9,147 research outputs found
Experiments with a fully instrumented split Stirling cryocooler
A practical model that can be used to accurately size and optimally split stirling cryocoolers is discussed. A practical model that could be used to extrapolate existing designs to meet different specifications was developed. However, to do this detailed knowledge of the dynamic operating parameters of this type of cryocooler is required. The first stage is to fully instrument a refrigerator so that various dynamic parameters can be measured. The second stage involves the application of these measurements to the design and optimization of a range of coolers
Quasi-classical rate coefficient calculations for the rotational (de)excitation of H2O by H2
The interpretation of water line emission from existing observations and
future HIFI/Herschel data requires a detailed knowledge of collisional rate
coefficients. Among all relevant collisional mechanisms, the rotational
(de)excitation of H2O by H2 molecules is the process of most interest in
interstellar space. To determine rate coefficients for rotational de-excitation
among the lowest 45 para and 45 ortho rotational levels of H2O colliding with
both para and ortho-H2 in the temperature range 20-2000 K. Rate coefficients
are calculated on a recent high-accuracy H2O-H2 potential energy surface using
quasi-classical trajectory calculations. Trajectories are sampled by a
canonical Monte-Carlo procedure. H2 molecules are assumed to be rotationally
thermalized at the kinetic temperature. By comparison with quantum calculations
available for low lying levels, classical rates are found to be accurate within
a factor of 1-3 for the dominant transitions, that is those with rates larger
than a few 10^{-12}cm^{3}s^{-1}. Large velocity gradient modelling shows that
the new rates have a significant impact on emission line fluxes and that they
should be adopted in any detailed population model of water in warm and hot
environments.Comment: 8 pages, 2 figures, 1 table (the online material (4 tables) can be
obtained upon request to [email protected]
Collisional excitation of singly deuterated ammonia NHD by H
The availability of collisional rate coefficients with H is a
pre-requisite for interpretation of observations of molecules whose energy
levels are populated under non local thermodynamical equilibrium conditions. In
the current study, we present collisional rate coefficients for the NHD /
para--H() collisional system, for energy levels up to (735 K) and for gas temperatures in the range K. The
cross sections are obtained using the essentially exact close--coupling (CC)
formalism at low energy and at the highest energies, we used the
coupled--states (CS) approximation. For the energy levels up to
(215 K), the cross sections obtained through the CS formalism are
scaled according to a few CC reference points. These reference points are
subsequently used to estimate the accuracy of the rate coefficients for higher
levels, which is mainly limited by the use of the CS formalism. Considering the
current potential energy surface, the rate coefficients are thus expected to be
accurate to within 5\% for the levels below , while we estimate
an accuracy of 30\% for higher levels
Laser-induced electron emission from a tungsten nanotip: identifying above threshold photoemission using energy-resolved laser power dependencies
We present an experiment studying the interaction of a strongly focused 25 fs
laser pulse with a tungsten nanotip, investigating the different regimes of
laser-induced electron emission. We study the dependence of the electron yield
with respect to the static electric field applied to the tip. Photoelectron
spectra are recorded using a retarding field spectrometer and peaks separated
by the photon energy are observed with a 45 % contrast. They are a clear
signature of above threshold photoemission (ATP), and are confirmed by
extensive spectrally resolved studies of the laser power dependence.
