Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X35: Energy Landscapes of Gases, Clusters, Materials and Biomolecules |
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Sponsoring Units: DCP Chair: B. J. Hinch, Rutgers Room: LACC Room 511B |
Friday, March 25, 2005 8:00AM - 8:12AM |
X35.00001: Surface defect-mediated reactivity of TiO2(110) toward gold Minghu Pan, Ken Park, Vincent Meunier, William Shelton, Sergei Kalinin, Arthur Baddorf, E.W. Plummer The novel properties of metal clusters supported by transition metal oxides originate from the specific interactions of metal clusters mediated by an oxide substrate, including local reduction below the cluster. In this work, we present recent results on the interactions between line defects on a TiO$_{2}$(110) and their reactivity toward Au nano-particles studied by a combination of Scanning Tunneling Microscopy and Spectroscopy. High-resolution STM images, interpreted with first-principles theory, show that the observed one-dimensional strands have partially reduced Ti atoms coordinated at oxygen octahedral sites. When gold nano-particles of diameters 5 nm and less are deposited on the sub-stoichiometric rows of TiO$_{x}$, the strands, like point defects and step edges on TiO$_{2}$(110), serve as nucleation sites for gold nano-clusters. Further reactions of the Au/TiO$_{2}$ system with O$_{2}$ and CO are discussed. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X35.00002: Dependence of direct-current-induced Si(111) Step Bunching on Initial Surface Miscut: Experiments and Simulations Brian Gibbons, Sylvia Schaepe, Jonathan Pelz DC heating of Si(111) surfaces can produce step bunching, with a complicated
dependence on current direction and temperature. We have measured how the
average step bunch height $H_{B}$ and maximum bunch slope $m_{B}$ depend on
initial surface miscut \textit{$\theta $}$_{0}$ for 940\r{ }C$ |
Friday, March 25, 2005 8:24AM - 8:36AM |
X35.00003: Electron Counting Rules For The Stability Of Metal-Encapsulated Sin Cages Anil Kandalam, Qian Wang, S. N. Khanna, Puru Jena The possibility of stabilizing silicon cages by encapsulating metal atoms has received wide attention due to their potential applications as building blocks for novel silicon based materials. It was earlier proposed that the stability of such cages could be reconciled within a simple model where each Si contributes one electron to the valence manifold. It was suggested that cages where the sum of the valence electrons from the metal and the contributed electrons from the Si is 18 exhibit enhanced stability. Recent experiments have suggested that cages with 20 valence electrons also exhibit enhanced stability. In this work, we will present preliminary results on a systematic theoretical study of 3d-TM doped Si cages containing a wide variety of transition metals to examine the applicability of these rules. Our studies, not only cover clusters with different number of Si atoms but we also vary charge on these clusters. The results will be compared with recent experiments on a wide variety of clusters. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X35.00004: Magnetic Dynamics of Cobalt Nanoparticle Dispersions Sue Inderhees We have used inelastic neutron scattering studies to study the dynamics and spatial correlations of monodisperse suspensions of Co nanoparticles. At high temperatures, the scattering is broad and quasielastic, reflecting the reversal of nanoparticle moments between states separated by an anisotropy barrier. Surprisingly, the scattering is strong even at wave vectors q as large as 2-3 A$^{-1}$, suggesting that the reversal of the overall nanoparticle moment involves localized and rapid fluctuations of smaller groups of Co moments. Spectral weight is transferred to increasingly small energies as the blocking temperature T$_{B}$ is approached, with the slowest fluctuations associated with the smallest wave vectors. Below T$_{B}$ the quasielastic scattering in the dynamically frozen state becomes inelastic, growing in intensity and hardening with decreasing temperature. This excitation is analogous to a bulk spin wave, albeit at a much lower energy. [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X35.00005: Ordering of CN species on copper (001) B.J. Hinch, E.Z. Ciftlikli, I.G. Shuttleworth, S.E. Sysoev, A. V. Ermakov Work function