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

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 spectrum is now commonly used as a probe into (inter)stellar envi- ronments. The NASA Ames 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 far-IR spectra and the quest for identifying a unique member of the interstel- lar family has begun. To guide this research, the far-IR (>20 µm) spectra of different sets of PAHs are investigated using the NASA Ames IR Spectroscopic Database. These sets explore the influence of size, shape, charge and composition on the far-IR spectrum. The far-IR is also the domain of the so-called modes and other molecular vibrations involving low order bending vibrations of the car- bon 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 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) molecules. From this study, some consequences for an observing strategy are drawn.