109 research outputs found
Molecular Hydrogen Formation on Porous Dust Grains
Recent laboratory experiments on interstellar dust analogues have shown that
H_2 formation on dust grain surfaces is efficient in a range of grain
temperatures below 20 K. These results indicate that surface processes may
account for the observed H_2 abundance in cold diffuse and dense clouds.
However, high abundances of H_2 have also been observed in warmer clouds,
including photon-dominated regions (PDRs), where grain temperatures may reach
50 K, making the surface processes extremely inefficient. It was suggested that
this apparent discrepancy can be resolved by chemisorption sites. However,
recent experiments indicate that chemisorption processes may not be efficient
at PDR temperatures. Here we consider the effect of grain porosity on H_2
formation. It is found that porosity extends the efficiency of the
recombination process to higher temperatures. This is because H atoms that
desorb from the internal surfaces of the pores may re-adsorb many times and
thus stay longer on the surface. However, this porosity-driven extension may
enable efficient H_2 formation in PDRs only if porosity also contributes to
significant cooling of the grains, compared to non-porous grains.Comment: 17 pages, 4 figures. Minor changes. Accepted for publication in MNRA
Electron-phonon coupling in potassium-doped graphene: Angle-resolved photoemission spectroscopy
The electron-phonon coupling in potassium-doped graphene on Ir(111) is
studied via the renormalization of the pi* band near the Fermi level, using
angle-resolved photoemission spectroscopy. The renormalization is found to be
fairly weak and almost isotropic, with a mass enhancement parameter of lambda=
0.28(6) for both the K-M and the K-G direction. These results are found to
agree well with recent first principles calculations.Comment: 5 pages, 3 figure
Laser cooling of trapped ytterbium ions with an ultraviolet diode laser
We demonstrate an ultraviolet diode laser system for cooling of trapped
ytterbium ions. The laser power and linewidth are comparable to previous
systems based on resonant frequency doubling, but the system is simpler, more
robust, and less expensive. We use the laser system to cool small numbers of
ytterbium ions confined in a linear Paul trap. From the observed spectra, we
deduce final temperatures < 270 mK.Comment: submitted to Opt. Let
H2 reformation in post-shock regions
H2 formation is an important process in post-shock regions, since H2 is an
active participant in the cooling and shielding of the environment. The onset
of H2 formation therefore has a strong effect on the temperature and chemical
evolution in the post shock regions. We recently developed a model for H2
formation on a graphite surface in warm conditions. The graphite surface acts
as a model system for grains containing large areas of polycyclic aromatic
hydrocarbon structures. Here this model is used to obtain a new description of
the H2 formation rate as a function of gas temperature that can be implemented
in molecular shock models. The H2 formation rate is substantially higher at
high gas temperatures as compared to the original implementation of this rate
in shock models, because of the introduction of H atoms which are chemically
bonded to the grain (chemisorption). Since H2 plays such a key role in the
cooling, the increased rate is found to have a substantial effect on the
predicted line fluxes of an important coolant in dissociative shocks [O I] at
63.2 and 145.5 micron. With the new model a better agreement between model and
observations is obtained. Since one of the goals of Herschel/PACS will be to
observe these lines with higher spatial resolution and sensitivity than the
former observations by ISO-LWS, this more accurate model is very timely to help
with the interpretation of these future results.Comment: 12 pages, 3 figures, 1 table, accepted in MNRAS Letter
Ground state cooling, quantum state engineering and study of decoherence of ions in Paul traps
We investigate single ions of in Paul traps for quantum
information processing. Superpositions of the S electronic ground state
and the metastable D state are used to implement a qubit. Laser light
on the S D transition is used for the
manipulation of the ion's quantum state. We apply sideband cooling to the ion
and reach the ground state of vibration with up to 99.9% probability. Starting
from this Fock state , we demonstrate coherent quantum state
manipulation. A large number of Rabi oscillations and a ms-coherence time is
observed. Motional heating is measured to be as low as one vibrational quantum
in 190 ms. We also report on ground state cooling of two ions.Comment: 12 pages, 6 figures. submitted to Journal of Modern Optics, Special
Issue on Quantum Optics: Kuehtai 200
Self-assembly of ordered graphene nanodot arrays (vol 8, 47, 2017)
Change History: A correction to this article has been published and is linked from the HTML version of this article
Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling
Molecular hydrogen has an important role in the early stages of star
formation as well as in the production of many other molecules that have been
detected in the interstellar medium. In this review we show that it is now
possible to study the formation of molecular hydrogen in simulated
astrophysical environments. Since the formation of molecular hydrogen is
believed to take place on dust grains, we show that surface science techniques
such as thermal desorption and time-of-flight can be used to measure the
recombination efficiency, the kinetics of reaction and the dynamics of
desorption. The analysis of the experimental results using rate equations gives
useful insight on the mechanisms of reaction and yields values of parameters
that are used in theoretical models of interstellar cloud chemistry.Comment: 23 pages, 7 figs. Published in the J. Phys.: Conf. Se
Prospects for hydrogen storage in graphene
Hydrogen-based fuel cells are promising solutions for the efficient and clean
delivery of electricity. Since hydrogen is an energy carrier, a key step for
the development of a reliable hydrogen-based technology requires solving the
issue of storage and transport of hydrogen. Several proposals based on the
design of advanced materials such as metal hydrides and carbon structures have
been made to overcome the limitations of the conventional solution of
compressing or liquefying hydrogen in tanks. Nevertheless none of these systems
are currently offering the required performances in terms of hydrogen storage
capacity and control of adsorption/desorption processes. Therefore the problem
of hydrogen storage remains so far unsolved and it continues to represent a
significant bottleneck to the advancement and proliferation of fuel cell and
hydrogen technologies. Recently, however, several studies on graphene, the
one-atom-thick membrane of carbon atoms packed in a honeycomb lattice, have
highlighted the potentialities of this material for hydrogen storage and raise
new hopes for the development of an efficient solid-state hydrogen storage
device. Here we review on-going efforts and studies on functionalized and
nanostructured graphene for hydrogen storage and suggest possible developments
for efficient storage/release of hydrogen at ambient conditions
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