4,042 research outputs found
Theory of interfacial charge-transfer complex photophysics in -conjugated polymer-fullerene blends
We present a theory of the electronic structure and photophysics of 1:1
blends of derivatives of polyparaphenylenevinylene and fullerenes. Within the
same Coulomb-correlated Hamiltonian applied previously to interacting chains of
single-component -conjugated polymers, we find an exciplex state that
occurs below the polymer's optical exciton. Weak absorption from the ground
state occurs to the exciplex. We explain transient photoinduced absorptions in
the blend, observed for both above-gap and below-gap photoexcitations, within
our theory. Photoinduced absorptions for above-gap photoexcitation are from the
optical exciton as well as the exciplex, while for below-gap photoexcitation
induced absorptions are from the exciplex alone. In neither case are free
polarons generated in the time scale of the experiment. Importantly, the
photophysics of films of single-component -conjugated polymers and blends
can both be understood by extending Mulliken's theory of ground-state charge
transfer to the case of excited-state charge transfer.Comment: 9 pages, 8 figure
Correlated-electron description of the photophysics of thin films of -conjugated polymers
We extend Mulliken's theory of ground state charge transfer in a
donor-acceptor complex to excited state charge transfer between pairs of
identical -conjugated oligomers, one of which is in the optically excited
state and the other in the ground state, leading to the formation of a
charge-transfer exciton. Within our theory, optical absorptions from the
charge-transfer exciton should include a low energy intermolecular
charge-transfer excitation, as well as distinct intramolecular excitations from
both the neutral delocalized exciton component and the Coulombically bound
polaron-pair component of the charge-transfer exciton. We report high order
configuration-interaction calculations for pairs of oligomers of
poly-paraphenylenevinylene (PPV) that go beyond our previous single
configuration-interaction calculation and find all five excited state
absorptions predicted using heuristic arguments based on the Mulliken concept.
Our calculated excited state absorption spectrum exhibits strong qualitative
agreement with the complete wavelength-dependent ultrafast photoinduced
absorption in films of PPV derivatives, suggesting that a significant fraction
of the photoinduced absorption here is from the charge-transfer exciton. We
make detailed comparisons to experiments, and a testable experimental
prediction
Competition between Electromagnetically Induced Transparency and Raman Processes
We present a theoretical formulation of competition among electromagnetically
induced transparency (EIT) and Raman processes. The latter become important
when the medium can no longer be considered to be dilute. Unlike the standard
formulation of EIT, we consider all fields applied and generated as interacting
with both the transitions of the scheme. We solve Maxwell equations
for the net generated field using a fast-Fourier-transform technique and obtain
predictions for the probe, control and Raman fields. We show how the intensity
of the probe field is depleted at higher atomic number densities due to the
build up of multiple Raman fields.Comment: 3.5 pages, 7 figure
Globular Cluster Systems in Brightest Cluster Galaxies. III: Beyond Bimodality
We present new deep photometry of the rich globular cluster (GC) systems
around the Brightest Cluster Galaxies UGC 9799 (Abell 2052) and UGC 10143
(Abell 2147), obtained with the HST ACS and WFC3 cameras. For comparison, we
also present new reductions of similar HST/ACS data for the Coma supergiants
NGC 4874 and 4889. All four of these galaxies have huge cluster populations (to
the radial limits of our data, comprising from 12000 to 23000 clusters per
galaxy). The metallicity distribution functions (MDFs) of the GCs can still be
matched by a bimodal-Gaussian form where the metal-rich and metal-poor modes
are separated by ~0.8 dex, but the internal dispersions of each mode are so
large that the total MDF becomes very broad and nearly continuous from [Fe/H] =
-2.4 to Solar. There are, however, significant differences between galaxies in
the relative numbers of \emph{metal-rich} clusters, suggesting that they
underwent significantly different histories of mergers with massive, gas-rich
halos. Lastly, the proportion of metal-poor GCs rises especially rapidly
outside projected radii R > 4 R_eff, suggesting the importance of accreted
dwarf satellites in the outer halo. Comprehensive models for the formation of
GCs as part of the hierarchical formation of their parent galaxies will be
needed to trace the systematic change in structure of the MDF with galaxy mass,
from the distinctly bimodal form in smaller galaxies up to the broad continuum
that we see in the very largest systems.Comment: In press for Astrophysical Journa
A Link to the Past: Using Markov Chain Monte Carlo Fitting to Constrain Fundamental Parameters of High-Redshift Galaxies
We have a developed a new method for fitting spectral energy distributions
(SEDs) to identify and constrain the physical properties of high-redshift (4 <
z < 8) galaxies. Our approach uses an implementation of Bayesian based Markov
Chain Monte Carlo (PiMC^2) that allows us to compare observations to
arbitrarily complex models and to compute 95% credible intervals that provide
robust constraints for the model parameters. The work is presented in 2
sections. In the first, we test PiMC^2 using simulated SEDs to not only confirm
the recovery of the known inputs but to assess the limitations of the method
and identify potential hazards of SED fitting when applied specifically to high
