Photoelectron imaging is used to study the electronic states of chemical systems. Imaging allows not only the determination of electron kinetic energy, as in traditional photoelectron spectroscopy, but also yields information about parent orbital symmetry encoded in photoelectron angular distributions. Combining photoelectron imaging with pump-probe spectroscopy allows for the study of electronic rearrangements in real time.

This study focuses on the evolution of the electronic structure of I₂^- during dissociation. Excitation with an ultrashort (~100 fs) laser pump pulse brings the molecule to an excited dissociative state, from which an electron is detached by a second, probe pulse after a specific delay. Repeating this process for a series of pump-probe delays allows for complete mapping of the dissociation process. While the energetic asymptote is reached after 700 fs, the photoelectron anisotropy continues to evolve on a ps time scale. This can be explained as the persistence of I₂^- orbital inversion symmetry; even at an internuclear separation of ~35 Å, the extra electron remains delocalized around both nuclei. During detachment, the photoelectron behaves as a two-centered wavefront whose interference pattern depends upon the internuclear distance at the probe time. The system may thus be thought of as a molecular interferometer whose interference cycle is traced via the temporal variation in the photoelectron angular distribution.

We also present recent results in the study of the dissociation of N₂O₂^- via a similar excitation and photodetachment scheme.