Theoretical Energy Distributions Research Articles

Determination of fundamental parameters and circumstellar properties for a sample of B[e] stars

AbstractWe develop a simple model to derive theoretical continuum energy distributions for B[e] stars, consisting of a B star surrounded by an envelope made of gas and dust. We select a sample of B[e] objects for which we construct the observed energy distributions, from 0.1 to 100 μm, using available photometric and spectroscopic data. We present some preliminary fittings.

One-dimensional self-gravitating sheet model and Lynden-Bell statistics

The Lynden-Bell statistics has been proposed to explain common features among galaxies, which are not in thermal equilibrium. The statistics is not successful to reproduce energy distribution in the one-dimensional self-gravitating sheet model except for initial states near the virial equilibrium. The breakdown is caused by dynamically accelerated high-energy sheets, and hence a modified statistics is examined by focusing on low-energy sheets in order to clarify validity of the basic idea of the Lynden-Bell statistics. The modification improves agreement between the theoretical and numerical energy distributions in a wide interval of the initial virial ratio.

Critical distance for secondary ion formation: Experimental SIMS measurements

We have performed direct experimental measurements of the critical distance for the secondary ion formation process. To this end, we compared the experimentally measured energy distribution of secondary Si − ions with the theoretical energy distribution (Sigmund-Thompson relation) of secondary Si atoms. Our model states that the maxima positions of these two energy distributions differ by the Coulomb interaction potential between the outgoing ion (Si − in our case) and a charge with the opposite polarity formed at the surface after electron transition between the outgoing Si atom and the surface. Quite a reasonable value was obtained for the critical distance, but with a large scatter in experimental data. The conclusion has been made that the experimental technique should be improved to get more precise values of the critical distance, which is of high importance for practical purposes.

Ionization probability of sputtered particles as a function of their energy: Part II. Positive Si + ions

In this paper we represent the experimental ionization probability of sputtered silicon atoms as a function of their energy, which has been obtained for positive Si + ions sputtered from silicon by O 2 + ion beam. To explain the experimental data, we have considered ionization of an outgoing atom at a critical distance from the surface, which occurs due to the electron transition between this atom and the surface, and suggested the formation of a local surface charge with the polarity opposite to that of the outgoing ion that has just been formed. Then we have considered the interaction between those two charges, outgoing ion, and surface charge as a process of the particle passage through a spherical potential barrier; as a result, we have obtained the theoretical energy distribution of secondary ions. Together with the well-known Sigmund–Thompson energy distribution of sputtered atoms, the obtained ion energy distribution allowed us to derive the equation for the secondary ion yield versus the sputtered particle energy. Both equations derived have exhibited a quite good correlation with our experimental results and also with a large number of published experimental data.

New complexes of guanidine with acetic, trichloroacetic and trifluoroacetic acids: The DFT structural and vibrational investigations

The density functional theory (DFT) methods were used for theoretical studies of three compounds. The guanidine molecule is a main structural unit in these complexes, while the studied acids have similar chemical character. The family of simple guanidinium compounds is intensively investigated as potential materials with ferroic phase transitions. Among studied “guanidinium” complexes, those with crystal structure without macroscopic center of inversion, exhibit NLO properties. For three compounds: CN 3H 5CH 3COOH, CN 3H 5CCl 3COOH and CN 3H 5CF 3COOH the detailed theoretical calculations were performed. For each complex the equilibrium geometry was obtained. The calculated geometrical parameters (bond lengths and angles) of all investigated complexes are compared. The theoretical vibrational frequencies and potential energy distribution (PED) of three “guanidinium” compounds were calculated by B3LYP method. On the basis of PED calculations the detailed assignments of bands for new chemical complexes are presented. The real crystals were obtained in the case of CN 3H 5CH 3COOH compound, only. The theoretical results can be used in future, when the synthesis of trichloroacetic and trifluoroacetic analogues of CN 3H 5CH 3COOH will be done.

Buffer-gas influence on multiphoton absorption and dissociation in different gas mixtures

Buffer-gas influence on the multiphoton absorption and dissociation in different mixtures was investigated using the simple method based on the empirical and theoretical vibrational energy distribution, generalized coupled two-level model and photoacoustic cell especially designed for low pressures studies. Energy transfer efficiency was analyzed by means of pulsed photoacoustic spectroscopy technique. Collisional effects of buffer-gas (Ar) pressure are introduced to enhance the absorption and relaxation characteristics of irradiated absorbing molecules (SF6). Functional behavior of mean number of absorbed photons per molecule 〈n〉total and a dependence on buffer-gas pressure (pbuff) which enables us to confirm or predict some physical and chemical processes are presented. Limitation of proposed model was analyzed depending on both gas pressure and laser fluence. Results are compared with other previously obtained by the same experimental technique but for different absorber and different molecular buffer-gas.