Understanding these mechanisms opens the route to control experiment in the
strong-field regime on nanoscale objects.Comment: 9 pages, 6 figure
The IRAM-30m line survey of the Horsehead PDR: III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas
Complex (iso-)nitrile molecules, such as CH3CN and HC3N, are relatively
easily detected in our Galaxy and in other galaxies. We constrain their
chemistry through observations of two positions in the Horsehead edge: the
photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just
behind it. We systematically searched for lines of CH3CN, HC3N, C3N, and some
of their isomers in our sensitive unbiased line survey at 3, 2, and 1mm. We
derived column densities and abundances through Bayesian analysis using a large
velocity gradient radiative transfer model. We report the first clear detection
of CH3NC at millimeter wavelength. We detected 17 lines of CH3CN at the PDR and
6 at the dense core position, and we resolved its hyperfine structure for 3
lines. We detected 4 lines of HC3N, and C3N is clearly detected at the PDR
position. We computed new electron collisional rate coefficients for CH3CN, and
we found that including electron excitation reduces the derived column density
by 40% at the PDR position. While CH3CN is 30 times more abundant in the PDR
than in the dense core, HC3N has similar abundance at both positions. The
isomeric ratio CH3NC/CH3CN is 0.15+-0.02. In the case of CH3CN, pure gas phase
chemistry cannot reproduce the amount of CH3CN observed in the UV-illuminated
gas. We propose that CH3CN gas phase abundance is enhanced when ice mantles of
grains are destroyed through photo-desorption or thermal-evaporation in PDRs,
and through sputtering in shocks. (abridged)Comment: Accepted for publication in Astronomy & Astrophysic
Collisional excitation of water by hydrogen atoms
We present quantum dynamical calculations that describe the rotational
excitation of HO due to collisions with H atoms. We used a recent, high
accuracy potential energy surface, and solved the collisional dynamics with the
close-coupling formalism, for total energies up to 12 000 cm. From these
calculations, we obtained collisional rate coefficients for the first 45 energy
levels of both ortho- and para-HO and for temperatures in the range T =
5-1500 K. These rate coefficients are subsequently compared to the values
previously published for the HO / He and HO / H collisional
systems. It is shown that no simple relation exists between the three systems
and that specific calculations are thus mandatory
Rotational Excitation of HC_3N by H_2 and He at low temperatures
Rates for rotational excitation of HC3N by collisions with He atoms and H2
molecules are computed for kinetic temperatures in the range 5-20K and 5-100K,
respectively. These rates are obtained from extensive quantum and
quasi-classical calculations using new accurate potential energy surfaces
(PES)
Atomistic studies of transformation pathways and energetics in plutonium
One of the most challenging problems in understanding the structural phase
transformations in Pu is to determine the energetically favored, continuous
atomic pathways from one crystal symmetry to another. This problem involves
enumerating candidate pathways and studying their energetics to garner insight
into instabilities and energy barriers. The purpose of this work is to
investigate the energetics of two transformation pathways for the delta to
alpha' transformation in Pu that were recently proposed [Lookman et al., Phys.
Rev. Lett. 100:145504, 2008] on the basis of symmetry. These pathways require
the presence of either an intermediate hexagonal closed-packed (hcp) structure
or a simple hexagonal (sh) structure. A subgroup of the parent fcc and the
intermediate hexagonal structure, which has trigonal symmetry, facilitates the
transformation to the intermediate hcp or sh structure. Phonons then break the
translational symmetry from the intermediate hcp or sh structure to the final
monoclinic symmetry of the alpha' structure. We perform simulations using the
modified embedded atom method (MEAM) for Pu to investigate these candidate
pathways. Our main conclusion is that the path via hcp is energetically favored
and the volume change for both pathways essentially occurs in the second step
of the transformation, i.e. from the intermediate sh or hcp to the monoclinic
structure. Our work also highlights the deficiency of the current
state-of-the-art MEAM potential in capturing the anisotropy associated with the
lower symmetry monoclinic structure.Comment: 12 pages, 5 figures, accepted for publication in Philos. Ma
Topological properties of quantum periodic Hamiltonians
We consider periodic quantum Hamiltonians on the torus phase space
(Harper-like Hamiltonians). We calculate the topological Chern index which
characterizes each spectral band in the generic case. This calculation is made
by a semi-classical approach with use of quasi-modes. As a result, the Chern
index is equal to the homotopy of the path of these quasi-modes on phase space
as the Floquet parameter (\theta) of the band is varied. It is quite
interesting that the Chern indices, defined as topological quantum numbers, can
be expressed from simple properties of the classical trajectories.Comment: 27 pages, 14 figure
Collisional excitation of doubly and triply deuterated ammonia NDH and ND by H
The availability of collisional rate coefficients is a prerequisite for an
accurate interpretation of astrophysical observations, since the observed media
often harbour densities where molecules are populated under non--LTE
conditions. In the current study, we present calculations of rate coefficients
suitable to describe the various spin isomers of multiply deuterated ammonia,
namely the NDH and ND isotopologues. These calculations are based on
the most accurate NH--H potential energy surface available, which has
been modified to describe the geometrical changes induced by the nuclear
substitutions. The dynamical calculations are performed within the
close--coupling formalism and are carried out in order to provide rate
coefficients up to a temperature of = 50K. For the various
isotopologues/symmetries, we provide rate coefficients for the energy levels
below 100 cm. Subsequently, these new rate coefficients are used
in astrophysical models aimed at reproducing the NHD, NDH and ND
observations previously reported towards the prestellar cores B1b and 16293E.
We thus update the estimates of the corresponding column densities and find a
reasonable agreement with the previous models. In particular, the
ortho--to--para ratios of NHD and NHD are found to be consistent with
the statistical ratios
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