1998A&A...339..194B

Modelling the unidentified infrared dust bands at 6.2 7.7, 8.6, 11.3, and 12.7µm by Lorentz profiles is shown to produce excellent fits to the spectra obtained with the Continuous Variable Filter (CVF) of the instrument ISOCAM, on-board the Infrared Space Observatory (ISO). The feature pedestals are accounted for by the wide wings of the Lorentz curves which also account for a large fraction of the 10µm "continuum". The Lorentz shape of the features is believed to reveal intrinsic properties of the emitters, i.e. a very short life time of the excited state, of the order of a few 10^–13s. This is attributed to internal vibrational redistribution (IVR) between thermally accessible modes of the particles, which can be raised at high temperature by absorption of a single photon. Particles of a few hundred atoms may meet both requirements.