Ionization-Driven Chemistry in CO₂–H₂O Clusters: Cooperative Peroxide Bond Formation Relevant to Interstellar Ices and Icy Worlds

Chemical evolution in cold, radiation-rich environments such as interstellar ices and the surfaces of icy worlds may be strongly shaped by chemistry initiated upon ionization. Weakly bound molecular clusters provide a tractable molecular template for exploring such processes under conditions that mimic icy grain environments and the nanoscale structure of volatile deposits on planetary surfaces. Using synchrotron-based VUV photoionization mass spectrometry, we investigate mixed CO₂–H₂O clusters in the 10–14 eV energy range and observe new oxygen-bearing products formed through molecular growth and bond reorganization. Most notably, we detect [H₂CO₄]⁺, providing direct evidence of cooperative peroxide bond formation within the cluster, consistent with radiation-driven oxygen chemistry inferred from observations of Europa, Ganymede, and Charon. We further demonstrate the indirect formation of H₂O₂ via cluster rearrangement and evaporation, yielding the species m/z 112, (H₂O₂)₂(CO₂). The product branching depends sensitively on the CO₂:H₂O ratio, indicating that CO₂ actively mediates electron redistribution and enhances reactivity rather than acting as a passive spectator. Additional products including formyl radicals, formic acid, and carbonic acid confirm that ionization deposits sufficient energy to bypass barriers that suppress such reactions for isolated molecules under low-temperature astrophysical conditions.

To elucidate these pathways, we employ density functional theory at the ωB97X-D level with higher-level single-point refinements, revealing charge delocalization and barrierless rearrangements unique to the mixed cluster environment. Together, the experimental and theoretical results show that even nanoscale CO₂–H₂O aggregates can function as chemically active microreactors upon ionization. This work identifies a new mechanism capable of contributing to chemical complexity during the evolution of interstellar ices, within dense molecular clouds, and on the irradiated surfaces of icy worlds across the Solar System.