Probing free multiply charged anions using photodetachment photoelectron spectroscopy

Multiply charged anions are common in the condensed phase, and their existence has been generally taken for granted. But until very recently, these species have only rarely been observed in the gas phase. A new experimental technique, using photodetachment photoelectron spectroscopy, electrospray, and ion-trap mass spectrometry, has been developed in the author's laboratory to probe multiply charged anions in the gas phase. In this article, the principles of this technique and some of the initial results will be presented and discussed. The difference between photodetachment of singly and multiply charged anions is emphasized. Photodetachment photoelectron spectroscopy is ideal to probe the chemical and physical properties of free multiply charged anions. Among the initial findings, the repulsive Coulomb barriers existing in multiply charged anions have been directly observed. The relationship between the intramolecular electron-electron repulsion and the potential barrier is elucidated, culminating in the first observation of a negative electron binding energy in a quadruply charged anion. Electron tunneling effects through the repulsive Coulomb barriers are also observed and interpreted using a model Coulomb potential and the theory developed to understand a-decay in nuclear physics. We show that the electrospray technique provides a unique means that allow solution phase species and chemistry to be investigated in the gas phase with molecular details and specificity but without the complication of the condensed phase environments.