An extensive experimental search for the smallest stable gaseous multiply charged anions (MCAs) was pursued. Four penta-atomic dianions, $\mathrm{Pt}{X}_4^{2-}$ and $\mathrm{Pd}{X}_4^{2-}$ $(X=\mathrm{Cl},\mathrm{Br})$, were found to be the smallest observable using an electrospray ion source. Photodetachment experiments revealed, however, that ${\mathrm{PtCl}}_4^{2-}$ and ${\mathrm{PtBr}}_4^{2-}$ are electronically metastable with negative electron-binding energies. ${\mathrm{PdCl}}_4^{2-}$ and ${\mathrm{PdBr}}_4^{2-}$ 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.

• Received 29 June 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.3402