2,334 research outputs found
Quantum Smoluchowski equation: Escape from a metastable state
We develop a quantum Smoluchowski equation in terms of a true probability
distribution function to describe quantum Brownian motion in configuration
space in large friction limit at arbitrary temperature and derive the rate of
barrier crossing and tunneling within an unified scheme. The present treatment
is independent of path integral formalism and is based on canonical
quantization procedure.Comment: 10 pages, To appear in the Proceedings of Statphys - Kolkata I
The Ammount of Interstellar Carbon Locked in Solid Hydrogenated Amorphous Carbon
We review the literature and present new experimental data to determine the
amount of carbon likely to be locked in form of solid hydrogenated amorphous
carbon (HAC) grains. We conclude on the basis of a thorough analysis of the
intrinsic strength of the C-H stretching band at 3.4 micron that between 10 and
80 ppM H of carbon is in the form of HAC grains. We show that it is necessary
to know the level of hydrogenation (H/C) of the interstellar HAC to determine
more precisely the amount of carbon it ties up. We present optical constants,
photoluminescence spectroscopy, and IR absorption spectroscopy for a particular
HAC sample that is shown to have a 3.4 micron absorption feature that is
quantatively consistent with that observed in the diffuse interstellar medium.Comment: This paper is 14 pages long with 5 figures and will appear in the 1
December 1999 issue of Ap
The Excitation of Extended Red Emission: New Constraints on its Carrier From HST Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing the
extended red emission (ERE) in many dusty interstellar environments remains
unidentified. Several competing models are more or less able to match the
observed broad, unstructured ERE band. We now constrain the character of the
ERE carrier further by determining the wavelengths of the radiation that
initiates the ERE. Using the imaging capabilities of the Hubble Space
Telescope, we have resolved the width of narrow ERE filaments appearing on the
surfaces of externally illuminated molecular clouds in the bright reflection
nebula NGC 7023 and compared them with the depth of penetration of radiation of
known wavelengths into the same cloud surfaces. We identify photons with
wavelengths shortward of 118 nm as the source of ERE initiation, not to be
confused with ERE excitation, however. There are strong indications from the
well-studied ERE in the Red Rectangle nebula and in the high-|b| Galactic
cirrus that the photon flux with wavelengths shortward of 118 nm is too small
to actually excite the observed ERE, even with 100% quantum efficiency. We
conclude, therefore, that ERE excitation results from a two-step process. While
none of the previously proposed ERE models can match these new constraints, we
note that under interstellar conditions most polycyclic aromatic hydrocarbon
(PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eV
and that the electronic energy level structure of PAH di-cations is consistent
with fluorescence in the wavelength band of the ERE. Therefore, PAH di-cations
deserve further study as potential carriers of the ERE. (abridged)Comment: Accepted for Publication in the Ap
Critical behaviour of the Rouse model for gelling polymers
It is shown that the traditionally accepted "Rouse values" for the critical
exponents at the gelation transition do not arise from the Rouse model for
gelling polymers. The true critical behaviour of the Rouse model for gelling
polymers is obtained from spectral properties of the connectivity matrix of the
fractal clusters that are formed by the molecules. The required spectral
properties are related to the return probability of a "blind ant"-random walk
on the critical percolating cluster. The resulting scaling relations express
the critical exponents of the shear-stress-relaxation function, and hence those
of the shear viscosity and of the first normal stress coefficient, in terms of
the spectral dimension of the critical percolating cluster and the
exponents and of the cluster-size distribution.Comment: 9 pages, slightly extended version, to appear in J. Phys.
