69 research outputs found
Vibronic excitations of large molecules in solution studied by two-color pumpâprobe experiments on the 20 fs time scale
The ultrafast vibronic response of organic dye molecules in solution is studied in pumpâprobe experiments with 30 fs excitation pulses resonant to S0âSn transitions. The molecular dynamics is probed either by pulses at the same spectral position or by 20 fs pulses overlapping with both the S0âS1 absorption and emission bands. Three contributions on distinctly different time scales are observed in the temporally and spectrally resolved two-color measurements. In the regime below 50 fs, a strong coherent coupling of the S0âSn and the S0âS1 transitions occurs that is due to coherent vibrational motions in the electronic ground state. This signal is superimposed on the fast bleaching of the electronic ground state, resulting in a steplike increase of transmission. In the range of the S0âS1 emission band, one finds a subsequent picosecond rise of transmission that is due to stimulated emission from vibronic S1 states. The data demonstrate that the relaxation of Sn states directly populated by the pump pulses is much faster than the buildup of stimulated emission. This gives insight into different steps of intramolecular vibronic redistribution and is compared to the SnâS1 relaxation in other molecules
Polariton propagation in weak confinement quantum wells
Exciton-polariton propagation in a quantum well, under centre-of-mass
quantization, is computed by a variational self-consistent microscopic theory.
The Wannier exciton envelope functions basis set is given by the simple
analytical model of ref. [1], based on pure states of the centre-of-mass wave
vector, free from fitting parameters and "ad hoc" (the so called additional
boundary conditions-ABCs) assumptions. In the present paper, the former
analytical model is implemented in order to reproduce the centre-of-mass
quantization in a large range of quantum well thicknesses (5a_B < L < inf.).
The role of the dynamical transition layer at the well/barrier interfaces is
discussed at variance of the classical Pekar's dead-layer and ABCs. The Wannier
exciton eigenstates are computed, and compared with various theoretical models
with different degrees of accuracy. Exciton-polariton transmission spectra in
large quantum wells (L>> a_B) are computed and compared with experimental
results of Schneider et al.\cite{Schneider} in high quality GaAs samples. The
sound agreement between theory and experiment allows to unambiguously assign
the exciton-polariton dips of the transmission spectrum to the pure states of
the Wannier exciton center-of-mass quantization.Comment: 15 pages, 15 figures; will appear in Phys.Rev.
A Unifying Mechanism for Mitochondrial Superoxide Production during Ischemia-Reperfusion Injury.
Ischemia-reperfusion (IR) injury occurs when blood supply to an organ is disrupted--ischemia--and then restored--reperfusion--leading to a burst of reactive oxygen species (ROS) from mitochondria. It has been tacitly assumed that ROS production during IR is a non-specific consequence of oxygen interacting with dysfunctional mitochondria upon reperfusion. Recently, this view has changed, suggesting that ROS production during IR occurs by a defined mechanism. Here we survey the metabolic factors underlying IR injury and propose a unifying mechanism for its causes that makes sense of the huge amount of disparate data in this area and provides testable hypotheses and new directions for therapies.Work in our laboratories is supported by the Medical Research Council (UK) and the British Heart Foundation. E.T.C. is supported by a Human Frontiers Science Program fellowship.This is the author accepted manuscript. The final version is available from Cell Press via http://dx.doi.org/10.1016/j.cmet.2015.12.00
Vibrational and vibronic dynamics of large molecules in solution studied on a 20 fs timescale
The ultrafast dynamics of organic dye molecules in solution are studied in temporally and spectrally resolved experiments with 30 fs pump pulses resonant to short-wavelength S0-Sn transitions and 20 fs probe pulses overlapping with both the S0-S1 absorption and emission bands. Around zero delay, a pronounced coherent signal occurs with sign and amplitude depending on the spectral position within the probe pulses. The coherent coupling of the type electronic transitions is due to the coherent vibrational motion induced by an impulsive Raman-type excitation in the electronic ground state. The molecules promoted to Sn states give rise to a fast ground state bleaching on a 50 fs timescale that is followed by the picosecond onset of stimulated emission after accumulation of the excited molecules at the bottom of the S1 state
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