The infrared spectrum of interstellar dust: Surface functional groups on carbon

Abstract

A variety of chemical functional groups will be bound at reactive surface sites on small carbon grains in the diffuse interstellar medium. These groups give rise to infrared spectral features characteristic of aromatic molecules. In diffuse clouds the dominant group appears to be −CH3. When carbon grains are heated in emission objects aromatic CH groups are produced. It is shown that both the absorption and emission spectra of dust can be understood in terms of the existence of these and other simple functional groups combined with carbon at grain surfaces. Specific identifications are proposed for several absorption and emission features including the 3.4 µm absorption band and the 3.3 µm emission feature.