| Description: | Interstellar dust is a catalyst for the formation of many chemical species. One of them, molecular hydrogen, is the most abundant molecule of the universe and is formed for a wide range of physical conditions. During the last decades, many theoretical and experimental studies concentrated on understanding the formation of H2. In this colloquium, I will report these studies and show how they compare with measurements derived from photo-dominated regions. The formation of water can also be efficiently produced onto dust, which increases the abundances of warm water into the gas phase. The process that allow formation of water on dust and its ejection in the gas can impact the chemistry of X-rays dominated regions such as the one present in Markarian 231. |
