Rydberg-to-valence evolution in excited state molecular dynamics

We present an overview of experimental and theoretical investigations exploring the dynamical evolution of Rydberg-to-valence character in the electronically excited states of small polyatomic molecules. Time-resolved photoelectron imaging (TRPEI), in conjunction with high-level quantum chemistry calculations, permits detailed insight into the non-adiabatic processes operating in these systems and we review several case studies drawn from our own work in this area over the last few years. Electronically excited Rydberg states that develop significant valence character along specific molecular coordinates provide potentially important pathways for the rapid and efficient redistribution of excess energy following ultraviolet absorption. As such, there is considerable interest in developing better understanding of role of these states play within a broad range of different photochemical environments. A central theme of this review considers the way in which key energy - and angle-resolved observables in TRPEI measurements are influenced by different aspects of transitory Rydberg-to-valence behaviour. Several themes are discussed within a coherent narrative, drawing on experimental and theoretical findings in a selected series of small organic species containing nitrogen heteroatoms. Critically, many of the effects we highlight will also be generalisable to related studies interrogating non-adiabatic processes within a much broader range of molecular systems.