Binding energies of first and second shell water molecules in the Fe(H2O)(2)(+), Co(H2O)(2)(+) and Au(H2O)(2)(+) cluster ions

The fragmentation cross-section of the Fe(H2O)(1,2)(+), Co(H2O)(1,2)(+) and Au(H2O)(1,2)(+) ions were measured, as a function of the collision energy. Threshold energies of 1.4 +/- 0.2 eV, 1.4 +/- 0.2 eV and 1.7 +/- 0.1 eV we re measured for the monohydrated Fe+, Co+ and Au+ ions respectively, in fai r agreement with the existing literature. Small threshold energies of 0.7 /- 0.2 eV, 0.7 +/- 0.2 eV and 0.5 +/- 0.1 eV were found for the Fe(H2O)(2)( +), Co(H2O)(2)(+) and Au(H2O)(2)(+) clusters respectively. Secondary thresh olds were observed on the cross-section, respectively at 1.7 +/- 0.3 eV and 2.0 +/- 0.2 eV for the Co(H2O)(2)(+) and Au(H2O)(2)(+) clusters. This doub le threshold behavior could be attributed to the presence of two kinds of i somers in the beam. The upper threshold is associated with clusters where b oth water molecules are linked to the metal ion (first solvation shell), wh ereas the lower threshold corresponds to clusters with one water. molecule in the first solvation shell and the other in the second shell, Such an ana lysis documents the binding energy of either a first shell or a second shel l water molecule in the M(H2O)(2)(+) cluster ions.