12 research outputs found
Elemental nitrogen partitioning in dense interstellar clouds
Many chemical models of dense interstellar clouds predict that the majority
of gas-phase elemental nitrogen should be present as N2, with an abundance
approximately five orders of magnitude less than that of hydrogen. As a
homonuclear diatomic molecule, N2 is difficult to detect spectroscopically
through infrared or millimetre-wavelength transitions so its abundance is often
inferred indirectly through its reaction product N2H+. Two main formation
mechanisms each involving two radical-radical reactions are the source of N2 in
such environments. Here we report measurements of the low temperature rate
constants for one of these processes, the N + CN reaction down to 56 K. The
effect of the measured rate constants for this reaction and those recently
determined for two other reactions implicated in N2 formation are tested using
a gas-grain model employing a critically evaluated chemical network. We show
that the amount of interstellar nitrogen present as N2 depends on the
competition between its gas-phase formation and the depletion of atomic
nitrogen onto grains. As the reactions controlling N2 formation are
inefficient, we argue that N2 does not represent the main reservoir species for
interstellar nitrogen. Instead, elevated abundances of more labile forms of
nitrogen such as NH3 should be present on interstellar ices, promoting the
eventual formation of nitrogen-bearing organic molecules.Comment: Accepted for publication in the Proceedings of the National Academy
of Sciences of the United States of America - published online since June 11,
201
Interstellar chemistry of atomic nitrogen: Low temperature kinetics of the N + OH, N + CN and N + NO reactions
More than 100 reactions between stable molecules and free radicals have been shown to
remain rapid at low temperatures. In contrast, reactions between two unstable radicals
have received much less attention due to the added complexity of producing and measuring
excess radical concentrations. We performed kinetic experiments on the barrierless N + OH
and N + CN reactions in a supersonic flow (Laval nozzle) reactor. The results provide
insight into the gas-phase formation mechanisms of molecular nitrogen in interstellar
clouds (ISCs)
Vertically Stacked-NanoWires MOSFETs in a Replacement Metal Gate Process with Inner Spacer and SiGe Source/Drain
International audienceWe report on vertically stacked horizontal Si NanoWires (NW) p-MOSFETs fabricated with a replacement metal gate (RMG) process. For the first time, stacked-NWs transistors are integrated with inner spacers and SiGe source-drain (S/D) stressors. Recessed and epitaxially re-grown SiGe(B) S/D junctions are shown to be efficient to inject strain into Si p-channels. The Precession Electron Diffraction (PED) technique, with a nm-scale precision, is used to quantify the deformation and provide useful information about strain fields at different stages of the fabrication process. Finally, a significant compressive strain and excellent short-channel characteristics are demonstrated in stacked-NWs p-FETs