UNIDENTIFIED INFRARED-EMISSION BANDS - MODELS FOR THE CARRIERS OF THE SATELLITES OF THE 3.3-MU-M-BAND

Different suggestions have been made in the literature for the satellite bands observed in high resolution spectra of the 3.3 mum band assigned to the CH stretching vibration of PAHs. One of the most appealing is an anharmonic progression of this vibration, though considerations on the size of the possible carriers implied, made the hypothesis highly questionable for a number of authors. In this paper it is shown that this contradiction can be relieved by considering smaller molecules presenting the same band at 3.3 mum and most probably present in the same places as the hypothesized large PAHs. An ab-initio quantum mechanical treatment of the anharmonic progression of the symmetrical and asymmetrical CH stretchings of cyclopropylidene C3 H-2, anthracene and phenanthrene neutral and positive ions has been carried out. This study points to a direct link between the anharmonic progressions and the presence of isolated ''duo'' hydrogens on the possible carriers. It leads to the conclusion that part of the satellite features observed are most probably due to the anharmonicity of the 3.3 mum band which superimpose to features arising from aliphatic fundamental vibrations, providing one more piece of evidence for PAHs being at the origin of the IR emission.