408 research outputs found
Experiments on Quantum and Thermal Desorption from ^4He Films
Desorption of He atoms from thin films may be resolved experimentally into quantum and thermal components. We show that quantum desorption becomes the dominant part of the signal in submonolayer films. We also show that, when all effects of collisions between desorbed atoms are eliminated, quantum desorption is not focused normal to the surface of optically polished sapphire crystals
Probing the metal-nonmetal transition in thin metal overlayers using resonant photoemission
We have studied one and two monolayers of barium on Ni(111) and of mercury on Cu(100). Using resonant photoemission, we have found core excited electrons become delocalized with increasing barium coverage. Similarly, upon formation of the mercury bilayer (as determined by low-energy electron diffraction and by atom-beam scattering), there is a substantial increase in the screening of the photohole. A transition of the electronic structure akin to a metal-nonmetal (metal-insulator) transition is apparent in these final-state effects. The band structure for Hg is similar to the band structure expected for a free-standing film with a free-electron sd band. The delocalization of the core excited electrons resembles the exciton unbinding that occurs at the metal-nonmetal Mott transition
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
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
Role of the interferon-inducible gene IFI16 in the etiopathogenesis of systemic autoimmune disorders
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