Investigation of the neutron energy distribution of the IRSN 241Am-Be( ,n) source

The neutron energy distribution of the IRSN standard 241Am-Be(alpha,n) source was measured using a proton recoil liquid scintillator, BC501A, >1.65 MeV. The experimental data were compared with the ISO recommended neutron energy distribution for an Am-Be source and some significant discrepancies were observed. Monte Carlo simulations were then performed to investigate on the neutron source term in order to consider the different parameters between the IRSN Am-Be source and the one used to establish the neutron emission spectrum recommended by the ISO standard. The variation of the parameters of the source did not explain the remaining discrepancies. A good agreement with the experimental results was observed when the theoretical neutron energy distribution from Geiger and Van der Zwan was introduced in the study as new source term. These investigations showed that the ISO recommended Am-Be distribution might not be well suited to represent the neutron energy distribution of all Am-Be sources, and that the manufacturing of the sources might play a major role in the neutron fluence energy distribution.

Continuum Energy Distribution of Bright Be Stars

This paper presents the spectrophotometric observations of five Be stars belonging to spectral type from B2.5 to B5 and luminosity class from III to V. The continuum energy distribution data of these stars are extracted from the spectrophotometric measurements over the wavelength range of λλ 3200–8000 A. The observed data of continuum energy distribution are compared with the theoretical energy distribution curves to derive the value of effective temperatures of these stars. We have also examined the effect of circumstellar envelopes of Be stars on their continuum energy distribution. It has been found that the extended circumstellar envelope of Be stars affect the underlying continua in the near-ultraviolet and near-infrared regions. Three Be stars are found to exhibit near-ultraviolet and near-infrared flux deficiency in their continua. For one of the Be, namely star HR 1289, we report the first continuum energy distribution measurements. Another normal B star HR 1363, which was observed as comparison star, was found to exhibit near-ultraviolet and excess emission, which are signatures of a Be star, thus putting this star in the category of Be stars.

The equilibrium structures, vibrational spectra, NLO and directional properties of transition dipole moments of diguanidinium arsenate monohydrate and diguanidinium phosphate monohydrate: The theoretical DFT calculations

For diguanidinium arsenate monohydrate and diguanidinium phosphate monohydrate the energies were minimized and the theoretical vibrational frequencies and potential energy distribution (PED) were calculated by density functional method. The 6-31++G(d,p) basis set was used. The assignment of the bands has been made on the basis of the calculated PED. For calculated equilibrium geometries two methods for determination of direction of transition dipole moments (TDM) were used. The oriented gas model was used for calculation of “static” TDM and from the other side the “dynamic” approximation of TDM by analysis of changes in internal coordination during characteristic vibrations was performed. The restricted Hartree–Fock (RHF) methods were used for calculation of the hyperpolarizability for both investigated molecules. The theoretical results are compared with experimental value of β.

How extreme are the Wolf-Rayet clusters in NGC 3125?

