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F. Salama (NASA Ames Research Center), X. Tan (NRC & NASA Ames Research Center), L. Biennier (PALMS, Univ. Rennes, France), J. Cami (NRC & NASA Ames Research Center)
We present the gas-phase spectroscopy of neutral and ionized polycyclic aromatic hydrocarbons (PAHs) measured in the UV-Visible-NIR range in an astrophysically relevant environment. These measurements provide data on PAHs and nanometer-sized particles that can now be directly compared to astronomical observations. The harsh physical conditions of the IS medium - characterized by a low temperature, an absence of collisions and strong UV radiation fields - are simulated in the laboratory by associating a molecular beam with an ionizing discharge to generate a cold plasma expansion. PAH ions are formed from the neutral precursors in an isolated environment at low temperature (~100 K). The spectra of neutral and ionized PAHs are measured using the high sensitivity methods of cavity ring down spectroscopy (CRDS) and multiplex integrated cavity output spectroscopy (MICOS). These experiments provide unique information on the spectra of free, cold large carbon molecules and ions in the gas phase. The electronic bands measured for ionized PAH are found to be intrinsically broad (> 20 cm-1) while the bands associated with the neutral precursors are narrower (of the order of 2 - 10 cm-1). The laboratory data are discussed and compared with recent astronomical spectra of large and narrow DIBs and with the spectra of circumstellar environments of selected carbon stars (see contribution of Cami et al.) and the implications for the interstellar PAH population are derived. Preliminary results also show that carbon nanoparticles are formed during the short residence time of the precursors in the plasma. This finding holds great potential for understanding the formation process of interstellar grains.
Acknowledgements: This work is supported by the NASA Astronomy and Physics Research and Analysis (APRA) Program of the Science Mission Directorate. This research was performed while X.T., L.B. and J.C. held a National Research Council Research Associateship Award at NASA-Ames Research Center.
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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.