Inert Gas Crystals Research Articles

The effect of matrices on the low-temperature IR spectra of a formic acid molecule isolated in inert gas crystals

Using the DFT/M06-2X method, we simulated the structure and vibrational spectra of inert gas (Ne, Ar, Kr, Xe) clusters with an isolated formic acid molecule. The impact of the matrix environment on the vibrational spectra of formic acid is established. The values of the matrix shifts of the vibrational frequencies predicted by calculations matched those obtained experimentally. We found that the best agreement between the calculated and experimental shifts occurred for clusters with the smallest deformation energy of the inert gas crystal. At the same time, the ratio of the volume of the molecule embedded in the matrix, and the volume of substituted matrix gas atoms, allowed one to determine only the minimum possible size of the matrix site. The calculated and experimental values of the matrix shifts are in good agreement, indicating the computation method matches the actual experimental conditions.

Novel approaches in low energy threshold detectors for Dark Matter searches

Low energy threshold detectors are necessary in many frontier fields of the experimental physics. In particular these are extremely important for probing Dark Matter (DM) possible candidates. We present a novel detection approach that exploits the energy levels of atoms embedded into solid crystals of inert gases maintained at low temperature. We exploit laser-assisted transitions that are triggered by the absorption of the incident particle in the material and leads to a photon or an electron emission . Two possible schemes are thus possible: one is based on light signal while the other takes advantage of high efficiency-in vacuum single-electron detection using microchannel plate or channeltron sensors. Through these schemes, we could be able to detect low energy release in the range from sub eV to tens of eV in large volume crystals opening thus the possibility to investigate light DM candidates.

Experimental setup for the growth of solid crystals of inert gases for particle detection.

Low energy threshold detectors are necessary in many frontier fields of the experimental physics. In this work, we present a novel detection approach based on pure or doped matrices of inert gases solidified at cryogenic temperatures. The small energy release of the incident particle can be transferred directly (in pure crystals) or through a laser-driven ionization (in doped materials) to the electrons of the medium that are then converted into free electrons. The charge collection process of the electrons that consists in their drift within the crystal and their extraction through the solid-vacuum interface gives rise to an electric signal that we exploit for preliminary tests of charge collection and crystal quality. Such tests are carried out in different matrices of neon and methane using an UV-assisted apparatus for electron injection in crystals.

Lennard-Jones crystal: Complete vibrational spectrum, its local approximation, and thermodynamic functions

Different methods of taking into account the vibrational partition function in Helmholtz free energy for imperfect crystals of inert gases are compared. These methods are based on the complete vibrational spectrum of the fcc lattice or on few local frequencies. Thermodynamic properties of the Lennard- Jones crystal are considered in the framework of the lattice gas model with the potential interaction in the quasi-chemical approximation. In frequency calculations in the quasi-dimeric model, the role of local averaging, which leads to a threefold degeneracy of the vibrational contribution, is compared to the role of the direct diagonalization of the matrix of the second derivatives of potential energy in an imperfect cluster. The effects of different vibrational contributions on internal energy, entropy, and heat capacity at constant volume are considered as a function of temperature. The results of calculating thermal expansion in a vacuum for four inert gas crystals are compared with experimental data.

On the surface energy of crystals of inert gases

The dependences of the specific surface energy (σ) and its isochoric temperature derivative (∂σ/∂T) V on the degree of compression (V/V 0) of the crystal are calculated on the basis of the Mie-Lennard-Jones pair potential of interatomic interaction. The calculations are performed for all face-centered cubic crystals of inert gases (from Ne to Rn) to the degree of compression V/V 0 = 0.016 along three isotherms: 1K, T m and 300 K, where T m is the melting temperature at zero pressure (V/V 0 = 1). The activation processes such as the creation of vacancies and self-diffusion are taken into account in the calculations. It is shown that the isotherm σ(V/V 0) reaching its maximum at (V/V 0)max sharply decreases upon further compression. The surface energy becomes negative (σ(V/V 0) fr =0) at V/V 0 ≤ (V/V 0) fr < (V/V 0)max which should stimulate the process of crystal fragmentation, i.e., an increase in the specific (per atom) intercrystallite surface. It is shown that at high temperatures the condition of fragmentation holds in the crystal in the case of uniform tension, but it is already in the region of the liquid phase. The values of σ, (∂σ/∂T) V , the vacancy concentration and the fraction of the diffusion atoms are estimated at the points: V/V 0 = 1, (V/V 0)max and (V/V 0) fr at 1 K, Tm and 300 K. The size evolution of the surface and activation parameters is studied using neon as an example.

