The spacing of the interstellar 6.2 and 7.7 micron emission features as anindicator of polycyclic aromatic hydrocarbon size

A database of astrophysically relevant, infrared spectral measurements on a wide variety of neutral as well as positively and negatively charged polyc yclic aromatic hydrocarbons (PAHs), ranging in size from C10H8 through C48H 20, is now available to extend the interstellar PAH model. Beyond simply in dicating general characteristics of the carriers, this collection of data n ow makes it possible to conduct a more thorough interpretation of the detai ls of the interstellar spectra and thereby derive deeper insights into the nature of the emitting material and conditions in the emission zones. This Letter is the first such implementation of this spectral database. The infr ared spectra of PAH cations, the main PAH form in the most energetic emissi on zones, are usually dominated by a few strong features in the 1650-1100 c m(-1) (6.1-9.1 mu m) region that tend to cluster in the vicinity of the int erstellar emission bands at 1610 and 1320 cm(-1) (6.2 and 7.6 mu m), but wi th spacings typically somewhat less than that observed in the canonical int erstellar spectrum. However, the spectra in the database show that this spa cing increases steadily with molecular size. Extrapolation of this trend in dicates that PAHs in the 50-80 carbon atom size range are entirely consiste nt with the observed interstellar spacing. Furthermore, the profile of the 1610 cm(-1) (6.2 mu m) interstellar band indicates that PAHs containing as few as 20 carbon atoms contribute to this feature.