Mobility transition of solid rare gases in confined environments

We report the results of x-ray diffraction and small angle scattering studies of Ar and Kr confined in sol-gel and Vycor glasses. The confined liquid crystallizes in a disordered hcp structure on freezing. Upon further cooling a sharp transition occurs at a reduced temperature of T/T(m) approximately 0.65, where the crystalline structure disappears and the total scattering decreases. This behavior marks the onset of a well-defined mobility transition, where the confined sample migrates out of the pore space.     

Molecular simulation study of triangle-well fluids confined in slit pores

Grand canonical Monte Carlo simulations are used to study the behaviour of triangle-well (TW) fluids with variable well widths confined inside slit pores. The effect of individual factors influencing the properties of confined fluids such as fluid–fluid interactions, pore size and pore wall–fluid interactions are obtained using simulations as it is difficult to experimentally determine the same. An interesting observation of this study is that inside the narrow pore of slit height h* = 5 at the high-pressure condition of P* = 0.8, for the TW fluid with long-range attraction or for the fluid at a low temperature for even a short-range attraction, the density profiles show layering such that there is a sticking tendency of the particles at centre, while there is a depletion of particles near the wall (as the layers at the centre have higher density peak heights than near the walls).     

Study on the synthesis of ε-Fe3N-based magnetic fluid

In this work, stable high-saturation magnetization ε-Fe₃N magnetic fluid was synthesized successfully by the chemical reaction of iron carbonyl (Fe(CO)₅) and ammonia gas (NH₃). The experiment results have shown that the reactive conditions, such as the nitriding temperature, the gas flux ratio of Ar1:Ar2:NH₃, the reactive time, the content of surfactant and the hole size of the porous plate used, have important effects on the phase composition, the size of magnetic particles, the magnetic properties and the stability of ε-Fe₃N magnetic fluid. Also it was found that the synthetic time of stable high saturation magnetization ε-Fe₃N magnetic fluid could be shortened by adding n-heptane into the carrier, and the size of ε-Fe₃N magnetic particles could be decreased by decreasing the pore size of the porous plate used in our experiment. Finally, stable ε-Fe₃N magnetic fluid with the saturation magnetization 1663 Gs and the mean particle size 12 nm was synthesized successfully.     

Size-dependent freezing of n-alcohols in silicon nanochannels

We present a study on the phase behavior of several linear n-alcohols (heptanol, nonanol and undecanol) in their bulk state as well as confined in mesoporous silicon. We were able to vary the mean pore radii of the nanochannels from r = 3.5  nm to 7 nm and to determine the respective temperatures of the freezing and melting transitions using infrared and dielectric spectroscopy. The smaller the chain length the lower the freezing point, both in the bulk and in the confined state. Under confinement the freezing temperature decreases by up to 28 K compared to the bulk value. In accordance with the Gibbs-Thompson model the lowering is proportional to the inverse pore radius, ΔT _fr ∝ 1/r. Moreover, the ratio of freezing temperature depression to melting temperature depression is close to the theoretical value of ΔT _fr /ΔT _melt = 3/2. The spectra also indicate a structural change: while the solid bulk alcohols are a polycrystalline mixture of the orthorhombic β- and monoclinic γ-form, geometrical confinement forces the alcohol-chains into the more simple orthorhombic structure. In addition, a part of the material does not crystallize. Such an additional amorphous phase seems to be a logical consequence of the size mismatch between molecular crystals and irregular shaped pores.     

Demixing transitions in a binary Gaussian-core fluid confined in narrow slit-like pores

Using a mean-field density functional approach we investigate phase separation transition in a binary mixture of Gaussian-core molecules confined in narrow slit-like pores. We consider pores with repulsive and attractive walls. In the case of fluid confinement in pores with repulsive and non-selective attracting walls, no phase separation in the confined fluid, prior to the bulk separation transition, was observed. However, in the case of pores with the walls selectively attracting fluid particles, we reveal that the separation transition may take place as a two-step process. During the first step the composition change occurs within a few layers adjacent to the pore walls, whereas in the second step, it takes place in the pore interior.     

From localised to delocalised electronic states in free Ar, Kr and Xe clusters

We present new results for the inner valence levels of clusters of the three inert gases Ar, Kr and Xe based on photoelectron spectroscopy studies. The inner valence levels are compared to the localised core levels and to the delocalised outer valence levels. This comparison shows a gradual change from a relatively localised behaviour for Ar inner valence 3s, over the intermediate case of Kr inner valence 4s, to a more delocalised behaviour for Xe inner valence 5s. This change correlates well with the ratio between the orbital sizes and the interatomic distances. The Kr4s intermediate case is found to exhibit characteristics of both localised and delocalised behaviour.Received: 4 May 2004, Published online: 24 August 2004PACS: 36.40.-c Atomic and molecular clusters - 36.40.Wa Charged clusters - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystalsA. Naves de Brito: On leave from Dept. of Physics, University of Brasília, 70910-900 Brasília, Brazil.     

