185 research outputs found
Ground state structures and photoelectron spectroscopy of [Com(coronene)]− complexes
A synergistic approach involving theory and experiment has been used to study the structure and properties of neutral and negatively charged cobalt-coronene [Com(coronene)] complexes. The calculations are based on density functional theory with generalized gradient approximation for exchange and correlation potential, while the experiments are carried out using photoelectron spectroscopy of mass selected anions. The authors show that the geometries of neutral and anionic Co(coronene) and Co2(coronene) are different from those of the corresponding iron-coronene complexes and that both the Co atom and the dimer prefer to occupy η2-bridge binding sites. However, the magnetic coupling between the Co atoms remains ferromagnetic as it is between iron atoms supported on a coronene molecule. The accuracy of the theoretical results is established by comparing the calculated vertical detachment energies, and adiabatic electron affinities with their experimental data
Structure and stability of Con(pyridine)m − clusters: Absence of metal inserted structures
A synergistic approach combining the experimental photoelectron spectroscopy and theoretical electronic structure studies is used to probe the geometrical structure and the spin magnetic moment of Con(pyridine)−m clusters. It is predicted that the ground state of Co(pyridine)− is a structure where the Co atom is inserted in a CH bond. However, the insertion is marked by a barrier of 0.33eV that is not overcome under the existing experimental conditions resulting in the formation of a structure where Co occupies a site above the pyridine plane. For Co2(pyridine)−, a ground-state structure is predicted in which the Co2 diametric moiety is inserted in one of the CH bonds, but again because of a barrier, the structure which matches the photoelectron spectrum is a higher-energy isomer in which the Co2 moiety is bonded directly to nitrogen on the pyridine ring. In all cases, the Co sites have finite magnetic moments suggesting that the complexes may provide ways of making cluster-based magnetic materials
Locally U(1)*U(1) Symmetric Cosmological Models: Topology and Dynamics
We show examples which reveal influences of spatial topologies to dynamics,
using a class of spatially {\it closed} inhomogeneous cosmological models. The
models, called the {\it locally U(1)U(1) symmetric models} (or the {\it
generalized Gowdy models}), are characterized by the existence of two commuting
spatial {\it local} Killing vectors. For systematic investigations we first
present a classification of possible spatial topologies in this class. We
stress the significance of the locally homogeneous limits (i.e., the Bianchi
types or the `geometric structures') of the models. In particular, we show a
method of reduction to the natural reduced manifold, and analyze the
equivalences at the reduced level of the models as dynamical models. Based on
these fundamentals, we examine the influence of spatial topologies on dynamics
by obtaining translation and reflection operators which commute with the
dynamical flow in the phase space.Comment: 32 pages, 1 figure, LaTeX2e, revised Introduction slightly. To appear
in CQ
Al13H−: Hydrogen atom site selectivity and the shell model
Using a combination of anion photoelectron spectroscopy and density functional theory calculations, we explored the influence of the shell model on H atom site selectivity in Al13H−. Photoelectron spectra revealed that Al13H− has two anionic isomers and for both of them provided vertical detachment energies (VDEs). Theoretical calculations found that the structures of these anionic isomers differ by the position of the hydrogen atom. In one, the hydrogen atom is radially bonded, while in the other, hydrogen caps a triangular face. VDEs for both anionic isomers as well as other energetic relationships were also calculated. Comparison of the measured versus calculated VDE values permitted the structure of each isomer to be confirmed and correlated with its observed photoelectron spectrum. Shell model, electron-counting considerations correctly predicted the relative stabilities of the anionic isomers and identified the stable structure of neutral Al13H
Manufacture of Gowdy spacetimes with spikes
In numerical studies of Gowdy spacetimes evidence has been found for the
development of localized features (spikes) involving large gradients near the
singularity. The rigorous mathematical results available up to now did not
cover this kind of situation. In this work we show the existence of large
classes of Gowdy spacetimes exhibiting features of the kind discovered
numerically. These spacetimes are constructed by applying certain
transformations to previously known spacetimes without spikes. It is possible
to control the behaviour of the Kretschmann scalar near the singularity in
detail. This curvature invariant is found to blow up in a way which is
non-uniform near the spike in some cases. When this happens it demonstrates
that the spike is a geometrically invariant feature and not an artefact of the
choice of variables used to parametrize the metric. We also identify another
class of spikes which are artefacts. The spikes produced by our method are
compared with the results of numerical and heuristic analyses of the same
situation.Comment: 25 page
The Gowdy T3 Cosmologies revisited
We have examined, repeated and extended earlier numerical calculations of
Berger and Moncrief for the evolution of unpolarized Gowdy T3 cosmological
models. Our results are consistent with theirs and we support their claim that
the models exhibit AVTD behaviour, even though spatial derivatives cannot be
neglected. The behaviour of the curvature invariants and the formation of
structure through evolution both backwards and forwards in time is discussed.Comment: 11 pages, LaTeX, 6 figures, results and conclusions revised and
(considerably) expande
Communications: Chain and double-ring polymeric structures: Observation of AlnH3n+1 − (n=4–8) and Al4H14 −
A pulsed arc discharge source was used to prepare gas-phase, aluminum hydride cluster anions, AlnHm−, exhibiting enhanced hydrogen content. The maximum number of hydrogen atoms in AlnHm− species was m=3n+1 for n=5–8, i.e., AlnH3n+1−, and m=3n+2 for n=4, i.e., Al4H14−, as observed in their mass spectra. These are the most hydrogen-rich aluminum hydrides to be observed thus far, transcending the 3:1 hydrogen-to-aluminum ratio in alane. Even more striking, ion intensities for AlnHm− species with m=3n+1 and m=3n+2 hydrogen atoms were significantly higher than those of nearby AlnHm− mass peaks for which m\u3c3n+1, i.e., the ion intensities for AlnH3n+1− and for Al4H14− deviated from the roughly bell-shaped ion intensity patterns seen for most AlnHm−species, in which m ranges from 1 to 3n. Calculations based on density functional theory showed that AlnH3n+1− clusters have chain and/or double-ring polymericstructures
A Combined Theoretical and Photoelectron Spectroscopy Study of Al3Hn- (n=1-9) clusters
Combined photoelectron spectroscopic experiments and computational studies have been performed on Al3Hn- (n=1-9) clusters. Three modes of hydrogen bonding to the Al-3 moiety have been observed: terminal, bridging, and capping. Among various hydrides, Al3H5- and Al3H8- clusters have highest HOMO-LUMO gap and largest electron affinity, respectively. Our studies indicate that as the number of hydrogen atoms increase the presence of AlH2 groups, representing the tetrahedral coordination of the Al atom, which in turn led to the stoichiometric ring structure
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