120 research outputs found
Picosecond photofragment spectroscopy. II. The overtone initiated unimolecular reaction H_2O_2(v_(OH)=5)→2OH
This paper, second in the series, reports on the picosecond time‐resolved photofragmentation of the overtone (v_(OH)=5) initiated reaction: HOOH+hν→2OH. The hydrogen peroxide is initially excited by way of a picosecond laser pulse to the fourth overtone level of the OH‐stretch local mode. The subsequent unimolecular reaction behavior is obtained by monitoring the laser‐induced fluorescence, caused by the picosecond probe–pulse electronic excitation of the OH radical photoproduct (in a given rotational state). The two pulses are scanned relative to one another in time thereby mapping out the product yield for the given delay‐time interval. The resultant product formation behavior is found to be nonexponential, and may be modeled as a biexponential rise. Furthermore, the quasibiexponential behavior is sensitive to the exact excitation wavelength—slight variations of which result in large changes in the two time constants and the relative amplitudes of the fast and slow components. These experiments give direct evidence for the inhomogeneous nature of the overtone transition on the picosecond time scale, and provide the dissociation rate contribution to the homogeneous width (0.05–0.15 cm^(−1)).
The apparent width for the main band feature is about 200 cm^(−1). The rate of product formation (magnitude and form) is interpreted in terms of statistical and nonstatistical theories. The limitations of the applicability of each model is discussed. The fluctuations of the fitting parameters as a function of excitation wavelength may be simulated by a statistical model which considers all possible discrete optical transitions within the simulated laser bandwidth and the details of product formation from each state. For a nonstatistical interpretation, the biexponential form reflects a division of the vibrational phase space, and this is discussed in the spirit of a kinetic model. Finally, experimental results are reported for direct UV initiated photofragmentation. The observed dynamics indicate that a very different type of potential surface (repulsive) is involved, in contrast to the overtone initiated dissociation, which takes place on the ground state surface
Phase resetting reveals network dynamics underlying a bacterial cell cycle
Genomic and proteomic methods yield networks of biological regulatory
interactions but do not provide direct insight into how those interactions are
organized into functional modules, or how information flows from one module to
another. In this work we introduce an approach that provides this complementary
information and apply it to the bacterium Caulobacter crescentus, a paradigm
for cell-cycle control. Operationally, we use an inducible promoter to express
the essential transcriptional regulatory gene ctrA in a periodic, pulsed
fashion. This chemical perturbation causes the population of cells to divide
synchronously, and we use the resulting advance or delay of the division times
of single cells to construct a phase resetting curve. We find that delay is
strongly favored over advance. This finding is surprising since it does not
follow from the temporal expression profile of CtrA and, in turn, simulations
of existing network models. We propose a phenomenological model that suggests
that the cell-cycle network comprises two distinct functional modules that
oscillate autonomously and couple in a highly asymmetric fashion. These
features collectively provide a new mechanism for tight temporal control of the
cell cycle in C. crescentus. We discuss how the procedure can serve as the
basis for a general approach for probing network dynamics, which we term
chemical perturbation spectroscopy (CPS)
Ultrafast resonant optical scattering from single gold nanorods: Large nonlinearities and plasmon saturation
We measure nonlinear optical scattering from individual Au nanorods excited
by ultrafast laser pulses on resonance with their longitudinal plasmon mode.
Isolating single rods removes inhomogeneous broadening and allows the
measurement of a large nonlinearity, much greater than that of nanorod
ensembles. Surprisingly, the ultrafast nonlinearity can be attributed entirely
to heating of conduction electrons and does not exhibit any response associated
with coherent plasmon oscillation. This indicates a previously unobserved
damping of strongly driven plasmons.Comment: Revised tex
Single-gene tuning of Caulobacter cell cycle period and noise, swarming motility, and surface adhesion
We established that the sensor histidine kinase DivJ has an important role in the regulation of C. crescentus cell cycle period and noise. This was accomplished by designing and conducting single-cell experiments to probe the dependence of cell cycle noise on divJ expression and constructing a simplified cell cycle model that captures the dependence of cell cycle noise on DivJ with molecular details.In addition to its role in regulating the cell cycle, DivJ also affects polar cell development in C. crescentus, regulating swarming motility and surface adhesion. We propose that pleiotropic control of polar cell development by the DivJ–DivK–PleC signaling pathway underlies divJ-dependent tuning of cell swarming and adhesion behaviors.We have integrated the study of single-cell fluorescence dynamics with a kinetic model simulation to provide direct quantitative evidence that the DivJ histidine kinase is localized to the cell pole through a dynamic diffusion-and-capture mechanism during the C. crescentus cell cycle
Generation of Robust Entanglement in Plasmonically Coupled Quantum Dots Driven by Quantum Squeezed Light
Our cavity quantum electrodynamics calculations demonstrate generation of
steady-state entanglement between a plasmonically coupled pair of quantum dots
by using single-mode squeezed light source. We show that strong coupling of
plasmons to the incoming light source and the pairwise nature of squeezed
photon generation enable the formation of entanglement between the initially
unexcited quantum dots. The entanglement of quantum dots, measured as
concurrence, can be improved replacing a pulsed source of light to continuous
pumping of squeezed photons. Unlike previously introduced schemes the
concurrence is robust against variations in the system parameters.
Specifically, the generation of entanglement does not rely on fine tuning of
plasmon quantum dot coupling. This work provides a new perspective for robust
entangled state preparation in open quantum systems
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