the conventional gas atomization technology have been produced by this technology. The affecting factors of the particle size of as-prepared metallic powders have been analyzed, especially the influence of the state of NaCl particles.

First Identification of an Aliphatic cis-α,β-Dinitroso Compound: A Combined Experimental and Quantum Chemical Study

A combination of experimental and quantum chemical results has been used to characterize dimethylfuroxan, (CH3)2C2N2O2, isolated in a matrix of solid Ar atoms at 12 K, and then to study its photoinduced isomerization and decomposition. The less-stable isomer 2,3-dinitrosobut-2-ene can be generated by UV photolysis (λmax = 254 nm). However, this species is photolabile and decomposes upon prolonged photolysis times with light having λmax = 254 nm or with IR radiation (λmax = 700 nm) to give CH3CNO as the final, photostable product. The two photoproducts were characterized using a combination of experimental and quantum chemical results. The work presented herein sheds light on the physical and chemical properties of furoxans, which may gain considerable importance as a source of NO, which itself plays a significant role as a biological messenger.

Ag(1 0 0) surface density of states probed by metastable deexcitation spectroscopy: A comparison between experiment and theory

The metastable deexcitation spectrum of Ag(1 0 0) is measured and information on the effective density of states probed by the metastable atom is obtained by deconvolution of the experimental spectrum, under the hypothesis that it reflects the self-convolution of the surface density of states. Layer resolved semi-empirical LCAO calculations are compared to the deconvolution curve, showing poor agreement as far as the branching ratios of sp versus d contributions are concerned. The agreement between theory and experiment strongly improves if an energy dependent exponential decay factor is included in the first layer calculated density of states to account for the energy dependent increased overlap between electronic states of the solid and metastable atom outside the surface.

ESR investigation of hydrogen and deuterium atoms in impurity-helium solids

Impurity-helium solids (Im–He solids) are porous solids created by injecting a beam of mixed helium and impurity gases into superfluid He4. In this work we use electron spin resonance (ESR) techniques to investigate Im–He solids containing atoms and molecules of hydrogen and/or deuterium. We have performed studies of low temperature (T∼1.35 K) tunnelling chemical reactions in which deuterium atoms replace the hydrogen atoms bound in H2 or HD molecules to produce large (up to 7.5×1017 cm−3) and relatively stable concentrations of free hydrogen atoms. The time dependence of H and D atom concentrations has been investigated for Im–He samples with different initial ratios of hydrogen and deuterium ranging from 1:20 to 1:1. The satellite ESR lines associated with the dipolar coupling of electron spins of H and D atoms to the nuclear moments of the hydrogen nuclei found in neighboring molecules have been observed in Im–He solids. The forbidden hyperfine transition of atomic hydrogen involving the mutual spin flips of electrons and protons has also been observed.

Using Fluctuations To Locate The Surface Roughening Transition

AbstractAn analysis of the fluctuations of a {100} FCC interface has been used to locate the surface roughening transition in kinetic Monte Carlo simulations. These simulations were conducted using a spin 1/2 Ising model treatment of the solid-liquid interface. A temperature gradient was imposed on the simulation cell providing a restoring force to the interface. Fluctuations in the number of solid atoms in the system were measured using time correlations. The fluctuation dissipation theorem was employed to relate the characteristic times retrieved from the time correlation to the kinetic coeffcients of the system. A dramatic change was found in these characteristic times, which locates the surface roughening transition.

Using Fluctuations To Locate The Surface Roughening Transition

AbstractAn analysis of the fluctuations of a {100} FCC interface has been used to locate the surface roughening transition in kinetic Monte Carlo simulations. These simulations were conducted using a spin 1/2 Ising model treatment of the solid-liquid interface. A temperature gradient was imposed on the simulation cell providing a restoring force to the interface. Fluctuations in the number of solid atoms in the system were measured using time correlations. The fluctuation dissipation theorem was employed to relate the characteristic times retrieved from the time correlation to the kinetic coefficients of the system. A dramatic change was found in these characteristic times, which locates the surface roughening transition.