Fullerite Crystal Thermodynamic Characteristics and the Law of Corresponding States

The existence of single-wall carbon nanotubes in organic solvents in the form of clusters is discussed. A theory is developed based on a bundlet model, which enables describing the cluster-size distribution function. Comparison of calculated solubilities with experiments would permit obtaining energetic parameters, characterizing the interaction of a nanotube with its surrounding. Fullerenes and nanotubes are objects whose behaviour in many physical situations is characterized by peculiarities, which show up in that these systems represent the only soluble forms of carbon, what is related to their molecular structures. The fullerene molecule is a virtually uniform closed spherical-spheroidal surface and a nanotube is a smooth cylindrical unit. Both structures give rise to weak interactions between the neighbouring units in a crystal and promote their interaction with solvent molecules. The phenomena have a unified explanation in the bundlet model, in which the free energy of a nanotube in a cluster is combined from two components: a volume one proportional to the number of molecules n in a cluster and a surface one proportional to n1/2. Growth mechanisms of fractal clusters in fullerene solutions are analyzed along with similarity laws, determining the thermodynamic characteristics of fullerite crystals. In accordance with the similarity laws, the dimensionless Debye temperatures theta0 for all crystals belonging to the considered class should be close. Temperatures theta0 are determined by a similarity relation from experimental and estimated data. Fullerite theta0 is twice that for inert-gas crystals because, near the Debye point, the fullerite crystal is orientationally ordered so that its structure is dissimilar to face-centred cubic. A fullerene molecule whose thermal rotation is frozen cannot be considered as a spherically symmetric particle. The fulfilment of the similarity laws, which are valuable for particles with spherically symmetric interaction potential, would hardly be expected.

Empirical relation for the activation energy of diffusion in elemental substances

By examining the reported data on the self-diffusion in metals, inert-gas crystals, and silicon and the diffusion of impurities in silicon and silver, it is shown that the activation energy of diffusion can be related to the entropy of melting of the diffuser and the melting point of the host material

Effect of overlap of localized orbitals on the band structure of insulators under pressure

A new method for calculating the energy spectra of compressed insulators based on the cluster expansion is proposed. The dependence on the compression of the spectra of both the conduction band and the valence bands is uniquely determined by the values of the overlap integrals of the orbitals of pairs of isolated atoms. The overlap integrals and the matrices defined by them are calculated numerically for inert-gas crystals and their properties and values are analyzed for different lattice constants. Numerical calculations of the bands of the compressed crystal in the various proposed models are carried out and discussed for neon.

Clusters of inert gases of the close packed structures

This analysis shows that a short-range interaction potential approximates parameters of inert gas crystals better than the Lennard-Jones one. A method is developed for calculation of the atom binding energy of clusters of the face-centered cubic (fcc) and hexagonal structure. It uses the short-range character of interaction of atoms that leads to interaction between nearest neighbors of the cluster only. Symmetric figures of these clusters with magic numbers of atoms are considered. Energies of clusters of fcc, hexagonal and icosahedral structures are compared for a short-range interaction between atoms.

A Theory of Melting of Molecular Crystals I. Theory and Evaluation of the Thermodynamic Properties of Melting

Abstract The Pople and Karasz theory of melting of molecular crystals, which is based on the theory of melting of inert gas crystals by Lennard-Jones and Devonshire, is extended using a third energy parameter, W″. The extension is done as follows: the previous repulsive energy parameter W is divided into two parts. The first part is the interaction between molecules which are on different sites with the same orientations, called again W. The second is the interaction between molecules on different sites and also different orientations, namely W″. W″ is combined with the previous W′ and the new W energy parameters by the arithmetic mean including an adjustable parameter. The thermodynamic properties are evaluated by the Bragg-Williams approximation. The theory is applied to plastic crystals and compared with the Pople and Karasz theory. By introducing a physical realistic coupling between orientational and positional order, ν, the theory gives a solid state rotational transition and melting transition. For...

Investigation of the Thermodynamic Properties of Anharmonic Crystals by Momentum Method. IV. The Limiting of Absolute Stability and the Melting Temperature of Crystals

AbstractOn the basis of results obtained earlier the equation for the limiting of absolute stability in the crystalline state with f.c.c. structure is found. Using this the limiting temperatures of absolute stability and the melting temperatures of inert gas crystals at various pressures (including of very high pressures) are determined. Besides, the nearest neighbour distances and the Grüneisen constants at these temperatures are obtained. In comparison with the experimental data the calculated results deviate only by some per cent.