measurements during the adsorption of cyanogen (NCCN) on Cu(001) imply a dissociative adsorption of CN species over 130K. Helium atom reflectivity also indicates uptake of the radical species up to exposures of approximately 4 Langmuir, and also the onset of a weak ordering phenomena as adsorption temperatures exceed $\sim $ 240K. At 260K quasi hexagonal c(6x10) superstructures are observed with helium diffraction. At least two exposure ranges are observed with differing c(6x10) superstructure diffraction peak intensity ratios. The saturated phase shows strong long range ordering but also an intense diffuse scattering component. Inelastic scattering studies indicate a marked multiphonon scattering component to the diffuse intensity, and an absence of single phonon excitation features. Models for this simultaneously ordered/disordered surface will be discussed. The thermal stability of the CN superstructures will also be addressed. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X35.00006: Decoupling the Effect of Pseudomorphic Strain and Electron Confinement on Catalytic Properties of Supported Metal Films Raj Ganesh Pala, Feng Liu We present a model study of variations in reaction activation barriers as a function of thickness of a smooth metal film supported on a substrate. We demonstrate that the effect of pseudomorphic misfit strain and electron confinement can be completely decoupled by computing the adsorption-induced stress at the reactant's ground and transition state. This decoupling is based on linear elastic theory and is illustrated using first- principle calculations of CO dissociation on Ru film supported on the Os (0001) surface. The atomic coordination around the reactant is used to rationalize the nature of the adsorption-induced stress along the reaction coordinate. Furthermore, the effect of the different substrates (insulating/metallic) on the electron confinement is also addressed. The present model study will assist in making a quantitative estimate of the individual effects of misfit strain and electron confinement on chemical reactivity, thereby providing criteria for choosing catalytic metal films and their appropriate support. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X35.00007: Binding of Fe and S atoms to corannulene: A Theoretical Study Anil Kumar Kandalam, Bijan Rao, Purusottam Jena The equilibrium geometries and binding energies of neutral and positively charged Fe --corannulene and S-corannulene complexes are calculated under the framework of density functional theory and generalized gradient approximation for exchange and correlation functional. The global geometries are obtained by carrying out geometry optimizations using 6-311G** basis and Gaussian98 code without any symmetry constraints. In the case of neutral Fe-corannulene, the convex-$\eta ^{6}$ and convex-$\eta ^{6}$ binding sites are found to be energetically degenerate, hence are equally preferred by the iron atom. In cationic Fe-corannulene complex, however, the convex-$\eta ^{6}$ binding site is clearly preferred over the other binding sites. In the case of neutral and cationic S-corannulene complexes, the sulfur atom/ion prefers to occupy the edge $\eta ^{2}$-site on the convex face of corannulene. Here, also the energy difference between the binding sites on convex and concave faces of corannulene is very small. The preferred binding sites, calculated magnetic moments, binding energies, and ionization potentials of Fe-corannulene and S-corannulene complexes will be compared. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X35.00008: On the superhalogen character of Al13 in Al13I- N.O. Jones, T. Morisato, S.N. Khanna, D.E. Bergeron, A.W. Castleman It is shown that the reaction of aluminum cluster anions with HI in molecular beams leads to the generation of Al$_{n}$I$_{m}^{-}$ clusters. The stability of the clusters, examined by reacting the cluster products with oxygen, led to the observation of a magic cluster corresponding to the composition Al$_{13}$I$^{-.