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.