Photodetachment Spectroscopy of a Doubly Charged Anion: Direct Observation of the Repulsive Coulomb Barrier

Photoelectron spectra of citric acid doubly charged anion were measured. The repulsive Coulomb barrier due to the two excess charges was directly probed and estimated to be about ~1.9--2.5eV. The adiabatic binding energy of the second electron was measured to be 1.0 eV. Two detachment channels were observed, with the second channel at ~0.6eV higher in binding energy. The current study presents the first photodetachment experiments of a multiply charged anion and opens up new opportunities to study these novel molecular species.