}$. Ab initio calculations indicate that the cluster features a structurally unperturbed Al$_{13}^{-}$ icosahedric core with I occupying an on-top site. An examination of the electronic charge density indicates a region of high charge density on the aluminum vertex opposite from the I atom. As we discuss, this ionically bound magic cluster can be understood by considering that Al$_{13}$ has an electronic structure reminiscent of a halogen atom. Comparisons to polyhalides are shown to provide a sound explanation for our chemical observations. $^{\ast }$Supported by DOE Grant No. DE-FG02-02ER46009 [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X35.00009: Geometry and Electronic Structure of Nickel Benzene Complexes A. K. Kandalam, S. N. Khanna, Puru Jena A synergistic approach combining the negative ion photo- detachment spectroscopy and the first principles density functional calculations is used to probe the geometrical structure, nature of electronic bonding and the magnetic moment of anionic and neutral nickel- benzene (Bez) complexes. It is shown that, contrary to the previously published results, the ground state of a Ni-Bez anion is a structure where the Ni atom is inserted in one of the C-H bonds of the benzene molecule. However, a detailed investigation of the reaction barrier shows that this ground state may be inaccessible in ordinary experiments and the observed structures in negative ion photo- detachment spectra may be the higher energy structure where a Ni atom occupies a hollow site above the benzene molecule. The stability of inserted structures in multiple Ni-Bez complexes and how they may be attained in carefully planned experiments will be highlighted. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X35.00010: First Principles Study of Electronic Structure of BF3-NH3 Complex and Associated Properties Archana Dubey, Lee Chow, Mahendra K. Mahanti, Roger Pink, M. B. Huang, R. H. Scheicher (*) , T. P. Das (**) BF$_{3}$ is a planar molecule with three-fold symmetry which is widely used to promote various organic reactions such as Friedel-Crafts acylations and alkylations. To obtain a thorough understanding of the mechanisms for this role of BF$_{3}$, we are studying from first-principles the electronic structures of BF$_{3}$ and its complexes with NH$_{3}$. The procedure used is the first principles Hartree-Fock-Roothaan procedure combined with many body perturbation theory. The results for BF$_{3}$-NH$_{3}$ system will be reported, such as the binding energy and equilibrium geometry of the complex, the nature of the B-N bond and the changes in the B-F and N-H bond strengths on complex formation. The Nuclear Quadrupole Interactions of the $^{19}$F* (spin 5/2), $^{14}$N, $^{11}$B, and $^{2}$H will be presented and compared with available experimental data. (*) Present Address: Dept. of Physics, Uppsala University, Sweden (**) Also: Dept of Physics, University of Central Florida, Orlando, Florida [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X35.00011: Accurate global potential energy surfaces and ro-vibrational states of triatomic molecules Ant\'{o}nio Varandas This presentation focus on two case systems. The first is ground-state hydrogen cyanide for which we report a global single-sheeted double many-body expansion (DMBE) potential energy surface that fits accurately the best available {\em ab initio} energies while reproducing with near spectroscopic accuracy all vibrational levels up to the saddle point for the isomerization process $\rm HCN\rightleftharpoons HNC$. The problem of assigning ro-vibrational levels having wave functions delocalized over the HCN and HNC minima is then discussed. The second system is triplet ${\rm H_3^+}$, whose potential energy surface has two adiabatic sheets conically intersecting for nuclear arrangements with $D_{3h}$ symmetry. Using highly accurate {\em ab initio} calculations, a global double-sheeted DMBE potential energy surface has been modeled which should be reliable within a few $\rm cm^{-1}$. Ro- vibronic calculations both with the inclusion of the geometrical phase (GP) effect, and without it, have also been done. We will focus on the upper sheet case, as the GP effect plays there a key role. In this regard, we discuss the case of any homonuclear triatomic molecule subject to Jahn-Teller interaction. It will be shown that the GP effect can naturally be included in a generalization of the $G$ quantum number previously derived by Watson. This generalized $G$ will be demonstrated to provide an improved quantum number for classifying such cone states. [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X35.00012: Ab Initio Study of the Use of Ag2- for Catalytic Oxidation of CO. Pratik Dholabhai, Xueyuan Wu, Asok Ray The formalisms of density functional theory with Perdew and Wang exchange-correlation functional have been employed to study the electronic and geometric structures of silver anion dimer bonded with carbon monoxide and oxygen. Different possible structures for Ag$_{2}$O$_{2}^{-}$ and Ag$_{2}$CO$^{-}$ have been investigated in detail. An all electron 6-311++G** basis set for carbon and oxygen and a pseudopotential basis set for silver have been used for performing the calculations using the Gaussian 03 suite of programs.