Effective statistical weights of bound states in plasmas

Based on recent advances in the study of the statistics of interparticle distances and angles in plasmas, we develop an approach for the determination of the effective statistical weights of atomic (ionic) quantum states in ideal and nonideal plasmas. This approach allows one to account naturally for the effects of both the perturbation of the bound states by the neighboring ions and for the binding energy reduction due to the screening of the Coulomb interaction. We analyze the roles of tunneling and overbarrier escape of the optical electron from the parent ion potential well. The effects of neighbor ions and free electrons on these processes, and the simultaneous presence of several perturber ion species in the plasma are treated. We show that the present approach offers significantly more accurate effective-statistical-weight values in comparison to the existing theoretical treatments, and yields physical expressions for the empirical factors of the existing theories. Examples of calculations of effective statistical weights are given. The effects of the atomic (ionic) states collectivization on the collisional-radiative kinetics of dense plasmas are discussed. Received 10 August 2001     

Cross-section and rate coefficient calculation for electron impact excitation, ionisation and dissociation of H2 and OH molecules

The weighted total cross-section (WTCS) theory is used to calculate electron impact excitation, ionisation and dissociation cross-sections and rate coefficients of OH, H₂, OH⁺, H₂ ⁺, OH^- and H₂ ^- diatomic molecules in the temperature range 1500–15000 K. Calculations are performed for H₂(X, B, C), OH(X, A, B), H₂ ⁺(X), OH⁺(X, a, A, b, c), H₂ ^-(X) and OH^-(X) electronic states for which Dunham coefficients are available. Rate coefficients are calculated from WTCS assuming Maxwellian energy distribution functions for electrons and heavy particles. One and two temperature (θ_e and θ_g respectively for electron and heavy particles kinetic temperatures) results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θ^b)exp (-c/θ).     

Synthesis and characterization of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>–Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> magnetoelectric composites with dielectric and magnetic properties

Composite structures consisting of (001)-oriented SrTiO₃ (STO)/La_0.7Sr_0.3MnO₃ (LSMO) films of 30 nm thickness, grown on an (001) Pb(Mg_1/3Nb_2/3)TiO₃– 28 mol.% PbTiO₃ piezoelectric relaxor-ferroelectric single-crystalline wafer were investigated by means of Wide-Angle X-ray Diffraction (WAXRD) in situ under influence of a d.c. electric field with strength E up to ±18 kV/cm. The WAXRD measurements of the films and substrate reflection profiles resulted in a determination of the strain s in the films and the substrate separately. The strained state of the STO/LSMO films is effectively controlled by a huge converse piezoelectric effect of the PMN-PT substrate. The coefficients of coupling between electric-field-induced out-of-plane strain in the films and in the substrate for the composite system STO/LSMO/PMN-PT are obtained.     

Luminescence decays with underlying distributions: General properties and analysis with mathematical functions

In this work, an analysis of the general properties of the luminescence decay law is carried out. The conditions that a luminescence decay law must satisfy in order to correspond to a probability density function of rate constants are established. From an analysis of the general form of the luminescence decay law, it is concluded that the decay must be either exponential or sub-exponential for all times, in order to be represented by a distribution of rate constants H(k). Sub-exponentiality is nevertheless not a sufficient condition. Only decays that are completely monotonic have a probability density function of rate constants. The construction of the decay function from cumulant and moment expansions is studied, as well as the corresponding calculation of H(k) from a cumulant expansion. The asymptotic behavior of the decay laws is considered in detail, and the relation between this behavior and the form of H(k) for small k is explored. Several generalizations of the exponential decay function, namely the Kohlrausch, Becquerel, Mittag-Leffler and Heaviside decay functions, as well as the Weibull and truncated Gaussian rate constant distributions are discussed.     

