Infrared Emission from Interstellar PAHs

Abstract

The mid-infrared interstellar emission spectrum with features at 3050, 1610, 1300, 1150, and 885cm-1 (3.28, 6.2, 7.7 8.7 and 11.3 microns), spectroscopic details and continuum are discussed in terms of the Polycyclic Aromatic Hydrocarbon (PAH) hypothesis. This hypothesis is based on the similarity between the interstellar emission spectrum with the infrared absorption and Raman Spectra of PAHs and soots (collections of PAHs). The fundamental vibrations of PAHs and PAH-like species which determine the IR and Raman properties are discussed. Interstellar IR band emission is due to relaxation from highly vibrationally excited PAHs which have been excited by ultraviolet photons. The excitation/emission process is described in general and the IR fluorescence from one PAH, chrysene, is traced in detail. Generally, there is sufficient energy to populate several vibrational levels in each mode of the smallest PAHs. Molecular vibrational potentials are anharmonic and emission from these higher levels will fall at lower frequencies, producing weak features to the red of the stronger fundamentals. This accounts for some details of the interstellar emission spectra previously unexplained. Highly vibrationally excited molecules also produce a weak “quasi-continuum” that spans the entire mid-IR (4000–500cm-1, 2.5–20 microns). Analysis of the observed 3050cm-1to 2940 cm-1 intensity ratios (3.3 micron/3.4 micron) show that PAHs containing between 20 and 30 carbon atoms are responsible for the interstellar emission bands. The 885cm-1to 3050 cm-1 intensity ratio (11.3 micron/3.3 micron), which has previously been used to determine the size of the emitting PAHs, is a less appropriate measure since the 885cm-1(11.3 micron) feature is probably due to emission by fairly large PAHs (> 100 C atoms) which do not contribute much to the 3050 cm-1 (3.3 micron) band intensity.