$^{1}$ Using Ag$_{2}^{-}$ as a catalyst, a full catalytic cycle producing two CO$_{2}$ molecules will be presented. Also, presence of an intermediate state, Ag$_{2}$CO$_{3}^{-}$, which can be detected experimentally, is predicted. Presence of an energy barrier for certain reactions from the catalytic cycle has \textit{not} been observed. This is noteworthy since it has been claimed in the literature that for the same catalytic cycle to proceed using Au$_{2}^{-}$ as a catalyst, it has to overcome an energy barrier. Charge state effects have been studied by following similar steps for the catalytic cycle using the Ag$_{2}^{+}$ and Ag$_{2}$ as catalysts. We conclude that the Ag$_{2}^{-}$ performs better as a catalyst compared to the Ag$_{2}^{+}$ and Ag$_{2}$. \textbf{*}Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525). $^{1 }$\textit{Gaussian 03, }M. J. Frisch \textit{et al., }Gaussian Inc., Pittsburgh, PA. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X35.00013: Dual reactivity of iron-oxide clusters: self-stimulated CO oxidation as well as NO reduction Y. Matsuda, E.R. Bernstein, B.V. Reddy, F. Rasouli, S.N. Khanna It is shown that selected Fe$_{n}$O$_{m}$ clusters can accomplish the dual task of oxidizing CO and reducing NO. Our studies are based on the neutral iron-oxide clusters obtained by laser ablation of iron metal and subsequent reaction of the gas phase metal atoms/ions/clusters with an O$_{2}$/He mixture. A mass spectra of the clusters after a laser ionization with 118 nm , 193 nm or 355 nm photons shows that the most dominant species in the beam have the composition Fe$_{n}$O$_{n}$, followed by Fe$_{n}$O$_{n+1,2}$ for increasing n. The neutral clusters generated in the beam were subsequently reacted with CO and NO introduced in the reaction cell. The relative intensity of the specific clusters exiting the reaction chamber as a function of reacting gas pressure was evaluated through standard ionization and detection procedures of time of flight (TOF) mass spectrometry. The results indicate that while Fe$_{2}$O, Fe$_{2}$O$_{2}$ and Fe$_{3}$O$_{3}$ are all active towards both CO and NO elimination, the relative intensity of other clusters remain largely unchanged. Results of the gradient corrected density functional calculations to reveal the possible mechanisms involved in the cluster activated conversion of CO to CO$_{2}$ and NO to N$_{2}$ will be presented. [Preview Abstract] |
Friday, March 25, 2005 10:36AM - 10:48AM |
X35.00014: Semiclassical Scattering on Conical Intersections Andrei Piryatinski, Misha Stepanov, Sergei Tretiak, Vladimir Chernyak The problem of nonadiabatic vibrational dynamics in the vicinity of the electronic energy surface crossing is a key to understanding of variety of fundamental processes in photochemistry including radiativeless energy relaxation and photoisomerization in (bio)molecules. To address the problem, advanced theoretical methods have been developed and implemented as numerical techniques. In this contribution we focus on the photoexcited wavepacket scattering problem in the vicinity of conical intersection, and demonstrate that simple analytical expressions for the scattering matrix can be obtained in the semiclassical approximation. Simplicity of the latter expressions allow us to develop a clear quantitative picture of the photochemical processes taking place near the level crossing surface. This picture is verified using the numerical simulations, and good agreement is found for the realistic set of parameters. Therefore, it is now feasible to implement our computational method into the large scale molecular dynamics simulations significantly reducing the computational costs. [Preview Abstract] |
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