3,944 research outputs found
H2 Formation on Interstellar Grains in Different Physical Regimes
An analysis of the kinetics of H2 formation on interstellar dust grains is
presented using rate equations. It is shown that semi-empirical expressions
that appeared in the literature represent two different physical regimes. In
particular, it is shown that the expression given by Hollenbach, Werner and
Salpeter [ApJ, 163, 165 (1971)] applies when high flux, or high mobility, of H
atoms on the surface of a grain, makes it very unlikely that H atoms evaporate
before they meet each other and recombine. The expression of Pirronello et al.\
[ApJ, 483, L131 (1997)] -- deduced on the basis of accurate measurements on
realistic dust analogue -- applies to the opposite regime (low coverage and low
mobility). The implications of this analysis for the understanding of the
processes dominating in the Interstellar Medium are discussed.Comment: 4 pages, MN styl
Multiple Quantum NMR Dynamics in Pseudopure States
We investigate numerically the Multiple Quantum (MQ) NMR dynamics in systems
of nuclear spins 1/2 coupled by the dipole-dipole interactions in the case of
the pseudopure initial state. Simulations of the MQ NMR with the real molecular
structures such as six dipolar-coupled proton spins of a benzene, hydroxyl
proton chains in calcium hydroxyapatite and fluorine chains in calcium
fluorapatite open the way to experimental NMR testing of the obtained results.
It was found that multiple-spin correlations are created faster in such
experiments than in the usual MQ NMR experiments and can be used for the
investigation of many-spin dynamics of nuclear spins in solidsComment: 8 pages, 2 figure
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