83 research outputs found
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
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
Velocity Dispersion of Excited H2
We present a study of the high rotational bands (J > 2) of H2 toward 4 early
type galactic stars: HD 73882, HD 192639, HD 206267, and HD 207538. In each
case, the velocity dispersion - characterized by the spectrum fitting parameter
b - increases with the level of excitation, a phenomenon that has previously
been detected by the Copernicus and IMAPS observatories. In particular, we show
with 4 sigma confidence that for HD 192639 it is not possible to fit all J
levels with a single b value, and that higher b values are needed for the
higher levels. The amplitude of the line broadening, which can be as high as 10
km s^-1, makes explanations such as inhomogeneous spatial distribution
unlikely. We investigate a mechanism in which the broadening is due to the
molecules that are rotationally excited through the excess energy acquired
after their formation on a grain (H2-formation pumping). We show that different
dispersions would be a natural consequence of this mechanism. We note however
that such process would require a formation rate 10 times higher then what was
inferred from other observations. In view of the difficulty to account for the
velocity dispersion as thermal broadening (T would be around 10,000 K), we
conclude then that we are most certainly observing some highly turbulent warm
layer associated with the cold diffuse cloud. Embedded in a magnetic field, it
could be responsible for the high quantities of CH+ measured in the cold
neutral medium.Comment: accepted in Ap
Roadmap on dynamics of molecules and clusters in the gas phase
This roadmap article highlights recent advances, challenges and future prospects in studies of the dynamics of molecules and clusters in the gas phase. It comprises nineteen contributions by scientists with leading expertise in complementary experimental and theoretical techniques to probe the dynamics on timescales spanning twenty order of magnitudes, from attoseconds to minutes and beyond, and for systems ranging in complexity from the smallest (diatomic) molecules to clusters and nanoparticles. Combining some of these techniques opens up new avenues to unravel hitherto unexplored reaction pathways and mechanisms, and to establish their significance in, e.g. radiotherapy and radiation damage on the nanoscale, astrophysics, astrochemistry and atmospheric science
Relaxation processes in the ferrielectric and alpha phases of antiferroelectric liquid crystals
Dielectric measurements of a new antiferroelectric liquid crystal
series exhibiting different phase sequences have been carried
out as a function of frequency from 10 Hz to 10 MHz. Structural
properties of SCα* and ferrielectric SCFI* phases were discussed
on the basis of the experimental results of temperature and dc
bias field dependencies of the dielectric modes. Besides the
soft mode observed around the SA−SCα* phase transition, a Goldstone
mode was detected in the SCα* phase indicating a helicoidal structure
with small pitch values. In agreement with a bilayer ordering
model, the dielectric absorption in the SCFI* phase was splitted
up into two contributions: one related to a Goldstone mode and
the other to an azimuthal antiphase mode
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