An Ionic Look at Interstellar Grain Mantles

Abstract

Our knowledge of the interstellar medium and its components is still limited to the tip of an iceberg. However, with the coming of new and better observing facilities the amount of information has tremendously increased over the last decade and more and more people jump in. Observations (towards diffuse clouds) in the ultraviolet have revealed at least three populations of interstellar dust. Large grains (+/ - 0.15 µm) are responsible for the blocking of the visual star light. Small (0.01 µm) carbonaceous particles are required to explain the strong 216 nm hump. The exact nature of those small particle is still unknown and many explanations have been and are still being put forward. However, in the Leiden Laboratory strong indications have been found that they result from unsaturated carbon chain molecules like polyacetelyne. Finally, a third dust component consists of small (0.01 µm) silicate grains providing the far ultraviolet part of the extinction curve. How these grains evolve physically and chemically is a complicated story. In this contribution we want to focus on one aspect only: the chemical composition of the mantles that have accreted on large interstellar grains in molecular clouds.