Laboratory Astrophysics - Spectroscopy of Interstellar PAHs and Carbon Molecules and Ions. A New Generation of Laboratory & Astronomical Studies

PAHs are an important and ubiquitous component of carbon-bearing materials in space. We present gas-phase spectra of neutral and ionized PAHs measured in the UV-NIR range in astrophysically relevant environments. The harsh physical conditions of the ISM -low temperature, collisionless, 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 are measured using cavity ring down spectroscopy (CRDS) techniques. The bands measured for ionized PAH are found to be intrinsically broad (> 20 cm^-1) while the bands associated with the neutral precursors are narrower (2-10 cm^-1)^. The laboratory data are discussed and compared with recent astronomical spectra of DIBs and with the spectra of CS environments of selected carbon stars and the implications for the IS PAH population are derived. Preliminary results also indicate 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. These experiments provide unique information on the spectra of free, large carbon-containing molecules and ions in the gas phase. We are now in a position to directly compare laboratory spectra of free, cold, PAH ions and nanosized carbon particles with astronomical observations in the UV-NIR range (IS UV extinction, DIBs). This new phase offers tremendous opportunities for the data analysis of space missions geared toward the detection of large aromatic molecules (COS/HST, Spitzer, JWST, SOFIA).

Acknowledgements: This work is supported by NASA SMD (Planetary Science and APRA Research Programs). Parts of this research were performed while X.T., L.B. and J.C. held NRC/NPP fellowships at NASA-Ames Research Center.