Spectrophotometric observations of comet P/Giacobini-Zinner

Spectroscopic observations of the Giacobini-Zinner comet were performed on March 20, June 20 and 21, September 11, and October 19, 1985. The September observations were performed at perihelion, exactly at the time of the International Cometary Explorer (ICE) encounter with the comet. The March and June observations were obtained with an intensified image dissector scanner (IIDS) on the 2.1-meter Kitt Peak telescope and the September and the October observations were obtained with a charge-coupled device (CCD) on the 4-meter Kitt Peak telescope. The nucleus spectra from these observations are presented

Software Engineering Laboratory (SEL). Data base organization and user's guide, revision 1

The structure of the Software Engineering Laboratory (SEL) data base is described. It defines each data base file in detail and provides information about how to access and use the data for programmers and other users. Several data base reporting programs are described also

Spectroscopy and 3D imaging of the Crab nebula

Spectroscopy of the Crab nebula along different slit directions reveals the 3 dimensional structure of the optical nebula. On the basis of the linear radial expansion result first discovered by Trimble (1968), we make a 3D model of the optical emission. Results from a limited number of slit directions suggest that optical lines originate from a complicated array of wisps that are located in a rather thin shell, pierced by a jet. The jet is certainly not prominent in optical emission lines, but the direction of the piercing is consistent with the direction of the X-ray and radio jet. The shell's effective radius is ~ 79 seconds of arc, its thickness about a third of the radius and it is moving out with an average velocity 1160 km/s.Comment: 21 pages, 14 figures, submitted to ApJ, 3D movie of the Crab nebula available at http://www.fiz.uni-lj.si/~vidrih

Activated O2 dissociation and formation of oxide islands on the Be(0001) surface: Another atomistic model for metal oxidation

By simulating the dissociation of O2 molecules on the Be(0001) surface using the first-principles molecular dynamics approach, we propose a new atomistic model for the surface oxidation of sp metals. In our model, only the dissociation of the first oxygen molecule needs to overcome an energy barrier, while the subsequent oxygen molecules dissociate barrierlessly around the adsorption area. Consequently, oxide islands form on the metal surface, and grow up in a lateral way. We also discover that the firstly dissociated oxygen atoms are not so mobile on the Be(0001) surface, as on the Al(111) surface. Our atomistic model enlarges the knowledge on metal surface oxidations by perfectly explaining the initial stage during the surface oxidation of Be, and might be applicable to some other sp metal surfaces.Comment: 5 pages, 4 figure

First-principles calculations for the adsorption of water molecules on the Cu(100) surface

First-principles density-functional theory and supercell models are employed to calculate the adsorption of water molecules on the Cu(100) surface. In agreement with the experimental observations, the calculations show that a H2O molecule prefers to bond at a one-fold on-top (T1) surface site with a tilted geometry. At low temperatures, rotational diffusion of the molecular axis of the water molecules around the surface normal is predicted to occur at much higher rates than lateral diffusion of the molecules. In addition, the calculated binding energy of an adsorbed water molecule on the surfaces is significantly smaller than the water sublimation energy, indicating a tendency for the formation of water clusters on the Cu(100) surface.Comment: 5 pages, 3 figures, submitted to Phys. Rev.

Transferable Pair Potentials for CdS and ZnS Crystals

A set of interatomic pair potentials is developed for CdS and ZnS crystals. We show that a simple energy function, which has been used to describe the properties of CdSe [J. Chem. Phys. 116, 258 (2002)], can be parametrized to accurately describe the lattice and elastic constants, and phonon dispersion relations of bulk CdS and ZnS in the wurtzite and rocksalt crystal structures. The predicted coexistence pressure of the wurtzite and rocksalt structures, as well as the equation of state are in good agreement with experimental observations. These new pair potentials enable the study of a wide range of processes in bulk and nanocrystalline II-VI semiconductor materials

Metal-to-metal charge transfer between dopant and host ions: Photoconductivity of Yb-doped CaF2 and SrF2 crystals

Dopant-to-host electron transfer is calculated using ab initio wavefunction-based embedded cluster methods for Yb/Ca pairs in CaF2 and Yb/Sr pairs in SrF2 crystals to investigate the mechanism of photoconductivity. The results show that, in these crystals, dopant-to-host electron transfer is a two-photon process mediated by the 4fN-15d excited states of Y b2+: these are reached by the first photon excitation; then, they absorb the second photon, which provokes the Y b2+ + Ca2+ (Sr2+) → Y b3+ + Ca+ (Sr+) electron phototransfer. This mechanism applies to all the observed Y b2+ 4f-5d absorption bands with the exception of the first one: Electron transfer cannot occur at the first band wavelengths in CaF2:Y b2+ because the Y b3+-Ca+ states are not reached by the two-photon absorption. In contrast, Yb-to-host electron transfer is possible in SrF2:Y b2+ at the wavelengths of the first 4f-5d absorption band, but the mechanism is different from that described above: first, the two-photon excitation process occurs within the Y b2+ active center, then, non-radiative Yb-to-Sr electron transfer can occur. All of these features allow to interpret consistently available photoconductivity experiments in these materials, including the modulation of the photoconductivity by the absorption spectrum, the differences in photoconductivity thresholds observed in both hosts, and the peculiar photosensitivity observed in the SrF2 host, associated with the lowest 4f-5d bandThis work was partly supported by a grant from Ministerio de Economía y Competitividad, Spain (Dirección General de Investigación y Gestión del Plan Nacional de I+D+i, Grant Nos. MAT2011-24586 and MAT2014-54395-P

First Principles Studies on 3-Dimentional Strong Topological Insulators: Bi2Te3, Bi2Se3 and Sb2Te3

Bi2Se3, Bi2Te3 and Sb2Te3 compounds are recently predicted to be 3-dimentional (3D) strong topological insulators. In this paper, based on ab-initio calculations, we study in detail the topological nature and the surface states of this family compounds. The penetration depth and the spin-resolved Fermi surfaces of the surface states will be analyzed. We will also present an procedure, from which highly accurate effective Hamiltonian can be constructed, based on projected atomic Wannier functions (which keep the symmetries of the systems). Such Hamiltonian can be used to study the semi-infinite systems or slab type supercells efficiently. Finally, we discuss the 3D topological phase transition in Sb2(Te1-xSex)3 alloy system.Comment: 8 pages,17 figure