155 research outputs found
Statistical evaporation of rotating clusters. II. Angular momentum distribution
The change in the angular momentum of an atomic cluster following evaporation
is investigated using rigorous phase space theory and molecular dynamics
simulations, with an aim at the possible rotational cooling and heating
effects. Influences of the shape of the interaction potential, anharmonicity of
the vibrational density of states (DOS), and the initial distribution of
excitation energies are systematically studied on the example of the
Lennard-Jones cluster LJ_14. For this system, the predictions of PST are in
quantitative agreement with the results of the simulations, provided that the
correct forms for the vibrational density of states and the interaction
potential are used. The harmonic approximation to the DOS is used to obtain
explicit forms for the angular momentum distribution in larger clusters. These
are seen to undergo preferential cooling when thermally excited, and
preferential heating when subject to a strong vibrational excitation.Comment: 10 pages, 7 figure
Mapping the structural diversity of C60 carbon clusters and their infrared spectra
The current debate about the nature of the carbonaceous material carrying the
infrared (IR) emission spectra of planetary and proto-planetary nebulae,
including the broad plateaus, calls for further studies on the interplay
between structure and spectroscopy of carbon-based compounds of astrophysical
interest. The recent observation of C60 buckminsterfullerene in space suggests
that carbon clusters of similar size may also be relevant. In the present work,
broad statistical samples of C60 isomers were computationally determined
without any bias using a reactive force field, their IR spectra being
subsequently obtained following local optimization with the
density-functional-based tight-binding theory. Structural analysis reveals four
main structural families identified as cages, planar polycyclic aromatics,
pretzels, and branched. Comparison with available astronomical spectra
indicates that only the cage family could contribute to the plateau observed in
the 6-9 micron region. The present framework shows great promise to explore and
relate structural and spectroscopic features in more diverse and possibly
hydrogenated carbonaceous compounds, in relation with astronomical
observations
Statistical evaporation of rotating clusters. IV. Alignment effects in the dissociation of nonspherical clusters
Unimolecular evaporation in rotating, non-spherical atomic clusters is
investigated using Phase Space Theory in its orbiting transition state version.
The distributions of the total kinetic energy release epsilon_tr and the
rotational angular momentum J_r are calculated for oblate top and prolate top
main products with an arbitrary degree of deformation. The orientation of the
angular momentum of the product cluster with respect to the cluster symmetry
axis has also been obtained. This statistical approach is tested in the case of
the small 8-atom Lennard-Jones cluster, for which comparison with extensive
molecular dynamics simulations is presented. The role of the cluster shape has
been systematically studied for larger, model clusters in the harmonic
approximation for the vibrational densities of states. We find that the type of
deformation (prolate vs. oblate) plays little role on the distributions and
averages of epsilon_tr and J_r except at low initial angular momentum. However,
alignment effects between the product angular momentum and the symmetry axis
are found to be significant, and maximum at some degree of oblateness. The
effects of deformation on the rotational cooling and heating effects are also
illustrated.Comment: 15 pages, 9 figure
Second moment closure analysis of the backstep flow database
A Second Moment Closure computation (SMC) is compared in detail with the Direct Numerical Simulation (DNS) data of Le and Moin for the backstep flow at Re = 5,000 in an attempt to understand why the intensity of the backflow and, consequently, the friction coefficient in the recirculation bubble are severely underestimated. The data show that this recirculation bubble is far from being laminar except in the very near wall layer. A novel 'differential a priori' procedure was used, in which the full transport equation for one isolated component of the Reynolds stress tensor was solved using DNS data as input. Conclusions are then different from what would have been deduced by comparing a full simulation to a DNS. One cause of discrepancy was traced back to insufficient transfer of energy to the normal stress by pressure strain, but was not cured. A significant finding, confirmed by the DNS data in the core region of a channel flow, is that the coefficient that controls destruction of dissipation, C epsilon(sub 2), should be decreased by a factor of 2 when production is vanishing. This is also the case in the recirculation bubble, and a new formulation has cured 25% of the backflow discrepancy
