2,755 research outputs found
Time-resolved spectra of polar-polarizable chromophores in solution
A recently proposed model for steady-state spectra of polar-polarizable
chromophores is extended to describe time-resolved spectra. The model, based on
a two-state picture for the solute and on a continuum overdamped description
for the (polar) solvent, grasps the essential physics of solvation dynamics, as
demonstrated by the comparison with experimental spectra. The solute
(hyper)polarizability is responsible for spectroscopic features that cannot be
rationalized within the standard picture based on a linear perturbative
treatment of the solute-solvent interaction. In particular, the temporal
evolution of band-shapes and the appearance of temporary isosbestic points, two
common puzzling features of observed spectra, are natural consequences of the
molecular hyperpolarizability and of the consequent coupling between solvation
and vibrational degrees of freedom.Comment: 14pages, including 7 figure
Time-resolved spectra of a self-pulsing quantum dot laser
Self-sustained pulsations in the output of an InAs quantum dot laser diode in the MHz range are reported for the first time. The characteristics (shape, range and frequency) are presented for the free running laser and when optical feedback in the Littrow configuration is applied. The frequency resolved optical spectra reveal different envelope shifts between the two cases. This might be related to a change of phase-amplitude coupling across the gain maximum in agreement with the expectation for a two level system. The time scale and bifurcation scenario suggest that these are opto-thermal pulsation like those reported in quantum well amplifiers.(1
Time Resolved GRB Spectroscopy
We present the main results of a study of time-resolved spectra of 43 intense
GRBs detected by BATSE. We considered the 4-parameter Band model and the
Optically Thin Synchrotron Shock model (OTSSM). We find that the large majority
of time-resolved spectra of GRBs are in remarkable agreement with the OTSSM.
However, about 15 % of initial GRB pulses show an apparent low-energy photon
suppression. This phenomenon indicates that complex radiative conditions
modifying optically thin emission may occur during the initial phases of some
GRBs.Comment: 5 pages, 3 figures, Paper presented at the 5th Huntsville Symposium,
Huntsville (Alabama) Oct. 199
Nonlinear effects in time-resolved spectra of DAVs
Numerical simulations of light curves of variable DA white dwarfs (ZZ Ceti
stars) predict flux amplitudes with surface distributions different from the
spherical harmonics of the pulsation mode in deeper layers. In contrast to the
results of the perturbation analysis by Goldreich and Wu this is also true for
the fundamental period of the flux variation. As a consequence normalized
amplitude spectra depend not only on the mode number l but also on pulsation
amplitude and inclination. Another new result is that with increasing amplitude
of the pressure variation below the convection zone the flux variation at the
surface goes through a maximum and then decreases again
On the Shape of Pulse Spectra in Gamma-Ray Bursts
The discovery (Liang & Kargatis 1996), that the peak energy of time-resolved
spectra of gamma-ray burst (GRB) pulses decays exponentially with fluence, is
analytically shown to imply that the time-integrated photon number spectrum of
a pulse should have a unique shape, given by an underlying E^-1 behavior. We
also show that the asymptotic low energy normalization of the time-integrated
spectrum is equal to the exponential decay constant. We study analytically how
this general behavior is modified in more realistic situations and show that
diversity is then introduced in the properties of time-integrated GRB pulse
spectra. We argue that further diversity will occur in time-integrated
multi-pulse (complex) GRB spectra. The total energy received per cm^2 is
approximately the decay constant times the maximum peak energy of the pulse.
Our analytical results connect the properties of the time-integrated pulse
spectrum with those of the time-resolved spectra, and can thus be used when
studying observed GRB pulse spectra. We illustrate with the bright burst GRB
910807 and comment on GRB 910525 and GRB 921207.Comment: 7 pages, 6 postscript figures, accepted by the Astrophysical Journa
A fast data acquisition system for the study of transient events by high repetition rate time-of-flight mass spectrometry
Recent advances in commercially available data acquisition electronics embodying high speed A/D conversion coupled to increased memory storage have now made practical (at least within time intervals of a third of a millisecond or more) the capturing of all of the data generated by a high repetition rate time-of-flight mass spectrometer producing complete spectra every 25 to 35 microseconds. Such a system was assembled and interfaced with a personal computer for control and management of data. The applications are described for recording time-resolved spectra of individual vapor plumes induced from the pulsed-laser heating of material. Each laser pulse triggers the system to generate automatically a 3-dimensional (3-D) presentation of the time-resolved spectra with m/z labeling of the major mass peaks, plus an intensity versus time display of both the laser pulse and the resulting vapor pulse. The software also permits storing of data and its presentation in various additional forms
Synchrotron Cooling in Energetic Gamma-Ray Bursts Observed by the Fermi Gamma-Ray Burst Monitor
We study the time-resolved spectra of eight GRBs observed by Fermi GBM in its
first five years of mission, with 1 keV - 1 MeV fluence
erg cm and signal-to-noise level above 900 keV. We
aim to constrain in detail the spectral properties of GRB prompt emission on a
time-resolved basis and to discuss the theoretical implications of the fitting
results in the context of various prompt emission models. We perform
time-resolved spectral analysis using a variable temporal binning technique
according to optimal S/N criteria, resulting in a total of 299 time-resolved
spectra. We fit the Band function to all spectra and obtain the distributions
for the low-energy power-law index , the high-energy power-law index
, the peak energy in the observed spectrum , and
the difference between the low- and high-energy power-law indices . Using the distributions of and , the
electron population index is found to be consistent with the "moderately
fast" scenario which fast- and slow-cooling scenarios cannot be distinguished.
We also apply a physically motivated synchrotron model, which is a triple
power-law with constrained power-law indices and a blackbody component, to test
for consistency with a synchrotron origin for the prompt emission and obtain
the distributions for the two break energies and ,
the middle segment power-law index , and the Planck function temperature
. A synchrotron model is found consistent with the majority of
time-resolved spectra for these eight energetic Fermi GBM bursts with good
high-energy photon statistics, as long as both the cooling and injection break
are included and the leftmost spectral slope is lifted either by inclusion of a
thermal component or when an evolving magnetic field is accounted for.Comment: 20 pages, 7 figures, 8 tables, accepted for publication in A&
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