791 research outputs found
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
Effect of interchain coupling on conducting polymer luminescence: excimers in derivatives of poly(phenylene vinylene)
Optical excitation of a chain in a polymer film may result in formation of an
excimer, a superposition of on-chain excitons and charge-transfer excitons on
the originally excited chain and a neighboring chain. The excimer emission is
red-shifted compared to that of an on-chain exciton by an amount depending on
the interchain coupling . Setting up the excimer wavefunction and
calculating the red shift, we determine average values, referred to a
monomer, of 0.52 eV and 0.16 eV for poly(2,5-hexyloxy -phenylene
cyanovinylene), CN-PPV, and poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1, 4
p-phenylene vinylene], MEH-PPV, respectively, and use them to determine the
effect of interchain distance on the emission.Comment: 10 pages, RevTeX, 1 PS figure, replaced version of cond-mat/9707095,
accepted for publication in Phys. Rev. B, Rapid Communicatio
Hot-electron effect in spin dephasing in -type GaAs quantum wells
We perform a study of the effect of the high in-plane electric field on the
spin precession and spin dephasing due to the D'yakonov-Perel' mechanism in
-type GaAs (100) quantum wells by constructing and numerically solving the
kinetic Bloch equations. We self-consistently include all of the scattering
such as electron-phonon, electron-non-magnetic impurity as well as the
electron-electron Coulomb scattering in our theory and systematically
investigate how the spin precession and spin dephasing are affected by the high
electric field under various conditions. The hot-electron distribution
functions and the spin correlations are calculated rigorously in our theory. It
is found that the D'yakonov-Perel' term in the electric field provides a
non-vanishing effective magnetic field that alters the spin precession period.
Moreover, spin dephasing is markedly affected by the electric field. The
important contribution of the electron-electron scattering to the spin
dephasing is also discussed.Comment: 11 pages, 11 figures, accepted for publication in Phys. Rev.
Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure
The effect of pressure on acceptor levels and hole scattering mechanisms in
p-GaSe is investigated through Hall effect and resistivity measurements under
quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole
concentration is interpreted through a carrier statistics equation with a
single (nitrogen) or double (tin) acceptor whose ionization energies decrease
under pressure due to the dielectric constant increase. The pressure effect on
the hole mobility is also accounted for by considering the pressure
dependencies of both the phonon frequencies and the hole-phonon coupling
constants involved in the scattering rates.Comment: 13 pages, Latex, 4 ps figures. to appear in High Pressure Research 69
(1997
Suicide Prevention for Older Adults in Residential Communities: Implications for Policy and Practice
Examining an often under-appreciated area, Carol Podgorski and colleagues discuss the suicide risk and opportunities for suicide prevention in seniors' residential communities
Evidence for Excimer Photoexcitations in an Ordered {\pi}-Conjugated Polymer Film
We report pressure-dependent transient picosecond and continuous-wave
photomodulation studies of disordered and ordered films of
2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced
absorption (PA) bands in the disordered film exhibit very weak pressure
dependence and are assigned to intrachain excitons and polarons. In contrast,
the ordered film exhibits two additional transient PA bands in the midinfrared
that blueshift dramatically with pressure. Based on high-order configuration
interaction calculations we ascribe the PA bands in the ordered film to
excimers. Our work brings insight to the exciton binding energy in ordered
films versus disordered films and solutions. The reduced exciton binding energy
in ordered films is due to new energy states appearing below the continuum band
threshold of the single strand.Comment: 5.5 pages, 5 figure
Recommended from our members
Mass Spectrometry-Based Proteomics for Translational Research: A Technical Overview
Mass spectrometry-based investigation of clinical samples enables the
high-throughput identification of protein biomarkers. We provide an overview of
mass spectrometry-based proteomic techniques that are applicable to the
investigation of clinical samples. We address sample collection, protein
extraction and fractionation, mass spectrometry modalities, and quantitative
proteomics. Finally, we examine the limitations and further potential of such
technologies. Liquid chromatography fractionation coupled with tandem mass
spectrometry is well suited to handle mixtures of hundreds or thousands of
proteins. Mass spectrometry-based proteome elucidation can reveal potential
biomarkers and aid in the development of hypotheses for downstream investigation
of the molecular mechanisms of disease
- …