The widths of the unidentified infrared bands (UIBs) Molecular or solid-state carriers?

The molecular and solid-state models of the UIB widths are confronted with observations from ground-based telescopes and the ISO satellite. The PAH (Polycyclic Aromatic Hydrocarbons) model requires that free-flying molecules be excited by vacuum UV radiation. However, recent laboratory measurements of vibrational emission reveal that, in this case, band widths are several times larger than observed in the sky. This was also to be expected from older studies of Internal Vibrational Relaxation (IVR), which is the main cause of broadening in highly excited molecules. On the other hand, the widths of the vibration bands of coal correctly fit the spectrum of a number of Planetary or PrePlanetary Nebulae (PNe and PPNe) and Novae: those associated with a notable underlying continuum and a strong aliphatic feature at 3.4 μm. However, as compared with UIBs from the diffuse ISM, they are also too large (by a factor of ∼2 on average). But, here, the broadening is due to the intrinsic disorder and inhomogeneity of amorphous hydrogenated carbons. On this model, a partial cure may therefore be sought in reducing the disorder associated with the linear and aliphatic components (sp1 and sp3). However, it is proposed that, for the diffuse ISM, a more radical solution to the discrepancies lies in abandoning radiative excitation altogether, to the benefit of direct and selective excitation of the functional groups sitting at the surface of grains. This could be effected by interaction of impinging ambient H atoms with surface H atoms and/or carbon dangling bonds, which is already invoked to explain recombination of H atoms into H2 molecules. This mechanism could also explain the other characteristics of UIB emission from the diffuse ISM: insensitivity of spectral shape to intensity and hardness of the IS radiation field, weak underlying continuum, spatial correlation with H2 ir emission at PhotoDissociation Regions and emission from high galactic latitudes.