THE EMISSION BAND AT 5.25 MU-M AND ITS RELATIONSHIP TO THE UNIDENTIFIED EMISSION FEATURES AT 11-13 AND 3.4-3.6 MU-M

Spectra at 5-5.6 mu m are presented of three planetary nebulae with st rong unidentified infrared (UIR) emission bands. All three objects dis play the 5.25-mu m emission band which is attributed to a combination band arising from out-of-plane C-H bending modes. The profiles of the 5.25-mu m bands are closely similar to those of the 3.4- and 11.25-mu m emission features, confirming that the 5.25-mu m feature is a member of the UIR-band family. The origin of this band and its relationship to the UIR bands in the 3.4-3.6 and 11-13 mu m spectral regions are di scussed. An interpretation of these features in terms of combinations and overtones of the C-H out-of-plane modes is suggested, which, if co rrect, leads to a simplification of the UIR-band spectrum and suggests that the species that give rise to the bands must have a high degree of symmetry.