Surface formation of water in interstellar ice analogs

Water is an important oxygen reservoir in space and is often used as a tracer of large-scale gas-phase dynamics in star forming regions. Moreover, it is a crucial mol. for the existence of life on Earth. To date, the exact mechanisms leading to both water formation in space and its delivery to Earth are not fully understood. In 1982, Tielens and Hagen suggested low-temp. surface chem. on icy dust grains as the main water formation route in dense mol. clouds. Furthermore, deuterated water, specifically HDO, has recently been suggested to play an important role in the understanding of the origin of water on Earth (Morbidelli et al. In the past decade, several independent labs. performed systematic studies of specific reaction routes leading to water formation under interstellar relevant conditions: H/H_2 + O/O_2/O_3 (e.g., Miyauchi et al. 2008, Matar et al. 2008, Ioppolo et al. 2008, 2010, Cuppen et al. 2010, Romanzin et al. 2011, Lamberts et al. Here, I will present an overview of the specific reaction routes within the full astrochem. reaction network leading to the surface formation of water ice. Particular attention is paid to the work performed with SURFRESIDE in Leiden. The reactants, i.e., atoms and mols., are deposited on a gold substrate at temps. ranging between 13 K and 150 K. On the cold surface, reactants can diffuse and react. Sequential deposition and co- deposition expts. are used to investigate intermediate (HO_2 and OH) and final reaction products (H_2O_2 and H_2O). The specific case of H_2 + O, a surface reaction not fully investigated in the lab., is discussed in detail. Finally, I will show the importance of studying single reaction channels for the example of HDO formation, both at low (15 K) and high (145 K) temps