IR Selective Irradiations of Amorphous Solid Water Dangling Modes: Irradiation vs Annealing Effects

Amorphous solid water (ASW) is one of the most widely studied molecular systems. Ubiquitous in the interstellar medium (ISM) and potentially present in the upper layers of the Earth's atmosphere, ASW plays a major role in heterogeneous physical chemistry. Small molecules form or accrete at the ice surface, bonding to water molecules with an OH bond projecting from the surface, so-called dangling bonds. These dangling OH are of crucial importance in the quest to identify and quantify surface reactions. Water ices in the ISM or Earth's atmosphere undergo constant processing by thermal and irradiation effects, which can significantly affect both the bulk and surface structures and therefore the catalytic properties of the surface. In this work we have studied thermal and irradiation processing of ASW and determine that there is a photochemical processing pathway of the ice surface which is clearly distinct from purely thermal effects. Selective IR irradiations of each of the surface water modes led to the observation of a hole-burning at the irradiation frequency, counterbalanced by the production of a new band, identified as a water monomer interacting with the surface. The thermal effects, meanwhile, led to a global decrease of all the dangling modes due to global reorganization of the water ice structure. It is thus obvious that, depending on the processing history of an ice, its catalytic properties will not be affected in the same way. The fact that we observe an IR selective irradiation effect illustrates that some fraction of the vibrational energy, rather than being relaxed through the H-bonded network of the bulk ice, is trapped at the surface; this energy induces a reorganization of the surface structure, forming new trapping sites, and thus generating new catalytic properties.