  Published interpretations of the relative intensity variations of the
Unidentified Infrared Bands (UIBs) and their underlying continuum are
discussed. An alternative model is proposed, in which a single carrier for both
emits a) mostly a continuum when it is electronically excited by photons
(visible or UV), or b) exclusively the UIBs, when only chemical energy is
deposited by H capture on its surface, inducing only nuclear vibrations. The
bands will dominate in atomic H regions but will be overcome by thermal
continuum radiation when the ambient field is strong but lacks dissociating
photons (900-1100 Angstroms). The model applies to PDRs as well as to limbs of
molecular clouds in the ISM and agrees quantitatively with recent satellite
observations. It gives indications on atomic H density and UIB intensity
provided the ambient radiation field is known. It invokes no chemical,
electronic, structural or size change in order to interpret the observed
intensity variations.
