Excitation processes for the emission of the unidentified IR bands

Abstract

To our knowledge, no individual polycyclic aromatic hydrocarbon (PAH) molecule has yet been shown to carry all the ubiquitous and closely correlated unidentified IR (UIR) bands. More generally, no IR space observatory (ISO) instrument was able to split these bands into the narrower signatures expected from molecules. Again, the Japanese IRTS satellite has shown that the near-IR CH-stretching band observed near the galactic plane carries an important aliphatic component alien to aromatic molecules. On the other hand, a number of solid-state, disordered, more or less hydrogenated, carbonaceous materials have been shown to carry all the UIR bands. Moreover, their emission spectrum, computed under thermal equilibrium with typical circumstellar radiation fields, was shown to fit, satisfactorily and in detail, the observed near- and mid-IR spectra of a number of planetary nebulae (PNe) and proto-planetary nebulae (PPNe). However, the interstellar radiation field (ISRF) is not strong enough to heat this solid-state dust to the equilibrium temperatures required for emission observed from the galactic plane and reflection nebulae. Even stochastic heating is insufficient in these cases because nanometric grains are too small to retain the desirable optical properties of the bulk.

One way out of this deadlock is to look for other excitation mechanisms. Noting that spatial maxima of UIR emission usually occur in photodissociation regions between ionized and molecular gas maxima, where atomic H is most abundant, we have set up to study the interactions of atomic hydrogen with solid-state carbon surfaces. We consider, in particular, a mechanism in which, upon collision with a grain, the potential energy of recombination carried by an atom is delivered to one of the functional groups which will emit a UIR band. Such an IR chemiluminescence is found to be possible, in principle. A laboratory experiment dedicated to the quest of IR chemiluminescence has been built and is described.