Non-radiative decay and fragmentation in water molecules after 1a₁⁻¹4a₁ excitation and core ionization studied by electron-energy-resolved electron–ion coincidence spectroscopy

Abstract In this paper, we examine decay and fragmentation of core-excited and core-ionized water molecules combining quantum chemical calculations and electron-energy-resolved electron–ion coincidence spectroscopy. The experimental technique allows us to connect electronic decay from core-excited states, electronic transitions between ionic states, and dissociation of the molecular ion. To this end, we calculate the minimum energy dissociation path of the core-excited molecule and the potential energy surfaces of the molecular ion. Our measurements highlight the role of ultra-fast nuclear motion in the 1a₁⁻¹4a₁ core-excited molecule in the production of fragment ions. OH⁺ fragments dominate for spectator Auger decay. Complete atomization after sequential fragmentation is also evident through detection of slow H⁺ fragments. Additional measurements of the non-resonant Auger decay of the core-ionized molecule (1a₁⁻¹) to the lower-energy dication states show that the formation of the OH⁺ + H⁺ ion pair dominates, whereas sequential fragmentation OH⁺ + H⁺ → O + H⁺ + H⁺ is observed for transitions to higher dication states, supporting previous theoretical investigations