Is the Diffuse Interstellar Absorption Band at 4430 Å caused by Trivalent Iron ?

Abstract

ALTHOUGH the broad diffuse interstellar absorption bands have been known for more than 30 years, they still defy identification. It has been suggested that the bands may be caused by electronic transitions in impurity ions in grains1. Here I show that the wavelength, asymmetry and half-width of the 4430 A band, in particular, are compatible with absorption bands marking the 6A1(S)→4A14E(G) transitions in Fe3+ in some terrestrial silicates. The absorption spectra of Fe3+ in octahedral sites in some garnets are used for comparison, because these spectra are now reasonably well understood. Because Fe3+ is a 3d5 ion, however, the d–d spectra are similar for both octahedral and tetrahedral complexes.