The NASA Ames PAH IR Spectroscopic Database and the far-IR

¹ NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, USA
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² NASA Ames Research Center, MS 230-3, Moffett Field, CA 94035, USA
³ Department of Physics and Astronomy, PAB 213, The University of Western Ontario, London, ON N6A 3K7, Canada
⁴ SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA
⁵ NASA Headquarters, MS 3Y28, 300 E St. SW, Washington, DC 20546, USA

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) are widespread across the Universe and influence many stages of the Galactic lifecycle. The presence of PAHs has been well established and the rich mid-IR PAH spectrum is now commonly used as a probe into (inter)stellar environments. The NASA Ames PAH IR Spectroscopic Database has been key to test and refine the “PAH hypothesis”. This database is a large coherent set (>600 spectra) of laboratory measured and DFT computed infrared spectra of PAHs from C₁₀H₈ to C₁₃₀H₂₈ and has been made available on the web at (http://www.astrochem.org/pahdb). With a new spectral window opening up; the far-IR, the study of PAH far-IR spectra and the quest for identifying a unique member of the interstellar PAH family has begun. To guide this research, the far-IR (>20 μm) spectra of different sets of PAHs are investigated using the NASA Ames PAH IR Spectroscopic Database. These sets explore the influence of size, shape, charge and composition on the far-IR PAH spectrum. The far-IR is also the domain of the so-called “drumhead” modes and other molecular vibrations involving low order bending vibrations of the carbon skeleton as a whole. As with drums, these are molecule and shape specific and promise to be a key diagnostic for specific PAHs. Here, the sensitivity of these “drumhead” modes to size and shape is assessed by comparing the frequencies of the lowest drumhead modes of a family of circular shaped (the coronene “family”) and rhombus shaped (the pyrene “family”) PAH molecules. From this study, some consequences for an observing strategy are drawn.