1,095 research outputs found
Femtosecond spectroscopy of the first events of the photochemical cycle in bacteriorhodopsin
The first steps in the photochemistry of bacteriorhodopsin (BR) are investigated with light pulses of 160 fs duration. Four samples are studied: (i) the purple membrane, (ii) deuterated purple membrane, (iii) BR trimers and (iv) BR monomers. In all samples the first intermediate J is formed within 430±50 fs. No isotope effect is observed in the formation of J upon deuteration, in contrast to previous reports with much higher excitation energies. Thus proton movement to or from the retinal Schiff's base is not relevant during the first step. Comparing the data for trimeric and monomeric BR suggests an upper limit of 50 fs for the transfer of excitation energy from the excitonically coupled trimer to a single retinal chromophore
Small quark stars in the chromodielectric model
Equations of state for strange quark matter in beta equilibrium at high
densities are used to investigate the structure (mass and radius) of compact
objects. The chromodielectric model is used as a general framework for the
quark interactions, which are mediated by chiral mesons, and , and by a confining chiral singlet dynamical field, . Using a
quartic potential for , two equations of state for the same set of model
parameters are obtained, one with a minimum at around the nuclear matter
density and the other at . Using the latter
equation of state in the Tolman-Oppenheimer-Volkoff equations we found
solutions corresponding to compact objects with km and . The phenomenology of recently discovered X-ray sources is compatible
with the type of quark stars that we have obtained.Comment: 8 pages, AIP macros; Talk delivered at the Pan American Advanced
Studies Institute (PASI) Conference "New States of Matter in Hadronic
Interactions", Campos do Jordao, Brazil, January 200
Testing the validity of THz reflection spectra by dispersion relations
Complex response function obtained in reflection spectroscopy at terahertz
range is examined with algorithms based on dispersion relations for integer
powers of complex reflection coefficient, which emerge as a powerful and yet
uncommon tools in examining the consistency of the spectroscopic data. It is
shown that these algorithms can be used in particular for checking the success
of correction of the spectra by the methods of Vartiainen et al [1] and
Lucarini et al [2] to remove the negative misplacement error in the terahertz
time-domain spectroscopy.Comment: 17 pages, 4 figure
Optical Hall Effect in the Integer Quantum Hall Regime
Optical Hall conductivity is measured from the Faraday
rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz
frequency regime. The Faraday rotation angle ( fine structure constant
mrad) is found to significantly deviate from the Drude-like behavior to
exhibit a plateau-like structure around the Landau-level filling . The
result, which fits with the behavior expected from the carrier localization
effect in the ac regime, indicates that the plateau structure, although not
quantized, still exists in the terahertz regime.Comment: 4 pages, 4 figure
Cooling Dynamics of Photoexcited Carriers in Si Studied by Using Optical Pump and Terahertz Probe Spectroscopy
We investigated the photoexcited carrier dynamics in Si by using optical pump
and terahertz probe spectroscopy in an energy range between 2 meV and 25 meV.
The formation dynamics of excitons from unbound e-h pairs was studied through
the emergence of the 1s-2p transition of excitons at 12 meV (3 THz). We
revealed the thermalization mechanism of the photo-injected hot carriers
(electrons and holes) in the low temperature lattice system by taking account
of the interband and intraband scattering of carriers with acoustic and optical
phonons. The overall cooling rate of electrons and holes was numerically
calculated on the basis of a microscopic analysis of the phonon scattering
processes, and the results well account for the experimentally observed carrier
cooling dynamics. The long formation time of excitons in Si after the above-gap
photoexcitation is reasonably accounted for by the thermalization process of
photoexcited carriers.Comment: 8 pages, 8 figures, to be published in Phys. Rev.
Femtosecond spectroscopy of the first events of the photochemical cycle in bacteriorhodopsin
The first steps in the photochemistry of bacteriorhodopsin (BR) are investigated with light pulses of 160 fs duration. Four samples are studied: (i) the purple membrane, (ii) deuterated purple membrane, (iii) BR trimers and (iv) BR monomers. In all samples the first intermediate J is formed within 430±50 fs. No isotope effect is observed in the formation of J upon deuteration, in contrast to previous reports with much higher excitation energies. Thus proton movement to or from the retinal Schiff's base is not relevant during the first step. Comparing the data for trimeric and monomeric BR suggests an upper limit of 50 fs for the transfer of excitation energy from the excitonically coupled trimer to a single retinal chromophore
Detection and correction of the misplacement error in THz Spectroscopy by application of singly subtractive Kramers-Kronig relations
In THz reflection spectroscopy the complex permittivity of an opaque medium
is determined on the basis of the amplitude and of the phase of the reflected
wave. There is usually a problem of phase error due to misplacement of the
reference sample. Such experimental error brings inconsistency between phase
and amplitude invoked by the causality principle. We propose a rigorous method
to solve this relevant experimental problem by using an optimization method
based upon singly subtractive Kramers-Kronig relations. The applicability of
the method is demonstrated for measured data on an n-type undoped (100) InAs
wafer in the spectral range from 0.5 up to 2.5 THz.Comment: 16 pages, 5 figure
Genomic Insights Into the Ixodes Scapularis Tick Vector of Lyme Disease
Ticks transmit more pathogens to humans and animals than any other arthropod. We describe the 2.1 Gbp nuclear genome of the tick, Ixodes scapularis (Say), which vectors pathogens that cause Lyme disease, human granulocytic anaplasmosis, babesiosis and other diseases. The large genome reflects accumulation of repetitive DNA, new lineages of retro-transposons, and gene architecture patterns resembling ancient metazoans rather than pancrustaceans. Annotation of scaffolds representing ~57% of the genome, reveals 20,486 protein-coding genes and expansions of gene families associated with tick-host interactions. We report insights from genome analyses into parasitic processes unique to ticks, including host \u27questing\u27, prolonged feeding, cuticle synthesis, blood meal concentration, novel methods of haemoglobin digestion, haem detoxification, vitellogenesis and prolonged off-host survival. We identify proteins associated with the agent of human granulocytic anaplasmosis, an emerging disease, and the encephalitis-causing Langat virus, and a population structure correlated to life-history traits and transmission of the Lyme disease agent
Microwave conductivity of YBaCuO including inelastic scattering
The fluctuation spectrum responsible for the inelastic scattering in
YBaCuO which was recently determined from consideration of the
in-plane optical conductivity in the infrared, is used to calculate the
temperature dependence of the microwave conductivity at several measured
frequencies. Reasonable overall agreement can only be achieved if, in addition,
some impurity scattering is included within a model potential intermediate
between weak (Born) and strong (unitary) limit.Comment: 15 pages, 5 figures accepted for publication in Phys. Rev.
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