CHEM291D Seminar: SJSU, October 8, 1998

Duncan Hall 250: 430 to 520 PM

NASA Ames Research Center, Moffett Field, CA

The diffuse interstellar bands (DIBs) are the discrete absorption features in the visible and near-infrared region observed in the spectra of stars that are obscured by interstellar clouds. The origin of these bands remains the "oldest unsolved problem in astronomical spectroscopy" [1]. Though the DIBs were first detected in 1922 and their interstellar origin was recognized in 1934, the identity of the carrier(s) of the DIBs, which now number over 100, is still unknown [2, 3]. Polycyclic aromatic hydrocarbons (PAHs) are promising candidates for the DIBs. Laboratory studies have shown that PAH cations absorb in the visible and the near-infrared region where the DIBs are observed. Although anionic PAHs may also exist in the interstellar medium [2], the spectroscopy of negatively-charged PAHs in an astrophysically relevant environment has been largely unexplored. We report here the first studies of the electronic

absorption spectra of PAH anions isolated in noble gas matrices containing alkali metal atoms. Emission features from the interstellar medium have also been recently attributed to PAHs containing excess H atoms (hydrogenated PAHs) [4]. The UV/visible spectroscopy of several hydrogenated PAHs and their photoproducts isolated in neon matrices will also be reported.

(1) Snow, T. P. Nature 1993, 366, 407-408.

(2) Salama, F.; Bakes, E. L. O.; Allamandola, L. J. The Astrophysical Journal 1996, 458, 621-636.

(3) Salama, F. Origins of Life and Evolution of the Biosphere 1998, 28, 349-364.

(4) Bernstein, M. P.; Sandford, S. A.; Allamandola, L. J. The Astrophysical Journal 1996, 472, L127-L130.