Reactions of Singly Charged Alkaline-Earth Metal Ions with Water Clusters: Characteristic Size Distribution of Product Ions

The reactions of Mg⁺ and Ca⁺ ions with water clusters are examined using a reflectron time-of-flight mass spectrometer combined with a laser vaporization technique. Both the M⁺(H₂O)_n and M0H⁺(H₂O)_n-1 (M = Mg and Ca) ions are found to form as the reaction products with characteristic size distributions: the latter ions are produced via an H-atom elimination reaction (oxidation of M⁺). As for the Mg⁺ ion, the Mg⁺(H₂O)_n ions are predominantly produced for 1 ≤n ≤ 5 and n ≥ 15, while MgOH⁺(H20)_n-1 are exclusively observed for 6 ≤ n ≤ 14 in the mass spectrum. Similar product distributions are also observed for Mg⁺—D₂O, Ca⁺—H₂O, and Ca⁺—D₂O systems, though they are found to be affected by deuterium and metal substitutions. On the basis of these results as well as those on the photoinduced reaction of Mg⁺(H₂O)_n reported previously, the first product switching at n = 5 for Mg⁺(n = 4 for Ca⁺) is ascribed to the difference in the successive hydration energies of the M⁺ and MOH⁺ ions. As for the second product switching, two possible mechanisms are proposed such as the stabilization of a Rydberg-type ion-pair state and the involvement of a new product.