Weak metal-metal bonding in small manganese cluster ions, Mn(N)+ (N < or = 7).

The binding energies of manganese cluster ions Mn(N)+ (N = 5-7) were determined by the photodissociation experiments in the near-infrared and visible-photon-energy ranges. The bond dissociation energies of Mn(N)+, D0(Mn(N-1)+...Mn), were obtained to be 1.70+/-0.08, 1.04+/-0.10, and 1.46+/-0.11 eV, respectively, for N = 5, 6, and 7 from the threshold energies for the two-atom loss processes and the bond dissociation energies of Mn3(+) and Mn4(+) reported previously [A. Terasaki et al., J. Chem. Phys. 117, 7520 (2002)]. Correspondingly, binding energies per atom are obtained to be 0.99+/-0.03, 1.00+/-0.03, and 1.06+/-0.03 eV/at. for N = 5, 6, and 7, respectively. A gradual increase in the binding energy from N = 2 to N = 7 shows an increasing contribution of nonbonding 3d orbitals to the bonding via weak hybridization with valence 4s orbitals as the cluster size increases. These binding energies per atom are still much smaller than the bulk cohesive energy of manganese (2.92 eV/at.), and this finding indicates exceptionally weak metal-metal bonds in this size range.