249 research outputs found
Excitonic Funneling in Extended Dendrimers with Non-Linear and Random Potentials
The mean first passage time (MFPT) for photoexcitations diffusion in a
funneling potential of artificial tree-like light-harvesting antennae
(phenylacetylene dendrimers with generation-dependent segment lengths) is
computed. Effects of the non-linearity of the realistic funneling potential and
slow random solvent fluctuations considerably slow down the center-bound
diffusion beyond a temperature-dependent optimal size. Diffusion on a
disordered Cayley tree with a linear potential is investigated analytically. At
low temperatures we predict a phase in which the MFPT is dominated by a few
paths.Comment: 4 pages, 4 figures, To be published in Phys. Rev. Let
Disorder and Funneling Effects on Exciton Migration in Tree-Like Dendrimers
The center-bound excitonic diffusion on dendrimers subjected to several types
of non-homogeneous funneling potentials, is considered. We first study the
mean-first passage time (MFPT) for diffusion in a linear potential with
different types of correlated and uncorrelated random perturbations. Increasing
the funneling force, there is a transition from a phase in which the MFPT grows
exponentially with the number of generations , to one in which it does so
linearly. Overall the disorder slows down the diffusion, but the effect is much
more pronounced in the exponential compared to the linear phase. When the
disorder gives rise to uncorrelated random forces there is, in addition, a
transition as the temperature is lowered. This is a transition from a
high- regime in which all paths contribute to the MFPT to a low- regime
in which only a few of them do. We further explore the funneling within a
realistic non-linear potential for extended dendrimers in which the dependence
of the lowest excitonic energy level on the segment length was derived using
the Time-Dependent Hatree-Fock approximation. Under this potential the MFPT
grows initially linearly with but crosses-over, beyond a molecular-specific
and -dependent optimal size, to an exponential increase. Finally we consider
geometrical disorder in the form of a small concentration of long connections
as in the {\it small world} model. Beyond a critical concentration of
connections the MFPT decreases significantly and it changes to a power-law or
to a logarithmic scaling with , depending on the strength of the funneling
force.Comment: 13 pages, 9 figure
Quasiparticle Density-Matrix Representation of Nonlinear TDDFT Response Functions
The time-dependent density functional (TDDFT) equations may be written either
for the Kohn-Sham orbitals in Hilbert space or for the single electron density
matrix in Liouville space. A collective-oscillator, quasiparticle,
representation of the density response of many-electron systems which
explicitly reveals the relevant electronic coherence sizes is developed using
the Liouville space representation of adiabatic TDDFT. Closed expressions for
the nonlinear density-density response are derived, eliminating the need to
solve nonlinear integral equations, as required in the Hilbert space
formulation of the response.Comment: 24 page
Current Profiles of Molecular Nanowires; DFT Green Function Representation
The Liouville-space Green function formalism is used to compute the current
density profile across a single molecule attached to electrodes. Time ordering
is maintained in real, physical, time, avoiding the use of artificial time
loops and backward propagations. Closed expressions for molecular currents,
which only require DFT calculations for the isolated molecule, are derived to
fourth order in the molecule/electrode coupling.Comment: 21 page
Analytical solutions to the third-harmonic generation in trans-polyacetylene: Application of dipole-dipole correlation on the single electron models
The analytical solutions for the third-harmonic generation (THG) on infinite
chains in both Su-Shrieffer-Heeger (SSH) and Takayama-Lin-Liu-Maki (TLM) models
of trans-polyacetylene are obtained through the scheme of dipole-dipole ()
correlation. They are not equivalent to the results obtained through static
current-current () correlation or under polarization operator
. The van Hove singularity disappears exactly in the analytical forms,
showing that the experimentally observed two-photon absorption peak (TPA) in
THG may not be directly explained by the single electron models.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
Optoelectronic Properties of Carbon Nanorings: Excitonic Effects from Time-Dependent Density Functional Theory
The electronic structure and size-scaling of optoelectronic properties in
cycloparaphenylene carbon nanorings are investigated using time-dependent
density functional theory (TDDFT). The TDDFT calculations on these molecular
nanostructures indicate that the lowest excitation energy surprisingly becomes
larger as the carbon nanoring size is increased, in contradiction with typical
quantum confinement effects. In order to understand their unusual electronic
properties, I performed an extensive investigation of excitonic effects by
analyzing electron-hole transition density matrices and exciton binding
energies as a function of size in these nanoring systems. The transition
