Density functional theory (DFT) is widely applied in calculations of molecules and materials. Yet, it suffers from a well-known over-emphasis on charge delocalization arising from self-interaction error that destabilizes localized states. Here, using the symmetric diamine N,N′-dimethylpiperazine as a model, we have experimentally determined the relative energy of a state with positive charge localized on one of the two nitrogen atoms, and a state with positive charge delocalized over both nitrogen atoms. The charge-localized state was found to be 0.33 (0.04) eV higher in energy than the charge-delocalized state. This provides an important test of theoretical approaches to electronic structure calculations. Calculations with all DFT functionals commonly used today, including hybrid functionals with exact exchange, fail to predict a stable charge-localized state. However, the application of an explicit self-interaction correction to a semi-local functional identifies both states and gives relative energy in excellent agreement with both experiment and CCSD(T) calculations.

Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10–100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH+* and PAH2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

Two identical ionization centers, one on each nitrogen atom, make N,N′dimethylpiperazine an important model to explore how the transfer of a (partial) charge is linked to the structural deformations of the molecular skeleton. Time-resolved photoelectron spectroscopy uncovered that upon excitation to the 3p Rydberg level at 207 nm only one of the initially symmetry-equivalent nitrogen atoms acquires the charge, creating an asymmetric molecular structure with a localized charge. Rapid internal conversion to 3s leads to a multitude of conformeric structures with the charge localized on one nitrogen atom (230 fs time constant) and a rigid structure with the charge delocalized over both nitrogen atoms (480 fs time constant). Structural motions continue while the molecule samples the 3s potential energy landscape, leading to an equilibrium between charge-localized and charge-delocalized conformeric structures that is approached with a 2.65 ps time constant.