Wideband tuning of the blue-green XeF (C→A) laser

Efficient wavelength tuning from 446 to 524 nm with a minimum spectral linewidth of 1 nm was demonstrated for an electron beam pumped XeF(CA) laser. Energy densities of 0.1 J/l were obtained for an optimized Ar/Kr/Xe/F₂/NF₃ mixture.     

On the formation of Ar-Kr two-dimensional mixtures

Successive adsorption of submonolayer amounts of Kr and Ar onto a Pt(111) surface leads to the formation of separate 2D islands of almost pure Kr and Ar content. Upon annealing, the phase separated Kr and Ar start to mix resulting eventually in a complete stochastic mixture of the two components at about T_mix = 30–36 K, depending on the Ar:Kr mixing ratio. This continuous mixing transition proceeds primarily via the dissolution of Kr atoms into the Ar 2D islands and not vice versa, as might be expected for kinetic reasons. Instead, the intermixing behaviour can be explained by the fact that the binding between an Ar and a Kr atom is stronger than between two Ar atoms but weaker than between two Kr atoms. As a consequence the incorporation of a Kr atom from the edge of an Ar island into its center is energetically favored while the dissolution of Ar into Kr is hindered.     

Equation of state of mixtures of simple fluids at high pressures

Recently Lee and Levesque have extended the Weeks-Chandler-Andersen pure fluid perturbation theory to mixtures and have compared the results to the Leonard-Henderson-Barker mixture theory. The results seem to favour the Leonard-Henderson-Barker theory. This and other previous comparisons of mixture theories have been mostly confined to the study of ‘zero pressure’ thermodynamic properties. In this paper we compare the Lee-Levesque, Leonard-Henderson-Barker and the van der Waals one-fluid theories to high pressure equation of state data for helium-xenon mixtures. This system is modelled by a binary mixture of Lennard-Jones fluids and the hard sphere reference system is characterized by the Grundke-Henderson hard sphere mixture radial distribution function. The Lee-Levesque theory compares favourably with experimental equation of state data up to pressures of 2000 atmospheres. The Leonard-Henderson-Barker and van der Waals one theories are satisfactory. Although the van der Waals one theory yields the poorest results, it does offer the advantage of having the greatest ease of computation.

Previous theoretical and Monte Carlo calculations of mixture properties have assumed the geometric mean rule for the mixed interaction energy parameter, ε ₁₂, with consequent disagreement with experimental results. We point out that ε ₁₂ can be determined from mixed second virial coefficient data and use such improved determinations of ε ₁₂. We show that this method yields significantly improved theoretical predictions.     

Infrared spectra of matrix-isolated tungsten oxides

The infrared spectra of the WO, WO₂, and WO₃ molecules were observed in Kr and Ar matrices at 14 K. The vibrational constants of W¹⁶O, ω_e and ω_eχ_e, in both Ar and Kr matrices were derived from the measured frequencies of W¹⁶O and W¹⁸O. The ν₁ and ν₃ mode absorptions of three WO₂ isotopomers were identified in krypton matrices and an upper limit to the bond angle of 119.4 ± 0.5° was determined. The true bond angle is estimated to be 114 ± 3°. The different effects of tungsten isotopes on the linewidths of the ν₁ and ν₃ absorption peaks of WO₂ were observed and are consistent with the calculated effects. The WO₃ molecule, whose spectrum was observed in Ar matrices and possibly in Kr, was found to be planar symmetric (D_3h). The observed lineshapes of WO₃ absorption peaks are consistent with the expected effects of tungsten isotopes in natural abundance.     

Free energy studies of freezing in slit pores: an order-parameter approach using Monte Carlo simulation

We report a molecular simulation study of freezing transitions for simple fluids in narrow slit pores. A major stumbling block in previous studies of freezing in pores has been the lack of any method for calculating the free energy difference between the confined solid and liquid phases. Conventional thermodynamic integration methods often fail for confined systems, due to the difficulty in choosing a suitable path of integration. We use a different approach that involves calculating the Landau free energy as a function of a suitable order parameter, using the grand canonical Monte Carlo simulation method. The grand free energy for each phase can then be obtained by one-dimensional integration of the Landau free energy over the order parameter. These calculations are carried out for two types of wall—fluid interaction, a hard wall and a strongly attractive wall modelled on carbon. The grand free energy results for both cases clearly indicate a first order fluid to solid transition. In the case of the attractive carbon wall, there are three phases. Phase A corresponds to all layers having a liquid-like structure; phase B corresponds to the contact layers (the layers adjacent to the two pore walls) being frozen and the rest of the layers being fluid-like; phase C corresponds to all the layers being frozen. Our results for the angular structure function in the individual molecular layers show strong evidence of a transition from a two-dimensional liquid phase to a hexatic phase. This is followed by a transition from the hexatic to a crystal phase.     