redshift (z>4) galaxies. Our tests reveal five critical results: 1) the ability
to confidently constrain metallicity, population ages, and Av all require
photometric accuracy better than what is currently achievable (i.e. less than a
few percent); 2) the ability to confidently constrain stellar masses (within a
factor of two) can be achieved without the need for high-precision photometry;
3) the addition of IRAC photometry does not guarantee that tighter constraints
of the stellar masses and ages can be defined; 4) different assumptions about
the star formation history can lead to significant biases in mass and age
estimates; and 5) we are able to constrain stellar age and Av of objects that
are both young and relatively dust free. In the second part of the paper we
apply PiMC^2 to 17 4<z<8 objects, including the GRAPES Ly alpha sample (4<z<6),
supplemented by HST/WFC3 near-IR observations, and several broad band selected
z>6 galaxies. Using PiMC^2, we are able to constrain the stellar mass of these
objects and in some cases their stellar age and find no evidence that any of
these sources formed at a redshift much larger than z_f=8, a time when the
Universe was ~ 0.6 Gyr old.Comment: Submitted to ApJ (Full abstract, 47 pages, 17 figures, 7 tables
A Link to the Past: Using Markov Chain Monte Carlo Fitting to Constrain Fundamental Parameters of High-Redshift Galaxies
We have a developed a new method for fitting spectral energy distributions
(SEDs) to identify and constrain the physical properties of high-redshift (4 <
z < 8) galaxies. Our approach uses an implementation of Bayesian based Markov
Chain Monte Carlo (PiMC^2) that allows us to compare observations to
arbitrarily complex models and to compute 95% credible intervals that provide
robust constraints for the model parameters. The work is presented in 2
sections. In the first, we test PiMC^2 using simulated SEDs to not only confirm
the recovery of the known inputs but to assess the limitations of the method
and identify potential hazards of SED fitting when applied specifically to high
redshift (z>4) galaxies. Our tests reveal five critical results: 1) the ability
to confidently constrain metallicity, population ages, and Av all require
photometric accuracy better than what is currently achievable (i.e. less than a
few percent); 2) the ability to confidently constrain stellar masses (within a
factor of two) can be achieved without the need for high-precision photometry;
3) the addition of IRAC photometry does not guarantee that tighter constraints
of the stellar masses and ages can be defined; 4) different assumptions about
the star formation history can lead to significant biases in mass and age
estimates; and 5) we are able to constrain stellar age and Av of objects that
are both young and relatively dust free. In the second part of the paper we
apply PiMC^2 to 17 4<z<8 objects, including the GRAPES Ly alpha sample (4<z<6),
supplemented by HST/WFC3 near-IR observations, and several broad band selected
z>6 galaxies. Using PiMC^2, we are able to constrain the stellar mass of these
objects and in some cases their stellar age and find no evidence that any of
these sources formed at a redshift much larger than z_f=8, a time when the
Universe was ~ 0.6 Gyr old.Comment: Submitted to ApJ (Full abstract, 47 pages, 17 figures, 7 tables
Mechanism of β4 Subunit Modulation of BK Channels
Large-conductance (BK-type) Ca2+-activated potassium channels are activated by membrane depolarization and cytoplasmic Ca2+. BK channels are expressed in a broad variety of cells and have a corresponding diversity in properties. Underlying much of the functional diversity is a family of four tissue-specific accessory subunits (β1–β4). Biophysical characterization has shown that the β4 subunit confers properties of the so-called “type II” BK channel isotypes seen in brain. These properties include slow gating kinetics and resistance to iberiotoxin and charybdotoxin blockade. In addition, the β4 subunit reduces the apparent voltage sensitivity of channel activation and has complex effects on apparent Ca2+ sensitivity. Specifically, channel activity at low Ca2+ is inhibited, while at high Ca2+, activity is enhanced. The goal of this study is to understand the mechanism underlying β4 subunit action in the context of a dual allosteric model for BK channel gating. We observed that β4's most profound effect is a decrease in Po (at least 11-fold) in the absence of calcium binding and voltage sensor activation. However, β4 promotes channel opening by increasing voltage dependence of Po-V relations at negative membrane potentials. In the context of the dual allosteric model for BK channels, we find these properties are explained by distinct and opposing actions of β4 on BK channels. β4 reduces channel opening by decreasing the intrinsic gating equilibrium (L0), and decreasing the allosteric coupling between calcium binding and voltage sensor activation (E). However, β4 has a compensatory effect on channel opening following depolarization by shifting open channel voltage sensor activation (Vho) to more negative membrane potentials. The consequence is that β4 causes a net positive shift of the G-V relationship (relative to α subunit alone) at low calcium. At higher calcium, the contribution by Vho and an increase in allosteric coupling to Ca2+ binding (C) promotes a negative G-V shift of α+β4 channels as compared to α subunits alone. This manner of modulation predicts that type II BK channels are downregulated by β4 at resting voltages through effects on L0. However, β4 confers a compensatory effect on voltage sensor activation that increases channel opening during depolarization
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