308 research outputs found
Decoherence of Excitons in Multichromophore Systems: Thermal Line Broadening and Destruction of Superradiant Emission
We study the temperature-dependent dephasing rate of excitons in chains of
chromophores, accounting for scattering on static disorder as well as acoustic
phonons in the host matrix. From this we find a powerlaw temperature dependence
of the absorption line width, in excellent quantitative agreement with
experiments on dye aggregates. We also propose a relation between the line
width and the exciton coherence length imposed by the phonons. The results
indicate that the much debated steep rise of the fluorescence lifetime of
pseudo-isocyanine aggregates above 40 K results from the fact that this
coherence length drops below the localization length imposed by static
disorder.Comment: 4 pages, 2 figure
Observation of the one-exciton to two-exciton transition in a J aggregate
We report on the first observation of the one-exciton to two-exciton transition in J aggregates. A theoretical analysis supports our interpretation
Dynamics of Frenkel excitons in disordered molecular aggregates
This article reports on the optical dynamics in aggregates of pseudoisocyanine-bromide and iodide. For PIC-Br in an ethylene glycol/water glass, the results of resonance light scattering (RLS), time-resolved emission, and photon echo decay measurements are discussed. Band structure calculations based on a linear-chain model for the J aggregate have also been performed. The results show that the J band can be described as a disordered Frenkel exciton band in which superradiant states exist that extend over about 100 molecules. Numerical simulation studies of the J band, based on Anderson's Hamiltonian with uncorrelated diagonal site energies, show that the ratio kappa of the disorder parameter D over the nearest-neighbor coupling parameter J12 is about 0.11. Using the frequency dependence of the ratio between the yields of vibrational fluorescence and Raman scattering as a probe, the dephasing process and derived parameters for the bath correlation function at three different temperatures have also been examined. It is shown that at all temperatures the exciton dephasing process occurs in the fast modulation limit. For PIC-I in a Langmuir-Blodgett film the optical excitation can be described also in terms of a band transition but the disorder is much larger than in a glass. For this system, a low-temperature fluorescence lifetime of about 10 ps is measured, which suggests that the excitation is much more delocalized than in the case of self-assembled aggregates in a glass. Resonance Rayleigh scattering experiments on PIC in a bilayer show that the exciton-dephasing rate increases dramatically at energies above the renormalized band edge
Statistics of low-energy levels of a one-dimensional weakly localized Frenkel exciton: A numerical study
Numerical study of the one-dimensional Frenkel Hamiltonian with on-site
randomness is carried out. We focus on the statistics of the energy levels near
the lower exciton band edge, i. e. those determining optical response. We found
that the distribution of the energy spacing between the states that are well
localized at the same segment is characterized by non-zero mean, i.e. these
states undergo repulsion. This repulsion results in a local discrete energy
structure of a localized Frenkel exciton. On the contrary, the energy spacing
distribution for weakly overlapping local ground states (the states with no
nodes within their localization segments) that are localized at different
segments has zero mean and shows almost no repulsion. The typical width of the
latter distribution is of the same order as the typical spacing in the local
discrete energy structure, so that this local structure is hidden; it does not
reveal itself neither in the density of states nor in the linear absorption
spectra. However, this structure affects the two-exciton transitions involving
the states of the same segment and can be observed by the pump-probe
spectroscopy. We analyze also the disorder degree scaling of the first and
second momenta of the distributions.Comment: 10 pages, 6 figure
Observation of the one‐exciton to two‐exciton transition in a J
We report on the first observation of the one-exciton to two-exciton transition in J aggregates. A theoretical analysis supports our interpretation
Response of Ti microstructure in mechanical and laser forming processes
Microstructural deformation mechanisms present during three different forming processes in commercially pure Ti were analysed. Room temperature mechanical forming, laser beam forming and a combination of these two processes were applied to thick metal plates in order to achieve the same final shape. An electron backscatter diffraction technique was used to study the plate microstructure before and after applying the forming processes. Substantial differences among the main deformation mechanisms were clearly detected. In pure mechanical forming at room temperature, mechanical twinning predominates in both compression and tensile areas. A dislocation slip mechanism inside the compression and tensile area is characteristic of the pure laser forming process. Forming processes which subsequently combine the laser and mechanical approaches result in a combination of twinning and dislocation mechanisms. The Schmid factor at an individual grain level, the local temperature and the strain rate are factors that determine which deformation mechanism will prevail at the microscopic level. The final microstructures obtained after the different forming processes were applied are discussed from the point of view of their influence on the performance of the resulting formed product. The observations suggest that phase transformation in Ti is an additional microstructural factor that has to be considered during laser forming
The frequency of the predominant Jewish mutations in BRCA1 and BRCA2 in unselected Ashkenazi colorectal cancer patients
It is presently unclear whether carriers of BRCA1 mutations have an increased risk for colorectal cancer (CRC). To gain insight into this issue, 225 unselected Ashkenazi Jewish CRC patients were tested for the presence of the three common Jewish BRCA1/2 germline mutations: 185delAG and 5382insC (BRCA1) and 6174delT (BRCA2). A total of four carriers was found (4/225, 1.78%). This frequency is similar to the estimated normal Ashkenazi population frequency, thus suggesting that these specific mutations do not contribute to CRC predisposition.© 2001 Cancer Research Campaign http://www.bjcancer.co
Linear optical properties of one-dimensional Frenkel exciton systems with intersite energy correlations
We analyze the effects of intersite energy correlations on the linear optical
properties of one-dimensional disordered Frenkel exciton systems. The
absorption line width and the factor of radiative rate enhancement are studied
as a function of the correlation length of the disorder. The absorption line
width monotonously approaches the seeding degree of disorder on increasing the
correlation length. On the contrary, the factor of radiative rate enhancement
shows a non-monotonous trend, indicating a complicated scenario of the exciton
localization in correlated systems. The concept of coherently bound molecules
is exploited to explain the numerical results, showing good agreement with
theory. Some recent experiments are discussed in the light of the present
theory.Comment: 18 pages, 3 figues, REVTeX, to appear in Physical Review
Fluorescence decay in aperiodic Frenkel lattices
We study motion and capture of excitons in self-similar linear systems in
which interstitial traps are arranged according to an aperiodic sequence,
focusing our attention on Fibonacci and Thue-Morse systems as canonical
examples. The decay of the fluorescence intensity following a broadband pulse
excitation is evaluated by solving the microscopic equations of motion of the
Frenkel exciton problem. We find that the average decay is exponential and
depends only on the concentration of traps and the trapping rate. In addition,
we observe small-amplitude oscillations coming from the coupling between the
low-lying mode and a few high-lying modes through the topology of the lattice.
These oscillations are characteristic of each particular arrangement of traps
and they are directly related to the Fourier transform of the underlying
lattice. Our predictions can be then used to determine experimentally the
ordering of traps.Comment: REVTeX 3.0 + 3PostScript Figures + epsf.sty (uuencoded). To appear in
Physical Review
Semiconductor-cavity QED in high-Q regimes: Detuning effect
The non-resonant interaction between the high-density excitons in a quantum
well and a single mode cavity field is investigated. An analytical expression
for the physical spectrum of the excitons is obtained. The spectral properties
of the excitons, which are initially prepared in the number states or the
superposed states of the two different number states by the resonant
femtosecond pulse pumping experiment, are studied. Numerical study of the
physical spectrum is carried out and a discussion of the detuning effect is
presented.Comment: 7 pages, 8 figure
- …