Multipole-Bound Anions and Photodetachment Spectroscopy

Photoelectron spectra of MgO n n 1 5 reveal a surprising trend: The electron binding energy decreases from n 1 to 4, and then increases from 4 to 5. Ab initio calculations suggest this pattern is related to the electrostatic interaction between the extra electron and the charge distribution of the neutral cluster. This interaction is significant in MgO and MgO 2 5 , for which the lowest nonvanishing multipole moment (LNM) is a dipole; 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 2 4 is the first octopole-bound anion yet observed experimentally and characterized theoretically.