The mechanisms of photoexcitation and photoionization in small water clusters in gas phase, (H2O) n ; n = 2-3, are studied using the complete active-space second-order perturbation theory (CASPT2) with the aug-cc-pVDZ basis set. The present study characterizes for the first time the structures and energetics of common transition and intermediate complexes in the photoexcitation and photoionization mechanisms in the lowest singlet-excited state. The ab initio results showed that the photoexcitation of the water monomer by a single photon can directly generate [OH]˙ and [H]˙ in their respective electronic-ground states, and a single photon with approximately the same energy can similarly lead to the photoexcitation and also to the photoionization in the water clusters. The S0 → S1 excitation leads to strong polarization of the O-H⋯O H-bond and to the formation of the water dimer radical cation transition state complex [(H2O)2]+˙, from which [OH]˙, [H]˙, and [H3O]+˙ can be generated. These products are obtained from [(H2O)2]+˙ by the direct dissociation of the O-H bond upon photoexcitation and by proton transfer and the formation of a metastable charge-separated Rydberg-like H-bond complex ([H3O]+˙⋯[OH]˙) upon photoionization. The proposed mechanisms suggest that in the gas phase, the photoexcitation and photoionization processes are most likely bimolecular reactions, in which all the transition and intermediate charged species are more stabilized than in a unimolecular reaction. The theoretical results provide insights into the photoexcitation and photoionization mechanisms of molecular clusters and can be used as guidelines for further theoretical and experimental studies.
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