2,972 research outputs found
Analysis of the electron transfer from Pheoâ to QA in PS II membrane fragments from spinach by time resolved 325 nm absorption changes in the picosecond domain
AbstractAbsorption changes at 325 nm (ÎA325) induced by 15 ps laser flashes (λ = 650 nm) in PS II membrane fragments were measured with picosecond time-resolution. In samples with the reaction centers (RCs) kept in the open state (P I QA) the signals are characterized by a very fast rise (not resolvable by our equipment) followed by only small changes within our time window of 1.6 ns. In the closed state (P I QâA) of the reaction center the signal decays with an average half-life time of about 250 ps. It is shown that under our excitation conditions (E = 2 Ă 1014 photons/cm2 per pulse) subtraction of the absorption changes in closed RCs (ÎAclosed325) from those in open RCs (ÎAopen325) leads to a difference signal which is dominated by the reduction kinetics of QA. From the rise kinetics of this signal and by comparison with data in the literature it is inferred that QA becomes reduced by direct electron transfer from Pheoâ with a time constant of about 350 ± 100 ps
Rentabilidade fisica e econÎmica de inseticidas para pulgÔes safra de trigo de 1977.
bitstream/item/84146/1/CNPT-COM.-TEC.-1-77.pd
Choice of steel material for bridge bearings to avoid brittle fracture
Bridge bearings need verification against brittle failure at low temperatures. The design of bearings according to EN 1337 may lead to structural components with thicknesses no longer covered in the relevant technical construction regulations. Due to its specific geometry, the loading and stressing and the fabrication process the prerequisites for using the rules in EN 1993 1 10 lead to conservative restrictions or uneconomical choice of steel material. For an economical bearing design further modifications of the existing rules are necessary. This report adapts the fracture mechanical approach used in EN 1993 1 10 and gives information for a âsafe-sidedâ choice of steel material for bearings. The main modifications refer to the hypothetical design crack scenario and the definition of the ânominal design stressâ at the geometric âhot-spotâ. An advanced methodology using Finite Elements and a simplified method using linear bending theory are evaluated.JRC.G.5-European laboratory for structural assessmen
Measurement of electron-hole friction in an n-doped GaAs/AlGaAs quantum well using optical transient grating spectroscopy
We use phase-resolved transient grating spectroscopy to measure the drift and
diffusion of electron-hole density waves in a semiconductor quantum well. The
unique aspects of this optical probe allow us to determine the frictional force
between a two-dimensional Fermi liquid of electrons and a dilute gas of holes.
Knowledge of electron-hole friction enables prediction of ambipolar dynamics in
high-mobility electron systems.Comment: to appear in PR
Characterizing Quantum Microwave Radiation and its Entanglement with Superconducting Qubits using Linear Detectors
Recent progress in the development of superconducting circuits has enabled
the realization of interesting sources of nonclassical radiation at microwave
frequencies. Here, we discuss field quadrature detection schemes for the
experimental characterization of itinerant microwave photon fields and their
entanglement correlations with stationary qubits. In particular, we present
joint state tomography methods of a radiation field mode and a two-level
system. Including the case of finite quantum detection efficiency, we relate
measured photon field statistics to generalized quasi-probability distributions
and statistical moments for one-channel and two-channel detection. We also
present maximum-likelihood methods to reconstruct density matrices from
measured field quadrature histograms. Our theoretical investigations are
supported by the presentation of experimental data, for which microwave quantum
fields beyond the single-photon and Gaussian level have been prepared and
reconstructed.Comment: 14 pages, 5 figure
A Mechanical Mass Sensor with Yoctogram Resolution
Nanoelectromechanical systems (NEMS) have generated considerable interest as
inertial mass sensors. NEMS resonators have been used to weigh cells,
biomolecules, and gas molecules, creating many new possibilities for biological
and chemical analysis [1-4]. Recently, NEMS-based mass sensors have been
employed as a new tool in surface science in order to study e.g. the phase
transitions or the diffusion of adsorbed atoms on nanoscale objects [5-7]. A
key point in all these experiments is the ability to resolve small masses. Here
we report on mass sensing experiments with a resolution of 1.7 yg (1 yg =
10^-24 g), which corresponds to the mass of one proton, or one hydrogen atom.
The resonator is made of a ~150 nm long carbon nanotube resonator vibrating at
nearly 2 GHz. The unprecedented level of sensitivity allows us to detect
adsorption events of naphthalene molecules (C10H8) and to measure the binding
energy of a Xe atom on the nanotube surface (131 meV). These ultrasensitive
nanotube resonators offer new opportunities for mass spectrometry,
magnetometry, and adsorption experiments.Comment: submitted version of the manuscrip
Strong coupling between mechanical modes in a nanotube resonator
We report on the nonlinear coupling between the mechanical modes of a
nanotube resonator. The coupling is revealed in a pump-probe experiment where a
mode driven by a pump force is shown to modify the motion of a second mode
measured with a probe force. In a second series of experiments, we actuate the
resonator with only one oscillating force. Mechanical resonances feature exotic
lineshapes with reproducible dips, peaks, and jumps when the measured mode is
commensurate with another mode with a frequency ratio of either 2 or 3.
Conventional lineshapes are recovered by detuning the frequency ratio using the
voltage on a nearby gate electrode. The exotic lineshapes are attributed to
strong coupling between the mechanical modes. The possibility to control the
strength of the coupling with the gate voltage holds promise for various
experiments, such as quantum manipulation, mechanical signal processing, and
the study of the quantum-toclassical transition.Comment: manuscript and supporting material, 31 pages, 15 figure
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