We reinvestigate the massive stellar content of the irregular dwarf galaxy NGC 3125 (Tol 3) using Very Large Telescope (VLT)/FORS1 imaging and spectroscopy, plus archival VLT/ISAAC, Hubble Space Telescope (HST)/FOC and HST/STIS data sets. FORS1 narrow-band imaging confirms that the NGC 3125-A and -B knots represent the primary sites of Wolf–Rayet (WR) stars, whilst HST imaging reveals that both regions host two clusters. Both clusters within region A host WR stars (A1 and A2), for which the optically fainter cluster A2 is heavily reddened. It is not clear which cluster within region B hosts WR stars. Nebular properties are in good agreement with previous studies and infer a Large Magellanic Cloud (LMC) like metallicity of log(O/H)+12∼8.3. LMC template mid-type WN and early-type WC spectra are matched to the observed blue and red WR bumps of A1 and B, permitting the contribution of WC stars to the blue bump to be quantified. From our FORS1 spectroscopy, we obtain N(WN5–6:WC4) = 105:20, ∼55:0 and 40:20 for the clusters A1, A2 and B1 + 2, respectively. Our results are a factor of ∼3 lower than previously reported by optical studies as a result of a lower Hα/Hβ derived interstellar reddening. Using Starburst99 theoretical energy distributions to estimate O star populations for each cluster, we find N(WR)/N(O) = 0.2 for A1 and 0.1 for A2 and the clusters within region B. From Hα narrow-band imaging, the O star content of the Giant H II regions A and B is found to be a factor of 5–10 times higher than that derived spectroscopically for the ultraviolet (UV)/optically bright clusters, suggesting that NGC 3125 hosts optically obscured young massive clusters, further supported by VLT/ISAAC K-band imaging. Archival HST/STIS UV spectroscopy confirms the low interstellar reddening towards A1, for which we have determined a Small Magellanic Cloud (SMC) extinction law for NGC 3125, in preference to an LMC or starburst law. We obtain N(WN5–6) = 110 from the slit loss corrected He II λ1640 line flux. This is in excellent agreement with optical results, although it is a factor of 35 times lower than that inferred from the same data set by Chandar, Leitherer & Tremonti. The discrepancy is due to an anomalously high interstellar reddening derived from their use of the generic starburst extinction law. Highly discrepant stellar populations may result in spatially resolved star-forming regions from UV and optical studies through the use of different extinction laws.

The structure, vibrational spectra and nonlinear optical properties of the l-lysine × tartaric acid complex—Theoretical studies

The room temperature X-ray studies of l-lysine × tartaric acid complex are not unambiguous. The disorder of three atoms of carbon in l-lysine molecule is observed. These X-ray studies are ambiguous. The theoretical geometry study performed by DFT methods explain the most doubts which are connected with crystallographic measurements. The theoretical vibrational frequencies and potential energy distribution (PED) of l-lysine × tartaric acid were calculated by B3LYP method. The calculated frequencies were compared with experimental measured IR spectra. The complete assignment of the bands has been made on the basis of the calculated PED. The restricted Hartee–Fock (RHF) methods were used for calculation of the hyperpolarizability for investigated compound. The theoretical results are compared with experimental value of β.

The theoretical calculations of vibrational spectra of guanidine selenate and guanidinium sulphate. Determination of direction of transition dipole moments by two methods: oriented gas model and changes in displacement eigenvectors computed by DFT method

For guanidinium selenate and guanidinium sulphate the energy was minimized and the theoretical vibrational frequencies and potential energy distribution (PED) were calculated by density functional method. The 6-31++G(d,p) basis set was used. The assignment of the bands has been made on the basis of the calculated PED. For calculated equilibrium geometries two methods for determination of direction of transition dipole moments (TDM) were used. The oriented gas model was used for calculation of ‘static’ TDM and from the other side the ‘dynamic’ approximation of TDM by analysis of changes in internal coordination during characteristic vibrations was performed.

The structure, vibrational spectra and nonlinear optical properties of neutral melamine and singly, doubly and triply protonated melaminium cations—theoretical studies

The melaminium cation can occur in three form: singly, doubly or triply protonated. The geometry and nonlinear optical properties (NLO) for all forms were computed by ab initio method. The restricted open Hartree–Fock (ROHF) computational method with 6-311+G basis set was used. Comprehensive studies of electronic and molecular structures have been performed. The theoretical vibrational frequencies and potential energy distribution (PED) of melaminium cations were calculated by B3LYP method. The calculated frequencies were compared with measured one for newly obtained compounds where melaminium cations are common component. The assignment of the bands has been made on the basis of the calculated PED. The restricted Hartree–Fock (RHF) methods were used for calculation of the hyperpolarizability for each ion. For comparison, analogous calculations were carried out for the closed-shell neutral melamine. The theoretical results can be used in design of new molecular compounds with nonlinear optical properties.

Mass and kinetic energy distributions of fission fragments using the time dependent generator coordinate method

Low energy fission is described using a microscopic and time-dependent approach. Fission dynamics is derived from the evolution of a collective wave function in the restricted space generated by the quadrupole and octupole collective variables. Propagation is made by use of the Time Dependent Generator Coordinate Method with a basis set of Hartree Fock Bogoliubov states obtained from self-consistent calculations with the Gogny force. Theoretical fragment mass and kinetic energy distributions in actinide fission are presented. First calculations with selected initial states indicate that the main features of experimental mass distributions in238U are reproduced. The peak-to-valley ratio appears to significantly depend on the parity of the initial state chosen.