Size effect in the ionization energy of PAH clusters
We report the first experimental measurement of the near-threshold
photo-ionization spectra of polycyclic aromatic hydrocarbon clusters made of
pyrene C16H10 and coronene C24H12, obtained using imaging photoelectron
photoion coincidence spectrometry with a VUV synchrotron beamline. The
experimental results of the ionization energy are confronted to calculated ones
obtained from simulations using dedicated electronic structure treatment for
large ionized molecular clusters. Experiment and theory consistently find a
decrease of the ionization energy with cluster size. The inclusion of
temperature effects in the simulations leads to a lowering of this energy and
to a quantitative agreement with the experiment. In the case of pyrene, both
theory and experiment show a discontinuity in the IE trend for the hexamer
Molecular hydrogen formation on grain surfaces
We reconsider H2 formation on grain surfaces. We develop a rate equation
model which takes into account the presence of both physisorbed and chemisorbed
sites on the surface, including quantum mechanical tunnelling and thermal
diffusion. In this study, we took into consideration the uncertainties on the
characteristics of graphitic surfaces. We calculate the H2 formation efficiency
with the Langmuir Hinshelwood and Eley Rideal mechanisms, and discuss the
importance of these mechanisms for a wide range of grain and gas temperatures.
We also develop a Monte Carlo simulation to calculate the H2 formation
efficiency and compare the results to our rate equation models. Our results are
the following: (1) Depending on the barrier against chemisorption, we predict
the efficiency of H2 formation for a wide range of grain and gas temperatures.
(2) The Eley-Rideal mechanism has an impact on the H2 formation efficiency at
high grain and gas temperatures. (3) The fact that we consider chemisorption in
our model makes the rate equation and Monte Carlo approaches equivalent.Comment: in "Light, dust and chemical evolution", Journal of Physics:
Conference Serie
Formation and destruction of polycyclic aromatic hydrocarbon clusters in the interstellar medium
The competition between the formation and destruction of coronene clusters
under interstellar conditions is investigated theoretically. The unimolecular
nucleation of neutral clusters is simulated with an atomic model combining an
explicit classical force field and a quantum tight-binding approach.
Evaporation rates are calculated in the framework of the phase space theory and
are inserted in an infrared emission model and compared with the growth rate
constants. It is found that, in interstellar conditions, most collisions lead
to cluster growth. The time evolution of small clusters (containing up to 312
carbon atoms) was specifically investigated under the physical conditions of
the northern photodissociation region of NGC 7023. These clusters are found to
be thermally photoevaporated much faster than they are reformed, thus providing
an interpretation for the lowest limit of the interstellar cluster size
distribution inferred from observations. The effects of ionizing the clusters
and density heterogeneities are also considered. Based on our results, the
possibility that PAH clusters could be formed in PDRs is critically discussed.Comment: 14 pages, 14 figures. Astronomy & Astrophysics, accepted for
publicatio
Blending of nanoscale and microscale in uniform large-area sculptured thin-film architectures
The combination of large thickness ( m), large--area uniformity (75
mm diameter), high growth rate (up to 0.4 m/min) in assemblies of
complex--shaped nanowires on lithographically defined patterns has been
achieved for the first time. The nanoscale and the microscale have thus been
blended together in sculptured thin films with transverse architectures.
SiO () nanowires were grown by electron--beam evaporation onto
silicon substrates both with and without photoresist lines (1--D arrays) and
checkerboard (2--D arrays) patterns. Atomic self--shadowing due to
oblique--angle deposition enables the nanowires to grow continuously, to change
direction abruptly, and to maintain constant cross--sectional diameter. The
selective growth of nanowire assemblies on the top surfaces of both 1--D and
2--D arrays can be understood and predicted using simple geometrical shadowing
equations.Comment: 17 pages, 9 figure
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