3,394 research outputs found
Rotational Correlation Functions of Single Molecules
Single molecule rotational correlation functions are analyzed for several
reorientation geometries. Even for the simplest model of isotropic rotational
diffusion our findings predict non-exponential correlation functions to be
observed by polarization sensitive single molecule fluorescence microscopy.
This may have a deep impact on interpreting the results of molecular
reorientation measurements in heterogeneous environments.Comment: 5 pages, 4 figure
Subpicosecond exciton dynamics in polyfluorene films from experiment and microscopic theory
The authors acknowledge financial support from the UK EPSRC (Grants EP/E065066/1, EP/E062636/1, EP/J009318/1 and EP/J009019/1), from the EPSRC Scottish Centre for Doctoral training in Condensed Matter Physics and from the European Union Seventh Framework Programme under Grant Agreement 321305.Electronic energy transfer (EET) in organic materials is a key mechanism that controls the efficiency of many processes, including light harvesting antennas in natural and artificial photosynthesis, organic solar cells, and biological systems. In this paper we have examined EET in solid-state thin-films of polyfluorene, a prototypical conjugated polymer, with ultrafast photoluminescence experiments and theoretical modeling. We observe EET occurring on a 680 ± 300 fs time scale by looking at the depolarisation of photoluminescence. An independent, predictive microscopic theoretical model is built by defining 125 000 chromophores containing both spatial and energetic disorder appropriate for a spin-coated thin film. The model predicts time-dependent exciton dynamics, without any fitting parameters, using the incoherent Förster-type hopping model. Electronic coupling between the chromophores is calculated by an improved version of the usual line-dipole model for resonant energy transfer. Without the need for higher level interactions, we find that the model is in general agreement with the experimentally observed 680 ± 300 fs depolarisation caused by EET. This leads us to conclude that femtosecond EET in polyfluorene can be described well by conventional resonant energy transfer, as long as the relevant microscopic parameters are well captured. The implications of this finding are that dipole-dipole resonant energy transfer can in some circumstances be fully adequate to describe ultrafast EET without needing to invoke strong or intermediate coupling mechanisms.PostprintPeer reviewe
Resident research associateships. Postdoctoral and senior research awards: Opportunities for research at the Jet Propulsion Laboratory
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Molecular velocity auto-correlation of simple liquids observed by NMR MGSE method
The velocity auto-correlation spectra of simple liquids obtained by the NMR
method of modulated gradient spin echo show features in the low frequency range
up to a few kHz, which can be explained reasonably well by a long
time tail decay only for non-polar liquid toluene, while the spectra of polar
liquids, such as ethanol, water and glycerol, are more congruent with the model
of diffusion of particles temporarily trapped in potential wells created by
their neighbors. As the method provides the spectrum averaged over ensemble of
particle trajectories, the initial non-exponential decay of spin echoes is
attributed to a spatial heterogeneity of molecular motion in a bulk of liquid,
reflected in distribution of the echo decays for short trajectories. While at
longer time intervals, and thus with longer trajectories, heterogeneity is
averaged out, giving rise to a spectrum which is explained as a combination of
molecular self-diffusion and eddy diffusion within the vortexes of hydrodynamic
fluctuations.Comment: 8 pages, 6 figur
Selected properties of optical spatial solitons in photorefractive media and their important applications
Some selected important properties of photorefractive spatial solitons and
their applications have been reviewed in the present paper. Using band
transport model, the governing principle of photorefractive nonlinearity has
been addressed and nonlinear dynamical equations of spatial solitons owing to
this nonlinearity have been discussed. Mechanisms of formation of screening and
photovoltaic solitons of three different configurations i.e., bright, dark and
grey varieties have been examined. Incoherently coupled vector solitons due to
single and two-photon photorefractive phenomena have been highlighted.
Modulation instability, which is precursor to soliton formation has been also
discused briefly. Finally possible applications of photorefractive spatial
solitons have been highlighted.Comment: 17 pages, 3 figures. arXiv admin note: text overlap with
arXiv:1209.221
Barrier Softening near the onset of Non-Activated Transport in Supercooled Liquids: Implications for Establishing Detailed Connection between Thermodynamic and Kinetic Anomalies in Supercooled Liquids
According to the Random First Order Transition (RFOT) theory of glasses, the
barriers for activated dynamics in supercooled liquids vanish as the
temperature of a viscous liquid approaches the dynamical transition temperature
from below. This occurs due to a decrease of the surface tension between local
meta-stable molecular arrangements much like at a spinodal. The dynamical
transition thus represents a crossover from the low activated bevavior to a
collisional transport regime at high . This barrier softening explains the
deviation of the relaxation times, as a function of temperature, from the
simple dependence at the high viscosity to a
mode-mode coupling dominated result at lower viscosity. By calculating the
barrier softening effects, the RFOT theory provides a {\em unified} microscopic
way to interpret structural relaxation data for many distinct classes of
structural glass formers over the measured temperature range. The theory also
provides an unambiguous procedure to determine the size of dynamically
cooperative regions in the presence of barrier renormalization effects using
the experimental temperature dependence of the relaxation times and the
configurational entropy data. We use the RFOT theory framework to discuss data
for tri-naphthyl benzene, salol, propanol and silica as representative systems.Comment: Submitted to J. Chem. Phy
A topological approximation of the nonlinear Anderson model
We study the phenomena of Anderson localization in the presence of nonlinear
interaction on a lattice. A class of nonlinear Schrodinger models with
arbitrary power nonlinearity is analyzed. We conceive the various regimes of
behavior, depending on the topology of resonance-overlap in phase space,
ranging from a fully developed chaos involving Levy flights to pseudochaotic
dynamics at the onset of delocalization. It is demonstrated that quadratic
nonlinearity plays a dynamically very distinguished role in that it is the only
type of power nonlinearity permitting an abrupt localization-delocalization
transition with unlimited spreading already at the delocalization border. We
describe this localization-delocalization transition as a percolation
transition on a Cayley tree. It is found in vicinity of the criticality that
the spreading of the wave field is subdiffusive in the limit
t\rightarrow+\infty. The second moment grows with time as a powerlaw t^\alpha,
with \alpha = 1/3. Also we find for superquadratic nonlinearity that the analog
pseudochaotic regime at the edge of chaos is self-controlling in that it has
feedback on the topology of the structure on which the transport processes
concentrate. Then the system automatically (without tuning of parameters)
develops its percolation point. We classify this type of behavior in terms of
self-organized criticality dynamics in Hilbert space. For subquadratic
nonlinearities, the behavior is shown to be sensitive to details of definition
of the nonlinear term. A transport model is proposed based on modified
nonlinearity, using the idea of stripes propagating the wave process to large
distances. Theoretical investigations, presented here, are the basis for
consistency analysis of the different localization-delocalization patterns in
systems with many coupled degrees of freedom in association with the asymptotic
properties of the transport.Comment: 20 pages, 2 figures; improved text with revisions; accepted for
publication in Physical Review
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