Investigation of the Thermodynamic Properties of Anharmonic Crystals by the Momentum Method. II. Comparison of Calculations with Experiments for Inert Gas Crystals

AbstractOn the basis of results obtained in a previous paper by the moment method the thermodynamic quantities of the inert gas crystals are calculated. The obtained results are compared with the experimental data and with the ones of other authors.

Calculations for CH 4 in a tetrahedral site in an Xe matrix

The rotational splittings observed by others in the ν 3 band of methane in xenon are fitted to a model. Instead of the usually assumed octahedral site, this model assumes that the methane rotates in a tetrahedral site and it gives a reasonable explanation of the spectrum. Recent high resolution spectra have shown the presence of more than one site for methane in inert gas crystals, and the existence of multiple sites is the most likely explanation for the observed anomalies.

Computational analysis of positron experiments

A number of applications of the calculational scheme developed by Puska and Nieminen (1982-3) are reported and the predictive power of the scheme is substantiated. Effects on positron parameters of relaxation and of N or H impurities in vacancies in Mo are calculated and employed to analyse recent experiments. Predictions pertaining to H decoration of vacancies in Al and Ni suggest the use of positron lifetime studies of these systems. Positron responses to submicroscopic vacancy clusters decorated with Kr and to large Kr bubbles in Cu are calculated and used to analyse recent experiments. To accomplish this the scheme is generalised to incorporate crystals of inert gas. In turn this makes it possible to estimate the average enhancement factors of the inert-gas crystals. The cases considered in this paper indicate that the potential of the positron techniques can be enhanced further by computational analysis.

Electron spectrum of inert gas crystals under pressure

Changes in zone structure of inert gas crystals with pressure are studied. The zone structure of solid Ne, Ar, Kr, and Xe are calculated over a wide pressure range using the combined plane wave method. Stability limits of the basic dielectric phase are determined by overlap of direct gaps in the electron spectrum with pressure increase. No loss of stability upon transition to the metallic state was observed.

Self-Diffusion in Rare-Gas Solids

The process is considered of self-diffusion in inert gases. Vacancy systems, controlling the diffusion, are assumed to exist in crystallized foils of inert gases and in bulk crystals in various equilibrium and non-equilibrium thermodynamic states. The coefficients of self-diffusion obtained for crystals of inert gases agree well with the experimental results. [Russian Text Ignored].

Thermal expansion of anharmonic crystals composed of inert gases

The temperature dependence of the lattice constant and linear expansion coefficient for inert gas crystals are determined using the second-order approximation of Bogolyubov's temperature expansion. It is shown that when the temperature dependence of the Gruneisen constant is taken into account the agreement with experiment is improved over the thermodynamic perturbation theory and the theory based on a multiplicative binary distribution function.

The solid sphere model in studies of crystal stability

The possibility of employing the hard sphere model in studies of real, stable, inert-gas crystals is considered. On the basis of the firmly established phase transition criterion for a model system the stability curve of a real crystal is found, with attractive forces considered by first-order perturbation theory. The result obtained is compared with experiment.

On the dependence of the elastic properties of small crystals on size

This paper aims at determining the dependence of the mechanical properties—equilibrium atomic spacing, force constant in tension and shear, and Poisson's ratios—of small crystals on their sizes. Simple crystalline atomic configurations are considered: a finite number of atoms in f.c.c. packing forming an atomic plane whose external boundaries form a parallelogram, and a finite number of such planes in closest packing forming a parallelepiped. The analysis is based on the assumption that a potential energy exists for the crystal which is a function of the atomic positions, and whose minimum determines the stable configuration. In Section 1 the relevant mechanical properties are defined and expressed in terms of the potential function and its derivatives. In Section 2 the formalism developed in Section 1 is applied to Mie potentials defined in the paper. The expressions thus obtained have been evaluated for ( m, n) equal to (6, 12) (the so-called Lennard-Jones potential) which gives good results for inert gas crystals. The results are illustrated in the paper and show that the equilibrium spacings decrease, the force constants increase and Poisson's ratios fluctuate with increasing crystal size. Similar calculations have been carried out in Section 3 using Morse potentials, where parameters have been selected to represent respectively the metals Ni, Cu, Ag, Al, Ca, Pb and Sr. These results show tendencies similar to those found in the Lennard-Jones case.

On the octupole interactions in the inert gas crystals

The lattice dynamics theory in the long-wavelength limit in inert gas crystals is presented. Long-range octupole interactions are taken into account and their effect on the dispersion relations to within terms of q4 (q is the wavevector) at q to 0 is investigated. A method of direct experimental measurement of the octupole interaction constant is proposed. The octupole interaction constant is evaluated from experimental data for Ne crystals.