405 research outputs found
Sub-10 fs pulses tunable from 480 to 980 nm from a NOPA pumped by a Yb:KGW source
We describe two noncollinear optical parametric amplifier (NOPA) systems
pumped by either the second (515 nm) or the third (343 nm) harmonic of an
Yb:KGW amplifier, respectively. Pulse durations as short as 6.8 fs are readily
obtained by compression with commercially available chirped mirrors. The
availability of both second and third harmonic for NOPA pumping allows for
gap-free tuning from 520 to 980 nm. The use of an intermediate NOPA to generate
seed light at 780 nm extends the tuning range of the third-harmonic pumped NOPA
towards 450 nm
Vibrationally coherent crossing and coupling of electronic states during internal conversion in beta-carotene
Coupling of nuclear and electronic degrees of freedom mediates energy flow in
molecules after optical excitation. The associated coherent dynamics in
polyatomic systems, however, remain experimentally unexplored. Here, we
combined transient absorption spectroscopy with electronic population control
to reveal nuclear wavepacket dynamics during the S2-S1 internal conversion in
beta-carotene. We show that passage through a conical intersection is
vibrationally coherent and thereby provides direct feedback on the role of
different vibrational coordinates in the breakdown of the Born-Oppenheimer
approximation
Entropy production by resonance decays
We investigate entropy production for an expanding system of particles and
resonances with isospin symmetry -- in our case pions and mesons --
within the framework of relativistic kinetic theory. A cascade code to simulate
the kinetic equations is developed and results for entropy production and
particle spectra are presented.Comment: 17 pages, 10 ps-figures included, only change: preprint number adde
A systematic comparison of jet quenching in different fluid-dynamical models
Comparing four different (ideal and viscous) hydrodynamic models for the
evolution of the medium created in 200 AGeV Au-Au collisions, combined with two
different models for the path length dependence of parton energy loss, we study
the effects of jet quenching on the emission-angle dependence of the nuclear
suppression factor R_AA(phi) and the away-side per trigger yield I_AA(phi).
Each hydrodynamic model was tuned to provide a reasonable description of the
single-particle transverse momentum spectra for all collision centralities, and
the energy loss models were adjusted to yield the same pion nuclear suppression
factor in central Au-Au collisions. We find that the experimentally measured
in-plane vs. out-of-plane spread in R_AA(phi) is better reproduced by models
that shift the weight of the parton energy loss to later times along its path.
Among the models studied here, this is best achieved by energy loss models that
suppress energy loss at early times, combined with hydrodynamic models that
delay the dilution of the medium density due to hydrodynamic expansion by
viscous heating. We were unable to identify a clear tomographic benefit of a
measurement of I_AA(phi) over that of R_AA(phi).Comment: 17 pages, 11 figure
and production in Au+Au collisions at = 130 and 200 GeV
Mid-rapidity Omega and anti-Omega production in Au+Au collisions at RHIC is
studied with the STAR experiment. We report preliminary results on yields and
spectra at = 130 and 200 GeV. Production relative to negatively
charged hadrons (h-) as well as thermal freeze-out and collective expansion are
discussed.Comment: 4 pages, 5 figures, 1 table, Contribution to Quark Matter 2002,
Nantes, France, July 200
Particle production at very low and intermediate transverse momenta in d+Au and Au+Au collisions
The transverse momentum spectra of identified charged particles have been
measured at very low and intermediate transverse momenta in Au+Au collisions at
sqrt(s_NN) = 62.4 GeV and d+Au collisions at sqrt(s_NN) = 200 GeV using the
PHOBOS detector at RHIC. New results on charged particle production at very low
p_T in central Au+Au collisions at sqrt(s_{NN) = 200 GeV in the centrality
intervals 0-6% and 6-15% are presented. A comparison of the PHOBOS low-p_T data
with predictions of a recent optical model is shown. The shapes of m_T spectra
for d+Au and Au+Au collisions are compared.Comment: Presented at the 18th International Conference on Ultra-Relativistic
Nucleus-Nucleus Collisions, Quark Matter 2005, Budapest, Hungary, Aug. 4-9,
200
Direct observation of the coherent nuclear response after the absorption of a photon
How molecules convert light energy to perform a specific transformation is a
fundamental question in photophysics. Ultrafast spectroscopy reveals the
kinetics associated with electronic energy flow, but little is known about how
absorbed photon energy drives nuclear or electronic motion. Here, we used
ultrabroadband transient absorption spectroscopy to monitor coherent
vibrational energy flow after photoexcitation of the retinal chromophore. In
the proton pump bacteriorhodopsin we observed coherent activation of hydrogen
wagging and backbone torsional modes that were replaced by unreactive
coordinates in the solution environment, concomitant with a deactivation of the
reactive relaxation pathway
Results on Identified Hadrons from the PHENIX Experiment at RHIC
Recent results on identified hadrons from the PHENIX experiment in Au+Au
collisions at mid-rapidity at = 200 GeV are presented. The
centrality dependence of transverse momentum distributions and particle ratios
for identified charged hadrons are studied. The transverse flow velocity and
freeze-out temperature are extracted from spectra within the framework
of a hydrodynamic collective flow model. Two-particle HBT correlations for
charged pions are measured in different centrality selections for a broad range
of transverse momentum of the pair. Results on elliptic flow measurements with
respect to the reaction plane for identified particles are also presented.Comment: 10 pages, 10 figures. Talk presented at Quark Matter 2002, Nantes,
France, July 18-24, 2002. To appear in the proceedings (Nucl. Phys. A
Rapidity particle spectra in sudden hadronization of QGP
We show that the remaining internal longitudinal flow of colliding quarks in
nuclei offers a natural explanation for the diversity of rapidity spectral
shapes observed in Pb--Pb 158AGeV nuclear collisions. Thus QGP sudden
hadronization reaction picture is a suitable approach to explain the rapidity
spectra of hadrons produced.Comment: 3 pages including 2 figure
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