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 MOH(+)(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 less than or equal to n less than or equal to 5 and n greater than or equal to 15, while MgOH+(H2O)(n-1) are exclusively observed for 6 less than or equal to n less than or equal to 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.