PAH Emission Features in the 15 to 20 Micron Region: Emitting to the Beat of a Different Drummer

Details PAH Emission Features in the 15 to 20 Micron Region: Emitting to the Beat of a Different Drummer PAH Emission Features in the 15 to 20 Micron Region: Emitting to the Beat of a Different Drummer

Jun 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005sptz.prop20309a&link_type=abstract

Spitzer Proposal ID #20309

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Spitzer has added a new complex of bands near 17 um to the PAH emission band family. This 17 um band complex, the second most intense of the PAH features, carries unique information about the emitting species. Because these bands arise from drumhead vibrations of the hexagonal carbon skeleton, they carry information directly related to PAH shape, size, and charge. Further, since much of the emission at these wavelengths originates from PAHs at the small end of the very small grain (vsg) population, this provides the first glimpse into details of the vsg population on a molecular level. Since the information contained in this band is fundamentally different from that of the mid-IR features and it probes the vsg population, the 17 um band complex is an important new tool with which to probe conditions within the Galaxy and in extragalactic star forming regions and simultaneously gives insight into the cosmic history of the important biogenic element, carbon. However, since little is known about PAH spectroscopic properties at wavelengths longer than about 15 um, it is impossible to utilize the 17 um band complex as a new astronomical investigative tool. We propose a laboratory study to remedy this situation. We will measure the 15 to 20 um spectra of PAHs which vary in size, shape, and ionization state. Spanning the range from C30H14 to C132H34, the PAH sample varies from moderately sized PAHs to the largest PAHs available anywhere. We will combine these experimental studies with quantum computations to test and perfect the computational approach for this new wavelength and fundamental vibration regime. The laboratory work will provide data with which one can analyze the new 17 um emission band complex and will ultimately be used to develop computational techniques to model the longer wavelength spectra of much larger PAHs. We will make these spectra available on the web and develop protocols for analyzing the Spitzer spectra and publish this procedure so it is accessible to the community.

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