A General Predictive and Conceptual Model for Repulsive Electronic States in Valence Ionized Molecules

Valence photoionization generates photoions with excess internal energy, often resulting in statistical dissociation, well described by RRKM theory. Yet, numerous small photoions with electronegative substituents (e.g., F, Cl, Br, OH) have been shown to exhibit nonstatistical behavior, undergoing direct dissociation from repulsive electronic states to yield radicals such as F^• or OH^•, along with their corresponding fragment ions. Here, we present a general, predictive model that rationalizes this mechanism and extends it to molecules bearing electronegative substituents capable of forming ²P (F^•, Cl^•, Br^•) or ²Π (OH^•, SH^•, N₃^•, NCO^•) radicals. Nonstatistical dissociation arises from ionization of p- or π-localized orbitals on electronegative atoms, producing radical-like fragments unbound to the cationic core. The outcome is governed by three factors: excitation energy, the bonding character of the ionized orbital, and electronic degeneracy between states upon dissociation. Using these criteria, we define four classes of potential energy surfaces, enabling predictive classification of dissociative behavior. Extensive computations on over 50 molecules confirm the model's accuracy and generality.