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F. Salama (NASA-Ames Research Center)
Polycyclic Aromatic Hydrocarbons are considered to be an important and ubiquitous component of carbon-bearing materials in space. PAHs are the best known candidates to account for the IR emission bands (UIR bands). PAHs are also thought to be among the carriers of the diffuse interstellar bands (DIBs). In the model dealing with the interstellar spectral features, PAHs are present as a mixture of radicals, ions and neutral species. An extensive laboratory program has been developed to assess the physical and chemical properties of PAHs and to understand how they influence the radiation and energy balance in space and the interstellar chemistry. In particular, laboratory experiments have provided measurements of the spectral properties of interstellar PAH analogs in the ultraviolet and visible range by measuring the spectra of neutral and ionized PAHs isolated at low temperature (of the order of 5 K) in inert gas matrices. Substantial progress has been made using these laboratory data measured in solid, chemically inert, matrices. We report the information gained from a comparative study of the electronic spectra of a large sample of neutral and ionized PAHs with an extensive set of high-resolution optical astronomical data. No definitive identification can be made, however, until the diffuse interstellar environments can be fully simulated in the laboratory. A list of promising PAH candidates as DIB carriers is derived and discussed in the light of very recent advances in gas phase jet expansion experiments where the PAH molecular ions can be measured isolated and cold in the gas-phase thus providing the ultimate test required for comparison with spectra of the diffuse interstellar medium.
This work is supported through the Astrophysics Program (UVGA branch) of NASA's Office of Space Science.
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