2,334 research outputs found
Jahn-Teller instability in C6H6+ and C6H6- revisited
The benzene cation (C6H6+) has a doublet (e_{1g}) ground state in hexagonal
ring (D_{6h}) geometry. Therefore a Jahn-Teller (JT) distortion will lower the
energy. The present theoretical study yields a model Huckel-type Hamiltonian
that includes the JT coupling of the e_{1g} electronic ground state with the
two e_{2g} vibrational modes: in-plane ring-bending and C-C bond-stretching. We
obtain the JT couplings from density functional theory (DFT), which gives a JT
energy lowering of 970 cm^{-1} in agreement with previous quantum chemistry
calculations. We find a non-adiabatic solution for vibrational spectra and
predict frequencies shifts of both the benzene cation and anion, and give a
reinterpretation of the available experimental data.Comment: 6 pages, 3 figure
First-principle density-functional calculation of the Raman spectra of BEDT-TTF
We present a first-principles density-functional calculation for the Raman
spectra of a neutral BEDT-TTF molecule. Our results are in excellent agreement
with experimental results. We show that a planar structure is not a stable
state of a neutral BEDT-TTF molecule. We consider three possible conformations
and discuss their relation to disorder in these systems.Comment: 3 pages, 2 figures, submitted to the proceedings of ISCOM 200
Predicted Infrared and Raman Spectra for Neutral Ti_8C_12 Isomers
Using a density-functional based algorithm, the full IR and Raman spectra are
calculated for the neutral Ti_8C_12 cluster assuming geometries of Th, Td, D2d
and C3v symmetry. The Th pentagonal dodecahedron is found to be dynamically
unstable. The calculated properties of the relaxed structure having C3v
symmetry are found to be in excellent agreement with experimental gas phase
infrared results, ionization potential and electron affinity measurements.
Consequently, the results presented may be used as a reference for further
experimental characterization using vibrational spectroscopy.Comment: 6 pages, 5 figures. Physical Review A, 2002 (in press
Membrane solitons in eight-dimensional hyper-Kaehler backgrounds
We derive the BPS equations satisfied by lump solitons in -dimensional
sigma models with toric 8-dimensional hyper-K\"ahler () target spaces
and check they preserve 1/2 of the supersymmetry. We show how these solitons
are realised in M theory as M2-branes wrapping holomorphic 2-cycles in the
\bE^{1,2}\times {HK}_8 background. Using the -symmetry of a probe
M2-brane in this background we determine the supersymmetry they preserve, and
note that there is a discrepancy in the fraction of supersymmetry preserved by
these solitons as viewed from the low energy effective sigma model description
of the M2-brane dynamics or the full M theory. Toric manifolds are
dual to a Hanany-Witten setup of D3-branes suspended between 5-branes. In this
picture the lumps correspond to vortices of the three dimensional or theory.Comment: 12+1 pages. LaTex. v2: Typos corrected and references adde
Density-functional-based predictions of Raman and IR spectra for small Si clusters
We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems
The Need for Better Methods for Determinating the Purity of Stream Water
The prevention or control of stream pollution or the conservation of the quality of the water of streams has of late years become one of the primary objects of study in the United States. In the east a number of the states have been at work for several years and have arrived at some definite conclusion concerning the streams which flow through or border their states. A number of publications have been issued which clearly set forth the various problems of stream pollution and attempts have been made to explain a solution to the problem
Kondo resonances and anomalous gate dependence of electronic conduction in single-molecule transistors
We report Kondo resonances in the conduction of single-molecule transistors
based on transition metal coordination complexes. We find Kondo temperatures in
excess of 50 K, comparable to those in purely metallic systems. The observed
gate dependence of the Kondo temperature is inconsistent with observations in
semiconductor quantum dots and a simple single-dot-level model. We discuss
possible explanations of this effect, in light of electronic structure
calculations.Comment: 5 pages, four figures. Supplementary material at
http://www.ruf.rice.edu/~natelson/publications.htm
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