60 research outputs found
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
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
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
Imaging and Dynamics of Light Atoms and Molecules on Graphene
Observing the individual building blocks of matter is one of the primary
goals of microscopy. The invention of the scanning tunneling microscope [1]
revolutionized experimental surface science in that atomic-scale features on a
solid-state surface could finally be readily imaged. However, scanning
tunneling microscopy has limited applicability due to restrictions, for
example, in sample conductivity, cleanliness, and data aquisition rate. An
older microscopy technique, that of transmission electron microscopy (TEM) [2,
3] has benefited tremendously in recent years from subtle instrumentation
advances, and individual heavy (high atomic number) atoms can now be detected
by TEM [4 - 7] even when embedded within a semiconductor material [8, 9].
However, detecting an individual low atomic number atom, for example carbon or
even hydrogen, is still extremely challenging, if not impossible, via
conventional TEM due to the very low contrast of light elements [2, 3, 10 -
12]. Here we demonstrate a means to observe, by conventional transmision
electron microscopy, even the smallest atoms and molecules: On a clean
single-layer graphene membrane, adsorbates such as atomic hydrogen and carbon
can be seen as if they were suspended in free space. We directly image such
individual adatoms, along with carbon chains and vacancies, and investigate
their dynamics in real time. These techniques open a way to reveal dynamics of
more complex chemical reactions or identify the atomic-scale structure of
unknown adsorbates. In addition, the study of atomic scale defects in graphene
may provide insights for nanoelectronic applications of this interesting
material.Comment: 9 pages manuscript and figures, 9 pages supplementary informatio
Laboratory evidence for the non-detection of excited nascent H2 in dark clouds
There has always been a great deal of interest in the formation of H2 as well
as in the binding energy released upon its formation on the surface of dust
grains. The present work aims at collecting experimental evidence for how the
bond energy budget of H2 is distributed between the reaction site and the
internal energy of the molecule. So far, the non-detection of excited nascent
H2 in dense quiescent clouds could be a sign that either predictions of
emission line intensities are not correct or the de-excitation of the newly
formed molecules proceeds rapidly on the grain surface itself. In this letter
we present experimental evidence that interstellar molecular hydrogen is formed
and then rapidly de-excited on the surface of porous water ice mantles. In
addition, although we detect ro-vibrationally excited nascent molecules
desorbing from a bare non-porous (compact) water ice film, we demonstrate that
the amount of excited nascent hydrogen molecules is significantly reduced no
matter the morphology of the water ice substrate at 10 K (both on non-porous
and on porous water ice) in a regime of high molecular coverage as is the case
in dark molecular clouds.Comment: 15 pages, 3 figures, to be published in MNRA
Interaction of Hydrogen with Graphitic Surfaces, Clean and Doped with Metal Clusters
Producción CientíficaHydrogen is viewed as a possible alternative to the fossil fuels in transportation.
The technology of fuel-cell engines is fully developed, and the outstanding
remaining problem is the storage of hydrogen in the vehicle. Porous materials,
in which hydrogen is adsorbed on the pore walls, and in particular nanoporous
carbons, have been investigated as potential onboard containers. Furthermore,
metallic nanoparticles embedded in porous carbons catalyze the dissociation of
hydrogen in the anode of the fuel cells. For these reasons the interaction of
hydrogen with the surfaces of carbon materials is a topic of high technological
interest. Computational modeling and the density functional formalism (DFT)
are helping in the task of discovering the basic mechanisms of the interaction
of hydrogen with clean and doped carbon surfaces. Planar and curved graphene
provide good models for the walls of porous carbons. We first review work on
the interaction of molecular and atomic hydrogen with graphene and graphene nanoribbons, and next we address the effects due to the presence of metal clusters
on the surface because of the evidence of their role in enhancing hydrogen
storage.Ministerio de Economía, Industria y Competitividad (Grant MAT2014-54378-R
A far-ultraviolet-driven photoevaporation flow observed in a protoplanetary disk.
Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photodissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, which affects planet formation within the disks. We report James Webb Space Telescope and Atacama Large Millimeter Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modeling their kinematics and excitation allowed us to constrain the physical conditions within the gas. We quantified the mass-loss rate induced by the FUV irradiation and found that it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk
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