High-pressure Raman spectroscopy of antimony: As-type, incommensurate host-guest, and bcc phases

The vibrational dynamics of elemental solids that form incommensurate host-guest structures are of fundamental interest. High-pressure Raman scattering has been used to examine the vibrational spectrum of the group-V element Sb up to 33 GPa. A_1g and E_g phonons of the ambient pressure rhombohedral A7 phase display a marked decrease with pressure, i.e., prior to the transition to the tetragonal host-guest Sb-II phase at 8.6 GPa, via the monoclinic host-guest Sb-IV phase. The Raman spectrum of the incommensurate host-guest Sb-II phase, has five bands between 80 cm⁻¹ and 200 cm⁻¹ that increase with pressure. For the bcc structure stable above 28 GPa, we observe one weak disorder-induced band that increases with pressure.     

Individual bottle brush molecules in dense 2D layers restoring high degree of extension after collapse-decollapse cycle: Directly measured scaling exponent

We prepared dense films of adsorbed brush-like macromolecules on mica substrate by transfer of compressed Langmuir monolayers from water subphase. The main macromolecular contours in the dense films were clearly resolved by SFM. The films were subjected to successive treatments by ethanol and water vapours. In accordance with previous results for isolated macromolecules, the films underwent collapse and subsequent decollapse morphological transformations in the changing vapour environment. Statistical analysis of the macromolecular dimensions in the films allowed us to measure the values of the scaling exponent determining the correlation between mean lateral and linear dimensions of the macromolecules. The analysis showed that the macromolecular conformations in the film as transferred were similar to the previously described conformations of the same macromolecules deposited directly on mica as isolated chains at much lower surface densities. The determined was close to the 0.75 value corresponding to the 2D SAW statistics. We assumed that the molecules retained the high degree of extension during the compression step due to suppressed reorganisation of the side chains. Differently from previous observations for isolated macromolecules, the restored conformations in the dense films after collapse-decollapse cycle were more extended with the of about 0.73 value. A theoretical explanation of the high degree of re-extension is proposed.     

Thermal unparticles: a new form of energy density in the universe

An unparticle with scaling dimension has peculiar thermal properties due to its unique phase space structure. We find that the equation of state parameter , the ratio of pressure to energy density, is given by providing a new form of energy in our universe. In an expanding universe, the unparticle energy density evolves dramatically differently from that for photons. For , even if at a high decoupling temperature T _D is very small, it is possible to have a large relic density at present photon temperature T _γ ⁰, large enough to play the role of dark matter. We calculate T _D and using photon–unparticle interactions for illustration.     

Amorphization and grain size effect on milled PbTiO3 studied by Raman scattering and visible photoluminescence emission

Raman scattering and photoluminescence (PL) studies on milled PbTiO₃ are presented in this paper. The results suggest that the visible PL emission could be related to both the localized states in the interface between the amorphous layer and the crystalline core and the amorphous layer itself. The Raman spectrum of PbTiO₃ milled for a long time showed the vibrational density of states, and a detailed analysis of the soft mode allowed us to conclude that the PbTiO₃ crystalline core did not experience any structural phasetransition. Received: 2 February 2001 / Accepted: 23 July 2001 / Published online: 17 October 2001     

High temperature phase transition in monohydrated L-asparagine crystal

Raman scattering has been used to investigate monohydrated L-asparagine single crystals for temperatures above 300 K in the spectral region between 20 and 3500 cm⁻¹. It is found that several phonon peaks characterizing the orthorhombic room temperature phase are notably suppressed in intensity for temperatures higher than ∼363 K. This is evidence of a new phase in this hydrogen-bonded crystalline system, and the transition is compared with both the high-pressure and the low-temperature phase transitions undergone by the crystal.     

Emission analysis of expanding laser produced lithium plasma plume in presence of ambient gas

Atomic analysis of the emission from laser-produced plasma from bulk lithium (Li) block has been made. The observed changes in the emission from lithium neutral and ionic species have been explained by considering various atomic processes. We demonstrate that the excitation process in case of neutral lithium Li (I) is electron impact excitation whereas for singly ionized lithium Li (II), radiative recombination is the dominant mechanism. We also show that the ionized species are dominant in the LPP plume.