162 research outputs found
Density functional theory and DFT+U study of transition metal porphines adsorbed on Au(111) surfaces and effects of applied electric fields
We apply Density Functional Theory (DFT) and the DFT+U technique to study the
adsorption of transition metal porphine molecules on atomistically flat Au(111)
surfaces. DFT calculations using the Perdew-Burke-Ernzerhof (PBE) exchange
correlation functional correctly predict the palladium porphine (PdP) low-spin
ground state. PdP is found to adsorb preferentially on gold in a flat geometry,
not in an edgewise geometry, in qualitative agreement with experiments on
substituted porphyrins. It exhibits no covalent bonding to Au(111), and the
binding energy is a small fraction of an eV. The DFT+U technique, parameterized
to B3LYP predicted spin state ordering of the Mn d-electrons, is found to be
crucial for reproducing the correct magnetic moment and geometry of the
isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111)
substantially alters the Mn ion spin state. Its interaction with the gold
substrate is stronger and more site-specific than PdP. The binding can be
partially reversed by applying an electric potential, which leads to
significant changes in the electronic and magnetic properties of adsorbed MnP,
and ~ 0.1 Angstrom, changes in the Mn-nitrogen distances within the porphine
macrocycle. We conjecture that this DFT+U approach may be a useful general
method for modeling first row transition metal ion complexes in a
condensed-matter setting.Comment: 14 pages, 6 figure
Large scale numerical investigation of excited states in poly(phenylene)
A density matrix renormalisation group scheme is developed, allowing for the
first time essentially exact numerical solutions for the important excited
states of a realistic semi-empirical model for oligo-phenylenes. By monitoring
the evolution of the energies with chain length and comparing them to the
experimental absorption peaks of oligomers and thin films, we assign the four
characteristic absorption peaks of phenyl-based polymers. We also determine the
position and nature of the nonlinear optical states in this model.Comment: RevTeX, 10 pages, 4 eps figures included using eps
Ultrafast optical nonlinearity in quasi-one-dimensional Mott-insulator
We report strong instantaneous photoinduced absorption (PA) in the
quasi-one-dimensional Mott insulator in the IR spectral
region. The observed PA is to an even-parity two-photon state that occurs
immediately above the absorption edge. Theoretical calculations based on a
two-band extended Hubbard model explains the experimental features and
indicates that the strong two-photon absorption is due to a very large
dipole-coupling between nearly degenerate one- and two-photon states. Room
temperature picosecond recovery of the optical transparency suggests the strong
potential of for all-optical switching.Comment: 10 pages, 4 figure
A theoretical investigation of the low lying electronic structure of poly(p-phenylene vinylene)
The two-state molecular orbital model of the one-dimensional phenyl-based
semiconductors is applied to poly(p-phenylene vinylene). The energies of the
low-lying excited states are calculated using the density matrix
renormalization group method. Calculations of both the exciton size and the
charge gap show that there are both Bu and Ag excitonic levels below the band
threshold. The energy of the 1Bu exciton extrapolates to 2.60 eV in the limit
of infinite polymers, while the energy of the 2Ag exciton extrapolates to 2.94
eV. The calculated binding energy of the 1Bu exciton is 0.9 eV for a 13
phenylene unit chain and 0.6 eV for an infinite polymer. This is expected to
decrease due to solvation effects. The lowest triplet state is calculated to be
at ca. 1.6 eV, with the triplet-triplet gap being ca. 1.6 eV. A comparison
between theory, and two-photon absorption and electroabsorption is made,
leading to a consistent picture of the essential states responsible for most of
the third-order nonlinear optical properties. An interpretation of the
experimental nonlinear optical spectroscopies suggests an energy difference of
ca. 0.4 eV between the vertical energy and ca. 0.8 eV between the relaxed
energy, of the 1Bu exciton and the band gap, respectively.Comment: LaTeX, 19 pages, 7 eps figures included using epsf. To appear in
Physical Review B, 199
Excitonic Strings in one dimensional organic compounds
Important questions concern the existence of excitonic strings in organic
compounds and their signatures in the photophysics of these systems. A model in
terms of Hard Core Bosons is proposed to study this problem in one dimension.
