Ionization-Driven Dehydrogenation and Molecular Growth in Gas-Phase Adamantane Clusters

Abstract: We investigate fragmentation and molecular growth induced by keV ions in initially neutral gas-phase adamantane clusters, the simplest diamondoid molecule (Ad, C$<em>{10}$H$</em>{16}$). Using time-of-flight mass spectrometry in combination with quantum-chemical calculations, we find that intact cluster cations undergo extensive dehydrogenation. We also observe a distinct distribution of singly charged products with masses predominantly between those of the adamantane monomer and dimer cations. We attribute these species to ion-induced growth products and interpret their formation considering the smallest cluster cation, [Ad]$_2<sup>+$, using quantum-chemical calculations. Our results demonstrate that ionization strongly reorganizes the dimer geometry, facilitating H$_2$ elimination and intermolecular covalent C--C bond formation. Weakly bound adamantane clusters are thereby transformed into covalently bound carbonaceous species with a strong preference for specific molecular structures. These findings provide new insight into ionization-driven molecular growth in saturated hydrocarbon systems contributing to the understanding of molecular complexification, with implications for chemical physics and astrochemistry.