Invited review article for "Astrophysics of Dust"
Symposium (Estes Park, Colorado, USA; May 26-30, 2003), ed. A.N. Witt,
G.C. Clayton, and B.T. Draine.
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Abstract. Interstellar grains span a wide range of sizes from a few angstroms to a few micrometers. The presence of nanometer-sized or smaller particles in the interstellar medium is indicated directly by the interstellar far ultraviolet (UV) extinction, the ubiquitous 3.3, 6.2, 7.7, 8.6, and 11.3 µm polycyclic aromatic hydrocarbon (PAH) emission features, the near and mid infrared broadband emission seen in the IRAS 12 and 25 µm bands and the COBE-DIRBE 3.5, 4.9, 12 and 25 µm bands, the 10-100 GHz Galactic foreground microwave emission, and indirectly by the heating of interstellar gas. For nanoparticles under interstellar conditions, UV/visible photon absorption is the dominant excitation process. With a heat capacity smaller than or comparable to the energy of an energetic stellar photon, nanoparticles are subject to single-photon heating, followed by vibrational relaxation, photoionization, and photodestruction. With excited electrons spatially confined, semiconductor nanoparticles are expected to luminesce efficiently. This review focuses on the photophysics of nanoparticles with emphasis on the stochastic heating and the vibrational excitation of interstellar PAH molecules, and the excitation of photoluminescence with special attention given to silicon nanoparticles.
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