1,168 research outputs found
Ultrafast spatio-temporal dynamics of terahertz generation by ionizing two-color femtosecond pulses in gases
We present a combined theoretical and experimental study of spatio-temporal
propagation effects in terahertz (THz) generation in gases using two-color
ionizing laser pulses. The observed strong broadening of the THz spectra with
increasing gas pressure reveals the prominent role of spatio-temporal reshaping
and of a plasma-induced blue-shift of the pump pulses in the generation
process. Results obtained from (3+1)-dimensional simulations are in good
agreement with experimental findings and clarify the mechanisms responsible for
THz emission
Photophysics of closed- and open-ring isomers of a diarylethene with a carboxylic anchor group
We study the transient photophysical properties of a diarylethene with a
carboxylic anchor group by a combination of steady-state and ultrafast
emission and absorption spectroscopy. After excitation of the closed-ring
form, fluorescence with a quantum yield of 10â5 is observed and separated into
different spectro-temporal components. The S1 state of the closed-ring form
shows a lifetime of 1.3 ps and decays mainly by internal conversion to the S0
state of this isomer. This vibrationally hot ground state cools on a time
scale of 10 ps
TROPESS-CrIS CO single-pixel vertical profiles: intercomparisons with MOPITT and model simulations for 2020 western US wildfires
The new TROPESS (TRopospheric Ozone and its Precursors from Earth System Sounding) profile retrievals of carbon monoxide (CO) from the Cross-track Infrared Sounder (CrIS) are evaluated against Measurement of Pollution in the Troposphere (MOPITT) CO version 9 data. Comparison results that were adjusted to common a priori constraints in the retrieval processes have improved agreement between the two data sets over direct comparisons. TROPESS-CrIS CO profiles are within 5â% of MOPITT but have higher concentrations in the lower troposphere and lower concentrations in the upper troposphere. For the intense western US wildfire events in September 2020, we compare CO fields simulated by the GISS climate model to the two satellite CO observations. We show intermediate steps of the comparison process to illustrate the evaluation of model simulations by deriving the âretrievedâ model CO profiles as they would be observed by the satellite. This includes the application of satellite level-2 data along with their corresponding diagnostic operators provided in the TROPESS-CrIS and MOPITT products. The process allows a diagnosis of potential model improvements in modeling fire emissions and pollution transport.</p
Evaluating Models' Response of Tropical Low Clouds to SST Forcings Using CALIPSO Observations
Recent studies have shown that, in response to a surface warming, the marine tropical low-cloud cover (LCC) as observed by passive-sensor satellites substantially decreases, therefore generating a smaller negative value of the top-of-the-atmosphere (TOA) cloud radiative effect (CRE). Here we study the LCC and CRE interannual changes in response to sea surface temperature (SST) forcings in the GISS model E2 climate model, a developmental version of the GISS model E3 climate model, and in 12 other climate models, as a function of their ability to represent the vertical structure of the cloud response to SST change against 10 years of CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) observations. The more realistic models (those that satisfy the observational constraint) capture the observed interannual LCC change quite well ([delta]LCC]/[delta]SST= -3.491.01%K [negative 1 superscript] vs. [delta]LCC/[delta]SST[subscript obs]= -3.590.28%K[negative 1 superscript]) while the others largely underestimate it ([delta]LCC/[delta]SST= -1.321.28%K[negative 1 superscript]). Consequently, the more realistic models simulate more positive shortwave (SW) feedback ([delta]CRE/[delta]SST=2.601.13Wm[negative 2 superscript] K[negative 1 superscript]) than the less realistic models (CRE/SST=0.872.63Wm2K1), in better agreement with the observations ([delta]CRE/[delta]SST[subscript obs]=30.26Wm[negative 2 superscript] K[negative 1 superscript] ), although slightly underestimated. The ability of the models to represent moist processes within the planetary boundary layer (PBL) and produce persistent stratocumulus (Sc) decks appears crucial to replicating the observed relationship between clouds, radiation and surface temperature. This relationship is different depending on the type of low clouds in the observations. Over stratocumulus regions, cloud-top height increases slightly with SST, accompanied by a large decrease in cloud fraction, whereas over trade cumulus (Cu) regions, cloud fraction decreases everywhere, to a smaller extent
