Photodissociation of size-selected Mg⁺(H₂O)_n ions for n = 1 and 2

Electronically excited states of magnesium-water cluster ions, Mg⁺(H₂O)_n, are studied by photodissociation after mass selection. Dissociation spectra are obtained as a function of wavelength for n = 1 (250-370 nm) and 2 (280-470 nm). The spectra show absorption peaks at 28 300, 30 500, and 38 500 cm^-1 for n = 1, and 25 000, 29 400, and 32 000 cm^-1 for n = 2. These absorption bands are assigned to the ²P-²S type transitions localized on the Mg⁺ ion with the aid of ab initio CI calculations. In addition to evaporation of water molecules, photoinduced chemical reaction to produce MgOH⁺ is found to occur efficiently. Especially, for Mg⁺(H₂O)₂, the branching ratio between the former and latter processes has been found to depend sensitively on the excitation energy. On the basis of these results, the mechanism of the photodissociation of these cluster ions is discussed.