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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry