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 μm 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 μm 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 C 3 H 2 , anthracene and phenanthrene neutral and positive ions has been carried out