Effect of surface modification on the pore condensation of fluids: experimental results and density functional theory

The physisorption and pore condensation of a polar fluid (CHF₃) in a series of MCM-41 type mesoporous silica materials with native and chemically modified pore walls has been studied over the temperature range 168–293 K, corresponding to reduced temperatures T/T_c in the range 0.56-0.98, where T_c is the critical temperature of the fluid. Chemical modification of the pore walls by attachment of Si(CH₃)₃ groups causes a shift in pore condensation to higher relative pressures p/p0. This effect is most pronounced for materials with narrow pores (2.9 nm) at low temperatures. In the theoretical part of the work density functional theory based on a simple cubic lattice model of the confined fluid has been used to analyse the combined effect of a reduced pore width and weaker fluid-wall interaction caused by the surface coating. For realistic values of the model parameters it is found that the effect of the lower pore width is outweighed by the opposing effect of the lower fluid-wall interactions. The weaker temperature dependence of the pore existence curve observed experimentally for the surface modified materials can be traced back to a crossover from a two-step to a single-step process of pore filling predicted by the model.     

The Influence of Molecular Gas on the Apokamp Discharge Formation

Emission spectra of apokamp discharge plasma jets in CO₂, Ar, Kr, N₂, and their mixtures have been studied. It has been shown that the emission spectra of the Kr–N₂ mixture contain N₂ and \(\rm{N_2^+}\) bands, as well as Kr lines. The spectrum of the Ar–CO₂ mixture is presented by bands of the Fox–Duffendack–Barker system and lines of the exited argon atom. In all gas media under study, the reduction of the molecular gas quantity leads to the transition from an apokampic discharge in the form of a diffusion jet developing from the current channel to a volume discharge with a strong transverse glowing. The experimental apparatus described in this work is proposed for use in laboratory investigations of spectral characteristics of transient luminous events observed in atmospheres of planets of the Solar System.     

In situ X-ray diffraction investigation of porous silicon strains induced by the freezing of a confined organic fluid

High resolution X-ray diffraction is used to perform an in situ measurement of the variations of the lattice parameter of the nanometer size crystallites of porous silicon, induced by the freezing of a confined organic fluid, dodecane. Two p⁺ type PS layers of 60 and 70% porosity are investigated, and the variations of their lattice parameter with the temperature (in the range 150–300 K) are measured. The experimental curves are discussed in relation with the results of a previous calorimetric study of the freezing of confined dodecane. We explain the observed strains by the presence of capillary stresses, that appear in the layer due to the formation of internal liquid-vapour meniscus during the freezing process of the confined fluid. Received 22 October 1999 and Received in final form 25 March 2000     

Dynamics of water in molecular sieves by dielectric spectroscopy

A short review of our thermodynamic and structural work on Ar, N₂, CO, O₂, and the n-alkane C₁₉H₄₀ in mesoporous glasses is presented. The experimental information comes from X-ray diffraction, vapour pressure isotherms, optical transmission, heat capacity, and dielectric measurements. The filling in the pore centre formed by capillary condensation is distinguished from the adsorbed layers on the pore walls and is compared to the bulk state. Special attention is paid to metastable states and to the rearrangement of partial fillings that show up as a consequence of phase transitions.Received: 1 January 2003, Published online: 21 October 2003PACS: 61.43.Gt Powders, porous materials - 78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures - 75.75. + a Magnetic properties of nanostructures - 77.22.-d Dielectric properties of solids and liquids     

Dielectric and magnetic permittivities of three new ceramic tungstates MPr2W2O10 (M = Cd,Co, Mn)

Broadband dielectric spectroscopy measurements revealed an anomalously large relative permittivity value (ε _r?=?884) for MnPr₂W₂O₁₀, a smaller value (ε _r?=?156) for CoPr₂W₂O₁₀ and the smallest value (ε _r?=?22) for CdPr₂W₂O₁₀ at low frequency (ν?=?0.1?Hz) and above room temperature in the insulating and paramagnetic state. Below 273?K, the relative permittivity (ε _r?～?24) did not depend significantly on frequency for all the tungstates under study. Electrical resistivity, thermoelectric power, electron paramagnetic resonance, magnetic susceptibility and magnetization provided experimental evidence that the studies tungstates were paramagnetic insulators with low n-type conduction. Only in the case of MnPr₂W₂O₁₀ was a ferrimagnetic order below 45?K observed. These effects are discussed within the framework of Maxwell–Wagner polarization, chemical covalent bonds and porosity mechanism.     

