Infrared spectroscopy of gas-phase PAH molecules. 1: Role of the physical environment

Details Infrared spectroscopy of gas-phase PAH molecules. 1: Role of the physical environment Infrared spectroscopy of gas-phase PAH molecules. 1: Role of the physical environment

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...281..923j&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 281, no. 3, p. 923-936

Astronomy and Astrophysics

Astronomy

73

Astronomical Models, Emission Spectra, Infrared Astronomy, Infrared Spectra, Interstellar Gas, Polycyclic Aromatic Hydrocarbons, Absorption Cross Sections, Analogs, Astronomical Spectroscopy, Molecular Gases, Planetary Nebulae, Reflection Nebulae

Scientific paper

In the PAH model, polycyclic aromatic hydrocarbons are responsible for the Unidentified IR (UIR) bands observed in emission in many astronomical objects. Although this model states that these molecules are isolated, and at high temperatures (T approximately 1000 K), most laboratory spectra have been obtained for condensed molecules at low temperatures. In this paper, we present the IR absorption spectra (3-20 micrometers), measured for gas phase molecules at high temperatures. For the three studied species, pyrene (C16H10), coronene (C24H12), and ovalene (C32H14), these spectra are compared to those obtained in condensed phases, solid state and rare gas matrices. The relative band intensities clearly depend on the molecular environment. We report absorption cross-sections for gaseous molecules which are better laboratory analogues for interstellar neutral PAHs. The interpretation of the UIR bands observed in strongly irradiated objects with the spectra of neutral gaseous PAHs, leads to a good spectral agreement. A mean number of about 100 atoms per molecule is derived for the PAH distributions in these objects. The determined hydrogen coverage is curiously low (a few %) in general although it can be higher (approximately 40%) in objects that show a strong plateau in the (11-13 micrometers) range. However, PAHs are predicted to be ionized in these regions by current models and our laboratory data may be more relevant for future observations of the diffuse interstellar medium where PAHs are expected to be neutral.

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