A photoelectron–photoion coincidence study of H2O, D2O, and (H2O)2

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Abstract
Photoelectron–photoion coincidence (PEPICO) data for OH+(OD+), H+(D+), and H2O+ (D2O+) from H2O (D2O) have been obtained in the region of 625–700 Å. The PEPICO measurements allow the construction of breakdown diagrams for the unimolecular dissociation of energy‐selected H2O+ and D2O+ in the B̃ 2B2 state. The breakdown diagrams for H2O+(B̃ 2B2) and D2O+(B̃ 2B2) in the internal energy range of 129–166 kcal/mol are essentially identical. The branching ratios observed for H+ (D+) are higher than those reported previously. About 3%–5% of stable H2O+ (D2O+) is observed in the time scale of ≊10 μs. These stable H2O+ (D2O+) ions are attributed to ultrafast B̃ 2B2→Ã2A1 nonradiative relaxation followed by the radiative stabilization from H2O+(Ã2A1) [D2O+(Ã2A1)] to H2O+ (X̃2B1) [D2O+(X̃2B1)]. This observation also supports that the formation of H+ (D+) via the H2O+ (Ã 2A1)[D2O+(Ã2A1)] state is a viable process. The relative state‐selected cross sections for the reaction H2O+ (X̃2B1,Ã2A1; ν1, ν2)+H2O→H3O+ +H at center‐of‐mass collision energies ≤1.9 eV have been examined using an effusive beam arrangement. Experimental evidence supports that vibrational and electronic excitations suppress the cross section of the latter reaction. We have also performed PEPICO measurements of the processes, (H2O)2+hν→(H2O)+2+e− and H3O+ +OH +e−, in the region of 1020–1110 Å. The sums of the PEPICO intensities for H3O+ and (H2O)+2 at corresponding photoionization wavelengths yield the photoelectron spectrum for (H2O)2.