Negative electron binding energies observed in a triply charged anion: Photoelectron spectroscopy of 1-hydroxy-3,6,8-pyrene-trisulfonate

We report the observation of negative electron binding energies (BEs) in a triply charged anion, 1-hydroxy-3,6,8-pyrene-trisulfonate (HPTS3-). Low-temperature photoelectron spectra were obtained for HPTS3- at several photon energies, revealing three detachment features below 0 electron BE. The HPTS3- trianion was measured to possess a negative BE of -0.66 eV. Despite the relatively high excess energy stored in HPTS3-, it was observed to be a long-lived anion due to its high repulsive Coulomb barrier (RCB) (similar to 3.3 eV), which prevents spontaneous electron emission. Theoretical calculations were carried out, which confirmed the negative electron BEs observed. The calculations further showed that the highest occupied molecular orbital in HPTS3- is an antibonding pi orbital on the pyrene rings, followed by lone pair electrons in the peripheral -SO3- groups. Negative electron BE is a unique feature of multiply charged anions due to the presence of the RCB. Such metastable species may be good models to study electron-electron and vibronic interactions in complex molecules. (c) 2008 American Institute of Physics.