Mass and kinetic energy distributions of fission fragments using the time dependent generator coordinate method

Low energy fission is described using a microscopic and time-dependent approach. Fission dynamics is derived from the evolution of a collective wave function in the restricted space generated by the quadrupole and octupole collective variables. Propagation is made by use of the Time Dependent Generator Coordinate Method with a basis set of Hartree Fock Bogoliubov states obtained from self-consistent calculations with the Gogny force. Theoretical fragment mass and kinetic energy distributions in actinide fission are presented. First calculations with selected initial states indicate that the main features of experimental mass distributions in 238U are reproduced. The peak-to-valley ratio appears to significantly depend on the parity of the initial state chosen.

A new technique for obtaining the activation energy distribution of a relaxing system

This paper describes a robust and fast fitting procedure applicable for relaxing processes that cannot be understood as a discrete sum of single processes but require an activation energy distribution. The method is based on a set of closed-form expressions that allow the computation of the relaxation parameters directly from the isochronal curves obtained experimentally. The usefulness of this method is checked by analyzing the isochronal curves given by a theoretical energy distribution and the magnetic disaccommodation spectra observed in yttrium iron garnet (YIG) samples.

A Wavepacket Study on Translational Energy Distributions of the Photo-stimulated Desorbed Xe from an Oxidized Si(001) Surface

We report a quantum wavepacket study on the characteristic bimodal translational energy distribution of photostimulated desorbed Xe from an oxidized silicon (001) surface observed by Watanabe and Matsumoto, Faraday Discuss. 117 (2000) 203. We have simulated the theoretical translational energy distributions based on wavepacket calculations with a sudden transition and averaging model to reproduce the experiment. We discuss the desorption mechanism and suggest a very strong position dependence of the deexcitation processes for Xe/oxidized Si(001).

Basis of high-pressure water-ice jet creation and application for surface treatment

The high-pressure water-ice jet treatment is a new technology that has grown in popularity lately. The presented technology is based on a hgh-pressure water-ice jet with addition of dry-ice CO2 pellets. The theoretical basis, including velocity and kinetic energy distributions of ice particles also with thermodynamical particle states during the water-ice jet creation are discussed. Using theoretical results the experimental plant, accompanied with suitable equipment, was built. It enables adequate research to be achieved that makes it possible to determine the jet erosion mechanism and point out the effects of different material treatments.

Analysis of the field-electron energy distribution from amorphous carbon-nitride films

The study of field-electron energy distribution (FEED) from amorphous carbon-nitride (a-CN) films is essential to understanding the mechanism of field emission from this promising material. In this work, the FEED from an a-CN film was obtained by measuring the distribution of electrons field-emitted from individual emitting sites. The FEED are recorded at different electrical fields. We found that the peak shifts as well as the half-widths increase with increasing applied fields. Furthermore, multipeak features are observed at the low-energy side of the spectra. The FEED data are fitted by using a theoretical energy distribution of free-electron approximation. We assumed that the small peaks originate from the interband states of a-CN film. We propose that although the emission mainly originates from current injection into the conduction band of a-CN film, the electrons may also be emitted from interband states.

Origins of the energy distribution structure in coherent pion production at intermediate energies

New calculations have been done that now cover the intermediate energy range from 100 MeV/nucleon to 2.0 GeV/nucleon incident energy for the reaction 12C + 12C [Formula: see text] 12C + 12C (15.11 MeV) +π0, where constructive, coherent Δ-hole states are excited in either nucleus while the companion nucleus is excited to a coherent nucleon–hole state describing the spin–isospin, giant resonant state at 15.11 MeV. The Δ (1232 MeV) isobar then decays to a nucleon and pion. Theoretical pion energy distributions are calculated and, for the first time, results above 400 MeV/nucleon are shown. A theoretical basis for understanding how the shapes of the pion distributions change as a function of incident energy is described. The fundamental shape of the Δ-production amplitude as a function of momentum transfer is discussed and the effects of the energy-dependent nuclear width are examined. Furthermore, the connection between the origins of the pion distribution to the final pion shapes is made and the importance of the giant resonance in providing an important signature is pointed out. By pushing the calculations above 400 MeV/nucleon, it was discovered that sliding kinematics and kinematic turnarounds occur due to the two-to-three-body sequential nature of the reactions and these effects determine the final structure of the pion distributions at higher incident energies. PACS Nos.: 24.10Cn, 24.30Cz, 25.70-z, 25.80-e