Mainly the cases with two and three particles are studied for finite and
infinite lattices, where analytical results are accessible. It is shown that if
bi-excitonic states exist, three-excitonic and even, n-excitonic strings, at
least in a certain range of parameters, will exist. Moreover, the behaviour of
the transitions from one exciton to the biexciton is fully clarified. The
results are in agreement with exact finite cluster diagonalizations of several
model Hamiltonians.Comment: 36 pages, 4 eps figs. to appear in Phys. Rev.
Ab-initio calculation of the electronic and optical excitations in polythiophene: effects of intra- and interchain screening
We present an calculation of the electronic and optical excitations of an
isolated polythiophene chain as well as of bulk polythiophene. We use the GW
approximation for the electronic self-energy and include excitonic effects by
solving the electron-hole Bethe-Salpeter equation. The inclusion of interchain
screening in the case of bulk polythiophene drastically reduces both the
quasi-particle band gap and the exciton binding energies, but the optical gap
is hardly affected. This finding is relevant for conjugated polymers in
general.Comment: 4 pages, 1 figur
Excitons in quasi-one dimensional organics: Strong correlation approximation
An exciton theory for quasi-one dimensional organic materials is developed in
the framework of the Su-Schrieffer-Heeger Hamiltonian augmented by short range
extended Hubbard interactions. Within a strong electron-electron correlation
approximation, the exciton properties are extensively studied. Using scattering
theory, we analytically obtain the exciton energy and wavefunction and derive a
criterion for the existence of a exciton. We also systematically
investigate the effect of impurities on the coherent motion of an exciton. The
coherence is measured by a suitably defined electron-hole correlation function.
It is shown that, for impurities with an on-site potential, a crossover
behavior will occur if the impurity strength is comparable to the bandwidth of
the exciton, corresponding to exciton localization. For a charged impurity with
a spatially extended potential, in addition to localization the exciton will
dissociate into an uncorrelated electron-hole pair when the impurity is
sufficiently strong to overcome the Coulomb interaction which binds the
electron-hole pair. Interchain coupling effects are also discussed by
considering two polymer chains coupled through nearest-neighbor interchain
hopping and interchain Coulomb interaction . Within the
matrix scattering formalism, for every center-of-mass momentum, we find two
poles determined only by , which correspond to the interchain
excitons. Finally, the exciton state is used to study the charge transfer from
a polymer chain to an adjacent dopant molecule.Comment: 24 pages, 23 eps figures, pdf file of the paper availabl
Polar and hydrogen-bonding effects of alcohols on the emission spectrum of styrene-triethylamine system
The emission spectra of styrene (ST)-triethylamine (TEA) systems were measured under steady-state illumination conditions in some THF-protic solvent mixtures. The fluorescence spectrum of the ST-TEA system in THF consists of two bands (band A at 304 nm (fluorescence of ST) and band B at 460 nm (emission from an exciplex)). The intensity of band A increased and that of band B decreased with increasing amounts of protic solvents in THF-protic solvent mixtures. The increase in the intensity of band A was explained by the decrease in the concentration of free amine owing to the hydrogen-bonding interaction (or protonation) between TEA and protic solvents. The decrease in the intensity of band B was considered to be caused by the decrease in the concentration of free amine on the addition of protic solvents and the enhanced conversion of the exciplex to an ion pair with increasing solvent polarity. The polar effect was expressed as a function of the relative permittivity of the solution.</p
Ab-initio prediction of the electronic and optical excitations in polythiophene: isolated chains versus bulk polymer
We calculate the electronic and optical excitations of polythiophene using
the GW approximation for the electronic self-energy, and include excitonic
effects by solving the electron-hole Bethe-Salpeter equation. Two different
situations are studied: excitations on isolated chains and excitations on
chains in crystalline polythiophene. The dielectric tensor for the crystalline
situation is obtained by modeling the polymer chains as polarizable line
objects, with a long-wavelength polarizability tensor obtained from the
ab-initio polarizability function of the isolated chain. With this model
dielectric tensor we construct a screened interaction for the crystalline case,
including both intra- and interchain screening. In the crystalline situation
both the quasi-particle band gap and the exciton binding energies are
drastically reduced in comparison with the isolated chain. However, the optical
gap is hardly affected. We expect this result to be relevant for conjugated
polymers in general.Comment: 15 pages including 4 figures; to appear in Phys. Rev. B, 6/15/200
Formation of polycyclic aromatic hydrocarbon grains using anthracene and their stability under UV irradiation
- …