Experimental search for the smallest stable multiply charged anions in the gas phase

An extensive experimental search for the smallest stable gaseous multiply charged anions (MCAs) was pursued. Four penta-atomic dianions, PtX42- and PdX42- (X = Cl,Br), were found to be the smallest observable using an electrospray ion source. Photodetachment experiments revealed, however, that PtCl42- and PtBr42- are electronically metastable with negative electron-binding energies. PdCl42- and PdBr42- were found to be electronically stable, but thermodynamically unstable against fragmentation. Lifetimes were estimated using an ion-trap technique for the four observed metastable MCAs, providing further insight into their electronic and thermodynamic stability.