3,775 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
The U.S. Government's automotive safety program : a preliminary examination
Division of Policy Research and Analysis, National Science Foundation under Grant no. OEP 76-0028
MOND rotation curves for spiral galaxies with Cepheid-based distances
Rotation curves for four spiral galaxies with recently determined
Cepheid-based distances are reconsidered in terms of modified Newtonian
dynamics (MOND). For two of the objects, NGC 2403 and NGC 7331, the rotation
curves predicted by MOND are compatible with the observed curves when these
galaxies are taken to be at the Cepheid distance. For NGC 3198, the largest
distance for which reasonable agreement is obtained is 10% smaller than the
Cepheid-based distance; i.e., MOND clearly prefers a smaller distance. This
conclusion is unaltered when new near-infrared photometry of NGC 3198 is taken
as the tracer of the stellar mass distribution. For the large Sc spiral, NGC
2841, MOND requires a distance which is at least 20% larger than the
Cepheid-based distance. However, the discrepancy of the Tully-Fisher and SNIa
distances with the Cepheid determination casts some doubt upon the Cepheid
method in this case.Comment: Accepted for publication in astronomy and astrophysics 9 pages, 9
figure
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
In-situ electrochemical fabrication of natural contacts on single nanowires
We report a template-based in-situ electrochemical method for fabricating
natural electric contacts on single nanowires using a pair of cross-patterned
electrodes. Such electric contacts are highly stable upon thermal cycling
between room temperature and milli-Kelvin temperatures. Direct imaging of the
single-nanowire contacts using scanning electron microscopy is also
demonstrated.Comment: 13 pages, 4 figure
The role of intermolecular coupling in the photophysics of disordered organic semiconductors: Aggregate emission in regioregular polythiophene
We address the role of excitonic coulping on the nature of photoexcitations
in the conjugated polymer regioregular poly(3-hexylthiophene). By means of
temperature-dependent absorption and photoluminescence spectroscopy, we show
that optical emission is overwhelmingly dominated by weakly coupled
H-aggregates. The relative absorbance of the 0-0 and 0-1 vibronic peaks
provides a powerfully simple means to extract the magnitude of the
intermolecular coupling energy, approximately 5 and 30 meV for films spun from
isodurene and chloroform solutions respectively.Comment: 10 pages, 4 figures, published in Phys. Rev. Let
Photophysics of charge-transfer excitons in thin films of \pi-conjugated polymers
We develop a theory of the electronic structure and photophysics of
interacting chains of \pi- conjugated polymers to understand the differences
between solutions and films. While photoexcitation generates only the
intrachain exciton in solutions, the optical exciton as well as weakly allowed
charge-transfer excitons are generated in films. We extend existing theories of
the lowest polaronpair and charge-transfer excitons to obtain descriptions of
the excited states of these interchain species, and show that a significant
fraction of ultrafast photoinduced absorptions in films originate from the
lowest charge-transfer exciton. Our proposed mechanism explains the
simultaneous observation of polaronlike induced absorption features peculiar to
films in ultrafast spectroscopy and the absence of mobile charge carriers as
deduced from other experiments. We also show that there is a 1:1 correspondence
between the essential states that describe the photophysics of single chains
and of interacting chains that constitute thin films.Comment: 10 pages,4 figures, submitted to Phys Rev
Ultra Low Momentum Neutron Catalyzed Nuclear Reactions on Metallic Hydride Surfaces
Ultra low momentum neutron catalyzed nuclear reactions in metallic hydride
system surfaces are discussed. Weak interaction catalysis initially occurs when
neutrons (along with neutrinos) are produced from the protons which capture
``heavy'' electrons. Surface electron masses are shifted upwards by localized
condensed matter electromagnetic fields. Condensed matter quantum
electrodynamic processes may also shift the densities of final states allowing
an appreciable production of extremely low momentum neutrons which are thereby
efficiently absorbed by nearby nuclei. No Coulomb barriers exist for the weak
interaction neutron production or other resulting catalytic processes.Comment: 4 pages, ReVTeX forma
Buried AGNs in Advanced Mergers:Mid-infrared color selection as a dual AGN finder
A direct consequence of hierarchical galaxy formation is the existence of
dual supermassive black holes (SMBHs), which may be preferentially triggered as
active galactic nuclei (AGN) during galaxy mergers. Despite decades of
searching, however, dual AGNs are extremely rare, and most have been discovered
serendipitously. Using the all-sky WISE survey, we identified a population of
over 100 morphologically identified interacting galaxies or mergers that
display red mid-infrared colors often associated in extragalactic sources with
powerful AGNs. The vast majority of these advanced mergers are optically
classified as star-forming galaxies suggesting that they may represent an
obscured population of AGNs that cannot be found through optical studies. In
this work, we present Chandra/ACIS observations and near-infrared spectra with
the Large Binocular Telescope of six advanced mergers with projected pair
separations less than ~ 10 kpc. The combined X-ray, near-infrared, and
mid-infrared properties of these mergers provide confirmation that four out of
the six mergers host at least one AGN, with four of the mergers possibly
hosting dual AGNs with projected separations less than ~10 kpc, despite showing
no firm evidence for AGNs based on optical spectroscopic studies. Our results
demonstrate that 1) optical studies miss a significant fraction of single and
dual AGNs in advanced mergers, and 2) mid-infrared pre-selection is extremely
effective in identifying dual AGN candidates in late-stage mergers. Our
multi-wavelength observations suggest that the buried AGNs in these mergers are
highly absorbed, with intrinsic column densities in excess of N_H >10^24cm^-2,
consistent with hydrodynamic simulations.Comment: 23 pages, 11 figures, accepted for publication to Ap
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