density matrices allow a global view of electronic coherence during an
electronic excitation, and the exciton binding energies give a quantitative
measure of electron-hole interaction energies in the nanorings. Based on
overall trends in exciton binding energies and their spatial delocalization, I
find that excitonic effects play a vital role in understanding the unique
photoinduced dynamics in these carbon nanoring systems.Comment: Accepted by the Journal of Physical Chemistry
Structure, Photophysics and the Order-Disorder Transition to the Beta Phase in Poly(9,9-(di -n,n-octyl)fluorene)
X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy
have been used to study the well-known order-disorder transition (ODT) to the
beta phase in poly(9,9-(di n,n-octyl)fluorene)) (PF8) thin film samples through
combination of time-dependent and temperature-dependent measurements. The ODT
is well described by a simple Avrami picture of one-dimensional nucleation and
growth but crystallization, on cooling, proceeds only after molecular-level
conformational relaxation to the so called beta phase. Rapid thermal quenching
is employed for PF8 studies of pure alpha phase samples while extended
low-temperature annealing is used for improved beta phase formation. Low
temperature PL studies reveal sharp Franck-Condon type emission bands and, in
the beta phase, two distinguishable vibronic sub-bands with energies of
approximately 199 and 158 meV at 25 K. This improved molecular level structural
order leads to a more complete analysis of the higher-order vibronic bands. A
net Huang-Rhys coupling parameter of just under 0.7 is typically observed but
the relative contributions by the two distinguishable vibronic sub-bands
exhibit an anomalous temperature dependence. The PL studies also identify
strongly correlated behavior between the relative beta phase 0-0 PL peak
position and peak width. This relationship is modeled under the assumption that
emission represents excitons in thermodynamic equilibrium from states at the
bottom of a quasi-one-dimensional exciton band. The crystalline phase, as
observed in annealed thin-film samples, has scattering peaks which are
incompatible with a simple hexagonal packing of the PF8 chains.Comment: Submitted to PRB, 12 files; 1 tex, 1 bbl, 10 eps figure
The Reform of Employee Compensation in China’s Industrial Enterprises
Although employee compensation reform in Chinese industrial sector has been discussed in the literature, the real changes in compensation system and pay practices have received insufficient attention and warrant further examination. This paper briefly reviews the pre- and post-reform compensation system, and reports the results of a survey of pay practices in the four major types of industrial enterprises in China. The research findings indicate that the type of enterprise ownership has little influence on general compensation practices, adoption of profit-sharing plans, and subsidy and allowance packages. In general, pay is linked more to individual performance and has become an important incentive to Chinese employees. However, differences are found across the enterprise types with regard to performance-related pay. Current pay practices are positively correlated to overall effectiveness of the enterprise
Excitons in a Photosynthetic Light-Harvesting System: A Combined Molecular Dynamics/Quantum Chemistry and Polaron Model Study
The dynamics of pigment-pigment and pigment-protein interactions in
light-harvesting complexes is studied with a novel approach which combines
molecular dynamics (MD) simulations with quantum chemistry (QC) calculations.
The MD simulations of an LH-II complex, solvated and embedded in a lipid
bilayer at physiological conditions (with total system size of 87,055 atoms)
revealed a pathway of a water molecule into the B800 binding site, as well as
increased dimerization within the B850 BChl ring, as compared to the
dimerization found for the crystal structure. The fluctuations of pigment (B850
BChl) excitation energies, as a function of time, were determined via ab initio
QC calculations based on the geometries that emerged from the MD simulations.
From the results of these calculations we constructed a time-dependent
Hamiltonian of the B850 exciton system from which we determined the linear
absorption spectrum. Finally, a polaron model is introduced to describe quantum
mechanically both the excitonic and vibrational (phonon) degrees of freedom.
The exciton-phonon coupling that enters into the polaron model, and the
corresponding phonon spectral function are derived from the MD/QC simulations.
It is demonstrated that, in the framework of the polaron model, the absorption
spectrum of the B850 excitons can be calculated from the autocorrelation
function of the excitation energies of individual BChls, which is readily
available from the combined MD/QC simulations. The obtained result is in good
agreement with the experimentally measured absorption spectrum.Comment: REVTeX3.1, 23 pages, 13 (EPS) figures included. A high quality PDF
file of the paper is available at
http://www.ks.uiuc.edu/Publications/Papers/PDF/DAMJ2001/DAMJ2001.pd
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