The Photophysics of the Carrier of Extended Red Emission
Interstellar dust contains a component which reveals its presence by emitting
a broad, unstructured band of light in the 540 to 950 nm wavelength range,
referred to as Extended Red Emission (ERE). The presence of interstellar dust
and ultraviolet photons are two necessary conditions for ERE to occur. This is
the basis for suggestions which attribute ERE to an interstellar dust component
capable of photoluminescence. In this study, we have collected all published
ERE observations with absolute-calibrated spectra for interstellar
environments, where the density of ultraviolet photons can be estimated
reliably. In each case, we determined the band-integrated ERE intensity, the
wavelength of peak emission in the ERE band, and the efficiency with which
absorbed ultraviolet photons are contributing to the ERE. The data show that
radiation is not only driving the ERE, as expected for a photoluminescence
process, but is modifying the ERE carrier as manifested by a systematic
increase in the ERE band's peak wavelength and a general decrease in the photon
conversion efficiency with increasing densities of the prevailing exciting
radiation. The overall spectral characteristics of the ERE and the observed
high quantum efficiency of the ERE process are currently best matched by the
recently proposed silicon nanoparticle (SNP) model. Using the experimentally
established fact that ionization of semiconductor nanoparticles quenches their
photoluminescence, we proceeded to test the SNP model by developing a
quantitative model for the excitation and ionization equilibrium of SNPs under
interstellar conditions for a wide range of radiation field densities.Comment: 42 p., incl. 8 fig. Accepted for publication by Ap
The Dust in Lyman Break Galaxies
We present our analysis of UV attenuation by internal dust of a large sample
(N=906 galaxies) of Lyman Break Galaxies (LBGs). Using spectral energy
distributions (SEDs) from the P\'EGASE galaxy spectral evolution model we apply
dust attenuation corrections to the G-R colors using the Witt & Gordon (2000)
models for radiative transfer in dusty galactic environments to arrive at the
UV attenuation factors. We show that the dust in the LBGs exhibit SMC-like
characteristics rather than MW-like, and that the dust geometry in these
systems is most likely to be represented by a clumpy shell configuration. We
show that the attenuation factor exhibits a pronounced dependence on the
luminosity of the LBG, a_{1600}\propto (L/L_\sun)^\alpha, where
. The exponent depends on the initial parameters
of the stellar population chosen to model the galaxies and the dust properties.
We find that the luminosity weighted average attenuation factor is likely to be
in the range from , which is consistent with the upper limits to the
star formation rate at set by the FIR background. This implies that the
current UV/optical surveys do detect the bulk of the star formation during the
epoch , but require substantial correction for internal dust
attenuation.Comment: 17 pages, 12 figures, uses AASTEX, accepted for publication in the
Astrophysical Journa
Critical Dynamics of Gelation
Shear relaxation and dynamic density fluctuations are studied within a Rouse
model, generalized to include the effects of permanent random crosslinks. We
derive an exact correspondence between the static shear viscosity and the
resistance of a random resistor network. This relation allows us to compute the
static shear viscosity exactly for uncorrelated crosslinks. For more general
percolation models, which are amenable to a scaling description, it yields the
scaling relation for the critical exponent of the shear
viscosity. Here is the thermal exponent for the gel fraction and
is the crossover exponent of the resistor network. The results on the shear
viscosity are also used in deriving upper and lower bounds on the incoherent
scattering function in the long-time limit, thereby corroborating previous
results.Comment: 34 pages, 2 figures (revtex, amssymb); revised version (minor
changes
Force-Extension Relations for Polymers with Sliding Links
Topological entanglements in polymers are mimicked by sliding rings
(slip-links) which enforce pair contacts between monomers. We study the
force-extension curve for linear polymers in which slip-links create additional
loops of variable size. For a single loop in a phantom chain, we obtain exact
expressions for the average end-to-end separation: The linear response to a
small force is related to the properties of the unstressed chain, while for a
large force the polymer backbone can be treated as a sequence of Pincus--de
Gennes blobs, the constraint effecting only a single blob. Generalizing this
picture, scaling arguments are used to include self-avoiding effects.Comment: 4 pages, 5 figures; accepted to Phys. Rev. E (Brief Report
Viscoelasticity near the gel-point: a molecular dynamics study
We report on extensive molecular dynamics simulations on systems of soft
spheres of functionality f, i.e. particles that are capable of bonding
irreversibly with a maximum of f other particles. These bonds are randomly
distributed throughout the system and imposed with probability p. At a critical
concentration of bonds, p_c approximately equal to 0.2488 for f=6, a gel is
formed and the shear viscosity \eta diverges according to \eta ~ (p_c-p)^{-s}.
We find s is approximately 0.7 in agreement with some experiments and with a
recent theoretical prediction based on Rouse dynamics of phantom chains. The
diffusion constant decreases as the gel point is approached but does not
display a well-defined power law.Comment: 4 pages, 4 figure
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