From Large PAHs to Very Small Grains: The Mid-IR Spectrum of the Interstellar Medium
The mid-infrared spectrum of the interstellar medium is characterized by strong IR emission features at 3.3, 6.2, 7.7, 11.2, and 12.7 cm superimposed on broad underlying plateaus. These different emission components show large variations in relative strength from source to source and within sources. These spectral differences are generally thought to represent the effects of size, molecular structure, clustering, and charge on the intrinsic spectral characteristics of carbonaceous species.
However, theoretical or experimental verification of this has previously been lacking. In order to link the mid-infrared observations of the interstellar medium back to the characteristics of carriers present in space, a deep understanding is required of the effects of these factors on the intrinsic infrared characteristics of PAH molecules and clusters of PAH molecules. This is a comprehensive study of the intrinsic spectral characteristics of carbonaceous species ranging from molecular sizes to the nano-domain (50-1000 C-atoms) with the goal of understanding the emission characteristics of the interstellar medium. Specifically, a new quantum chemical method allows us to calculate reliably the intrinsic emission characteristics of species containing an order of magnitude more carbon atoms than previously possible or than can be easily studied systematically in the laboratory. These emission properties can then be used to calculate the emission spectrum of astrophysical objects. This allows us to develop an "astronomical tool" that relates the diagnostic differences of observed mid-IR spectra to the characteristics of the emitting species (size, 2D/3D structure, charge) and the physical conditions in the emitting region. In addition, this study provides an important step towards building a unified carbonaceous dust model from the small size up.