Velocity dependence of total Penning ionization cross sections

The velocity dependence of the total Penning ionization cross sections,σ(v), is measured in the thermal relative velocity region, using a time of flight method.σ(v) curves are reported for the collision systems He(2¹ S)/Ar, Kr, Xe, N₂, Hg, He(2³ S)/Ar, Kr, Xe, N₂, Hg, Ne(³ P _{2, 0})/Kr, Hg, and Ar(³ P _{2, 0})/Hg. In a qualitative discussion it is shown that all features of the measuredσ(v) curves may be explained within the frame of the theory of Penning ionization, allowing to extract information on the physical quantities governing the process: on the interaction potentialV(R) and on the transition probabilityW(R). A theoretical calculation for the He(2³ S)/Ar system shows good agreement with our experimentalσ(v) curve. On the basis of the present results earlier data onσ(v), and on absolute cross sections and rate constants obtained at certain relative velocity distributions are discussed.     

Excess Gibbs energy for six binary solid solutions of molecularly simple substances

In this paper we apply the method developed in a previous study of Ar + CH₄ to the evaluation of the excess Gibbs energy G^{E S} for solid solutions of two molecularly simple components The method depends on combining information on the excess Gibbs energy G^{E L} for the liquid mixture of the two components with a knowledge of the (T, x) solid-liquid phase diagram Certain thermal properties of the pure substances are also needed G^{E S} has been calculated for binary mixtures of Ar + Kr, Kr + CH₄, CO + N₂, Kr + Xe, Ar + N₂ and Ar + CO. In general, but not always, the solid mixtures are more non-ideal than the liquid mixtures of the same composition at the same temperature Except for the Kr + CH₄ system, the ratio $\text{r =}\text{G}{}^{\mathrm{E\; S}}\text{G}{}^{\mathrm{E\; L}}$ is larger the richer the solution in the component with the smaller molecules     

Simple-Complex Fluid Transition in Microemulsions

The aim of this work was to study the rheological behavior of water in-oil microemulsion formulated with AOT (sodium bis(2-ethylhexyl) sulfoccinate) in isooctane over a temperature range from 25°C to 55°C for the oil/AOT volume fraction φ _m =0.1 and various AOT and H₂O molar ratios, W₀, ranging from 0 to 45. The apparent viscosity was measured vs. shear rate by a rotational concentric cylinder viscometer. It was shown that above s^?1, the solutions utilized behaved as Newtonian liquids. The absolute viscosities were deduced from the shear stress- shear rate rheograms. For W₀ < 17, the solutions behaved as simple fluids, where the viscosities decrease as the temperature increases. However, an opposite behavior was observed for W₀ > 17; the viscosity remained constant and then increased above a critical temperature, T_c, depending on W₀. For the molar ratio W_0,c = 17; the studied viscosity remained constant, η(W_0,c) = (1.17 ± 0.04) cP, for the temperature range studied. Thus, a simple–complex fluid transition was evident in the studied system. For the simple fluids, W₀ < W_0,c, the total activation energy vs. W₀ showed three linear parts. The different slopes were attributed to the change of the microscopic structure of the formed micelles. For the critical molar ratio W_0,c = 17, the total activation energy vanished. So, according to the Eyring lattice model, the jump of the formed micelles from one site to another was not permitted and the entire molecular groups were in vibration and rotation motions. For W₀ > W_0,c, where the formed micelles were fully hydrated, the relative viscosity of the studied solutions vs. temperature was fitted according a polynomial law, where the exponent was found to be dependent on W₀.     

Investigation of electron-beam-excited molecular gas lasers

Physical processes are investigated in active media of high-pressure gas lasers operating on electronic transitions of molecules. Lasing was obtained in the VUV ( = 172 nm) region of the spectrum by exciting compressed xenon and an Ar:Xe mixture. The effect of the temperature and pressure on the kinetics of the processes in the active laser media Ar:N₂, Xe:0₂, and Ar:Kr:F₂ are investigated. A new class of excimers, consisting of one halide and two inert atoms, is observed for the first time ever, and the temperature dependence of the spontaneous and laser emissions of XrF* is investigated; this dependence is due to the redistribution of the energy drawn from the electron beam among the excimers KrF^* and Kr2^F*.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 142, pp. 172–202, 1983.