Reduced carrier cooling and thermalization in semiconductor quantum wires
By using a Monte Carlo analysis of the carrier relaxation in GaAs quantum wires following laser photoexcitation, we show that carrier cooling due to phonon emission and internal thermalization due to electron-electron interaction are significantly decreased with respect to bulk systems. This decreased thermalization is mainly attributed to the reduced efficiency of intersubband processes and to the reduced effect of electron-electron intrasubband scattering
Evaluating models' response of tropical low clouds to SST forcings using CALIPSO observations
Recent studies have shown that, in response to a surface warming,
the marine tropical low-cloud cover (LCC) as observed by passive-sensor
satellites substantially decreases, therefore generating a smaller negative
value of the top-of-the-atmosphere (TOA) cloud radiative effect (CRE). Here we
study the LCC and CRE interannual changes in response to sea surface
temperature (SST) forcings in the GISS model E2 climate model, a
developmental version of the GISS model E3 climate model, and in 12 other
climate models, as a function of their ability to represent the vertical
structure of the cloud response to SST change against 10Â years of CALIPSO (Cloud-Aerosol Lidar and Infrared
Pathfinder Satellite Observations) observations.
The more realistic models (those that satisfy the observational
constraint) capture the observed interannual LCC change quite well
(ÎLCC/ÎSST=-3.49±1.01 % Kâ1 vs.
ÎLCC/ÎSSTobs=-3.59±0.28 % Kâ1) while the
others largely underestimate it (ÎLCC/ÎSST=-1.32±1.28 % Kâ1). Consequently, the more realistic models simulate more
positive shortwave (SW) feedback (ÎCRE/ÎSST=2.60±1.13 W mâ2 Kâ1)
than the less realistic models (ÎCRE/ÎSST=0.87±2.63 W mâ2 Kâ1), in better agreement with the
observations (ÎCRE/ÎSSTobs=3±0.26 W mâ2 Kâ1),
although slightly underestimated. The ability of the models to
represent moist processes within the planetary boundary layer (PBL) and produce
persistent stratocumulus (Sc) decks appears crucial to replicating the observed
relationship between clouds, radiation and surface temperature. This
relationship is different depending on the type of low clouds in the
observations. Over stratocumulus regions, cloud-top height increases slightly
with SST, accompanied by a large decrease in cloud fraction, whereas over
trade cumulus (Cu) regions, cloud fraction decreases everywhere, to a smaller
extent.</p
Coherent Optical polarization of Bulk GaAs Studied by Femtosecond Photon-Echo Spectroscopy
The nonlinear polarization close to the band gap of GaAs is studied by spectrally and temporally resolved four-wave mixing. Excitonic and free carrier contributions both excited within the bandwidth of the 100 fs pulses are distinguished for the first time. The excitonic part dominates at carrier densities below 1016 cm-3. At higher density, nonthermalized free carriers give rise to an additional component resonant to the pulse that shows a photon-echo-like time behavior. Monte Carlo simulations including the coherent polarization and the scattering dynamics of the carriers account for the data
The Role of Nonequilibrium Dynamical Screening in Carrier Thermalization
We investigate the role played by nonequilibrium dynamical screening in the
thermalization of carriers in a simplified two-component two-band model of a
semiconductor. The main feature of our approach is the theoretically sound
treatment of collisions. We abandon Fermi's Golden rule in favor of a
nonequilibrium field theoretic formalism as the former is applicable only in
the long-time regime. We also introduce the concept of nonequilibrium dynamical
screening. The dephasing of excitonic quantum beats as a result of
carrier-carrier scattering is brought out. At low densities it is found that
the dephasing times due to carrier-carrier scattering is in picoseconds and not
femtoseconds, in agreement with experiments. The polarization dephasing rates
are computed as a function of the excited carrier density and it is found that
the dephasing rate for carrier-carrier scattering is proportional to the
carrier density at ultralow densities. The scaling relation is sublinear at
higher densities, which enables a comparison with experiment.Comment: Revised version with additional refs. 12 pages, figs. available upon
request; Submitted to Phys. Rev.
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