Water Cages as Chemical Reactors: VUV Photolysis of Dimethyl Ether Clathrate Hydrate Thin Films in Ultrahigh Vacuum

Clathrate hydrates (CHs) are crystalline molecular solids with hydrogen-bonded water cages where small molecules are trapped and are important in both terrestrial and extraterrestrial environments. While the structural stability and phase behavior of CHs and related ices are well characterized, key questions about their role as reactive environments under energetic radiation remain unanswered. Here, we show that dimethyl ether (DME) CH thin films in ultrahigh vacuum (∼10^-10 mbar) can be switched from passive ice cages into active chemical reactors under vacuum-ultraviolet (VUV) photolysis at cryogenic temperatures (10-130 K). Temperature-dependent irradiation reveals that at 10 K, VUV photolysis leads to rapid dissociation of the DME CH, dissociation of the cage framework, and formation of amorphous ice with no efficient trapping of photoproducts. In contrast, at 130 K the hydrate cages partially reform during or after photolysis, and photoproduced formaldehyde (H₂CO) becomes confined within the reformed cages, enabled by increased intermolecular mobility. A comparison of DME dissociation and CH₄ photoproduction in pure DME ice, DME-water amorphous ice mixtures, and DME CH at 10 K shows that CH₄ formation and DME dissociation rate are highest for the clathrate phase, indicating the cage effect where the interaction of DME and water is minimal even in the condensed phase. These findings demonstrate that CH cages can act as VUV-driven chemical nanoreactors in the condensed phase, providing a mechanistic link between ice cage structure, photolytic dissociation, and the formation of reactive products under space-relevant conditions.