(MgO)(n)(-) (n=1-5)clusters: Multipole-bound anions and photodetachment spectroscopy

Photoelectron spectra of (MgO)(n)(-) (n = 1-5) reveal a surprising trend: T he electron binding energy decreases from n = 1 to 4, and then increases fr om 4 to 5. Ab initio calculations suggest this pattern is related to the el ectrostatic interaction between the extra electron and the charge distribut ion of the neutral cluster. This interaction is significant in MgO- and (Mg O)(5)(-) for which the lowest nonvanishing multipole moment (LNM) is a dipo le; it is smaller for (MgO)(2)(-) and (MgO)(3)(-) for which a quadrupole is the LNM; and it is the smallest for (MgO)(4)(-), for which an octopole is the LNM. The cubic (MgO)(4)(-) is the first octopole-bound anion yet observ ed experimentally and characterized theoretically.