6,015 research outputs found
Screened electrostatic interactions between clay platelets
An effective pair potential for systems of uniformly charged lamellar
colloids in the presence of an electrolytic solution of microscopic co- and
counterions is derived. The charge distribution on the discs is expressed as a
collection of multipole moments, and the tensors which determine the
interactions between these multipoles are derived from a screened Coulomb
potential. Unlike previous studies of such systems, the interaction energy may
now be expressed for discs at arbitrary mutual orientation. The potential is
shown to be exactly equivalent to the use of linearized Poisson-Boltzmann
theory.Comment: 23 pages, 10 figures, created with Revtex. To appear in Molecular
Physic
A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows
The use of probability theory to determine the effects of turbulent fluctuations on reaction rates in turbulent combustion systems is briefly reviewed. Results are presented for the effect of species fluctuations in particular. It is found that turbulent fluctuations of species act to reduce the reaction rates, in contrast with the temperature fluctuations previously determined to increase Arrhenius reaction rate constants. For the temperature fluctuations, a criterion is set forth for determining if, in a given region of a turbulent flow field, the temperature can be expected to exhibit ramp like fluctuations. Using the above results, along with results previously obtained, a model is described for testing the effects of turbulent fluctuations of temperature and species on reaction rates in computer programs dealing with turbulent reacting flows. An alternative model which employs three variable probability density functions (temperature and two species) and is currently being formulated is discussed as well
The Global Star Formation Rate from the 1.4 GHz Luminosity Function
The decimetric luminosity of many galaxies appears to be dominated by
synchrotron emission excited by supernova explosions. Simple models suggest
that the luminosity is directly proportional to the rate of supernova
explosions of massive stars averaged over the past 30 Myr. The proportionality
may be used together with models of the evolving 1.4 GHz luminosity function to
estimate the global star formation rate density in the era z < 1. The local
value is estimated to be 0.026 solar masses per year per cubic megaparsec, some
50% larger than the value inferred from the Halpha luminosity density. The
value at z ~ 1 is found to be 0.30 solar masses per year per cubic megaparsec.
The 10-fold increase in star formation rate density is consistent with the
increase inferred from mm-wave, far-infrared, ultra-violet and Halpha
observations.Comment: 10 pages, 2 figures, Astrophysical Journal Letters (in press); new PS
version has improved figure placemen
The Redshift of GRB 970508
GRB 970508 is the second gamma-ray burst (GRB) for which an optical afterglow
has been detected. It is the first GRB for which a distance scale has been
determined: absorption and emission features in spectra of the optical
afterglow place GRB 970508 at a redshift of z >= 0.835 (Metzger et al. 1997a,
1997b). The lack of a Lyman-alpha forest in these spectra further constrains
this redshift to be less than approximately 2.3. I show that the spectrum of
the optical afterglow of GRB 970508, once corrected for Galactic absorption, is
inconsistent with the relativistic blast-wave model unless a second, redshifted
source of extinction is introduced. This second source of extinction may be the
yet unobserved host galaxy. I determine its redshift to be z =
1.09^{+0.14}_{-0.41}, which is consistent with the observed redshift of z =
0.835. Redshifts greater than z = 1.40 are ruled out at the 3 sigma confidence
level.Comment: Accepted to The Astrophysical Journal (Letters), 10 pages, LaTe
Doppler-Free Saturated Fluorescence Spectroscopy of Lithium Using a Stabilized Frequency Comb
The lineshapes of the D1 (22S1/2 → 22P1/2 ) and D2 (22S3/2 → 22P1/2) transitions in lithium were measured using a diode laser that was frequency-stabilized to a Ti:Sapphire 1 GHz optical frequency comb. The excitation was achieved by retroreflecting the diode laser, in effect producing the Doppler-free profiles for the center frequencies of transitions. The observed spectra were compared to density matrix calculations to gain insight into systematic effects including the dependence of Doppler-free profiles on power and polarization angle of the diode. For certain transitions, the method of saturated fluorescence spectroscopy inevitably leads to the presence of extra resonances known as crossover signals. Our preliminary data suggest that the presence of this complicating effect may render saturated fluorescence spectroscopy an ineffective technique for resolution of transitions whose relative separation is on the order of the natural linewidth of Li
Doppler-Free Saturated Fluorescence Spectroscopy of Lithium Using a Stabilized Frequency Comb
The lineshapes of the D1 (22S1/2 → 22P1/2 ) and D2 (22S3/2 → 22P1/2) transitions in lithium were measured using a diode laser that was frequency-stabilized to a Ti:Sapphire 1 GHz optical frequency comb. The excitation was achieved by retroreflecting the diode laser, in effect producing the Doppler-free profiles for the center frequencies of transitions. The observed spectra were compared to density matrix calculations to gain insight into systematic effects including the dependence of Doppler-free profiles on power and polarization angle of the diode. For certain transitions, the method of saturated fluorescence spectroscopy inevitably leads to the presence of extra resonances known as crossover signals. Our preliminary data suggest that the presence of this complicating effect may render saturated fluorescence spectroscopy an ineffective technique for resolution of transitions whose relative separation is on the order of the natural linewidth of Li
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