808 research outputs found
Determination of the Boltzmann constant by laser spectroscopy as a basis for future measurements of the thermodynamic temperature
In this paper, we present the latest results on the measurement of the
Boltzmann constant kB, by laser spectroscopy of ammonia at 10 ?m. The Doppler
absorption profile of a ro-vibrational line of an NH3 gas sample at thermal and
pressure equilibrium is measured as accurately as possible. The absorption cell
is placed inside a large 1m3 thermostat filled with an ice-water mixture, which
sets the temperature very close to 273.15 K. Analysing this profile, which is
related to the Maxwell-Boltzmann molecular speed distribution, leads to a
determination of the Boltzmann constant via a measurement of the Doppler width
(proportional tosqrt(kBT)). A spectroscopic determination of the Boltzmann
constant with an uncertainty as low as 37 ppm is obtained. Recent improvements
with a new passive thermostat lead to a temperature accuracy, stability and
homogeneity of the absorption cell better than 1 ppm over a day
Discreteness effects in a reacting system of particles with finite interaction radius
An autocatalytic reacting system with particles interacting at a finite
distance is studied. We investigate the effects of the discrete-particle
character of the model on properties like reaction rate, quenching phenomenon
and front propagation, focusing on differences with respect to the continuous
case. We introduce a renormalized reaction rate depending both on the
interaction radius and the particle density, and we relate it to macroscopic
observables (e.g., front speed and front thickness) of the system.Comment: 23 pages, 13 figure
Measurement of the Boltzmann constant by the Doppler broadening technique at a 3,8x10-5 accuracy level
In this paper, we describe an experiment performed at the Laboratoire de
Physique des Lasers and dedicated to an optical measurement of the Boltzmann
constant. With the proposed innovative technique, determining comes down to an
ordinary frequency measurement. The method consists in measuring as accurately
as possible the Doppler absorption profile of a rovibrational line of ammonia
in thermal equilibrium. This profile is related to the Maxwell-Boltzmann
molecular velocity distribution along the laser beam. A fit of the absorption
line shape leads to a determination of the Doppler width proportional to
sqrt(kT) and thus to a determination of the Boltzmann constant. The laser
source is an ultra-stable CO2 laser with a wavelength . The absorption cell is
placed in a thermostat keeping the temperature at 273.15 K within 1.4 mK. We
were able to measure with a relative uncertainty as small as 3.8x10-5, which
represents an improvement of an order of magnitude for an integration time
comparable to our previous measurement published in 2007 [1
Ultraviolet Radiation Constraints around the Circumstellar Habitable Zones
Ultraviolet radiation is known to inhibit photosynthesis, induce DNA
destruction and cause damage to a wide variety of proteins and lipids. In
particular, UV radiation between 200-300 nm becomes energetically very damaging
to most of the terrestrial biological systems. On the other hand, UV radiation
is usually considered one of the most important energy source on the primitive
Earth for the synthesis of many biochemical compounds and, therefore, essential
for several biogenesis processes. In this work, we use these properties of the
UV radiation to define the bounderies of an ultraviolet habitable zone. We also
analyze the evolution of the UV habitable zone during the main sequence stage
of the star. We apply these criteria to study the UV habitable zone for those
extrasolar planetary systems that were observed by the International
Ultraviolet Explorer (IUE). We analyze the possibility that extrasolar planets
and moons could be suitable for life, according to the UV constrains presented
in this work and other accepted criteria of habitability (liquid water, orbital
stability, etc.).Comment: 34 pages, 8 figures Accepted for publication by Icaru
Fronts dynamics in the presence of spatio-temporal structured noises
Front dynamics modeled by a reaction-diffusion equation are studied under the
influence of spatio-temporal structured noises. An effective deterministic
model is analytical derived where the noise parameters, intensity, correlation
time and correlation length appear explicitely. The different effects of these
parameters are discussed for the Ginzburg-Landau and Schl\"ogl models. We
obtain an analytical expression for the front velocity as a function of the
noise parameters. Numerical simulations results are in a good agreement with
the theoretical predictions.Comment: 11 pages, 6 figures; REVTEX; to be published in Phys.Rev.E, july 200
Emergence of pulled fronts in fermionic microscopic particle models
We study the emergence and dynamics of pulled fronts described by the
Fisher-Kolmogorov-Petrovsky-Piscounov (FKPP) equation in the microscopic
reaction-diffusion process A + A A$ on the lattice when only a particle is
allowed per site. To this end we identify the parameter that controls the
strength of internal fluctuations in this model, namely, the number of
particles per correlated volume. When internal fluctuations are suppressed, we
explictly see the matching between the deterministic FKPP description and the
microscopic particle model.Comment: 4 pages, 4 figures. Accepted for publication in Phys. Rev. E as a
Rapid Communicatio
High reflectivity grating waveguide coatings for 1064nm
We propose thin single-layer grating waveguide structures to be used as
high-reflectivity, but low thermal noise, alternative to conventional coatings
for gravitational wave detector test mass mirrors. Grating waveguide (GWG)
coatings can show a reflectivity of up to 100% with an overall thickness of
less than a wavelength. We theoretically investigate GWG coatings for 1064nm
based on tantala (Ta2O5) on a Silica substrate focussing on broad spectral
response and low thickness
Four-component united-atom model of bitumen
We propose a four-component molecular model of bitumen. The model includes
realistic chemical constituents and introduces a coarse-graining level that
suppresses the highest frequency modes. Molecular dynamics simulations of the
model are being carried out using Graphic-Processor-Units based software in
time spans in order of microseconds, and this enables the study of slow
relaxation processes characterizing bitumen. This paper focuses on the
high-temperature dynamics as expressed through the mean-square displacement,
the stress autocorrelation function, and rotational relaxation. The diffusivity
of the individual molecules changes little as a function of temperature and
reveals distinct dynamical time scales as a result of the different
constituents in the system. Different time scales are also observed for the
rotational relaxation. The stress autocorrelation function features a slow
non-exponential decay for all temperatures studied. From the stress
autocorrelation function, the shear viscosity and shear modulus are evaluated
at the highest temperature, showing a viscous response at frequencies below 100
MHz. The model predictions of viscosity and diffusivities are compared to
experimental data, giving reasonable agreement. The model shows that the
asphaltene, resin and resinous oil tend to form nano-aggregates. The
characteristic dynamical relaxation time of these aggregates is different from
the homogeneously distributed parts of the system, leading to strong dynamical
heterogeneity.Comment: 10 pages, 12 figure
The diffusion coefficient of propagating fronts with multiplicative noise
Recent studies have shown that in the presence of noise both fronts
propagating into a metastable state and so-called pushed fronts propagating
into an unstable state, exhibit diffusive wandering about the average position.
In this paper we derive an expression for the effective diffusion coefficient
of such fronts, which was motivated before on the basis of a multiple scale
ansatz. Our systematic derivation is based on the decomposition of the
fluctuating front into a suitably positioned average profile plus fluctuating
eigenmodes of the stability operator. While the fluctuations of the front
position in this particular decomposition are a Wiener process on all time
scales, the fluctuations about the time averaged front profile relax
exponentially.Comment: 4 page
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