Highly Excited Electronic States of the Dicarbon Molecule: From Spectroscopy to Dynamics

The dicarbon molecule (C₂) serves not only as a benchmark for studying chemical bonding, but also as an elementary building block in the bottom-up synthesis of carbon materials, and a key species in astrochemistry. The spectroscopic and dynamic studies of C₂ have lasted for longer than two centuries, which provide invaluable insights into its complex electronic structure manifested as dense manifolds of low-lying electronic states, quasi-degenerate frontier orbitals, and significant multi-configurational characters. From a materials perspective, spectroscopic and dynamical insights into the excited states of C₂ are pivotal for controlling the non-equilibrium carbon chemistry in chemical vapor deposition (CVD) processes, where C₂ acts as a primary building block. Here, we present a comprehensive review of recent progress on the spectroscopic and dynamic study of C₂, focusing on its highly excited states near or above the lowest dissociation threshold. These advances not only led to the identification of several new highly excited states but also provided important information on the decay dynamics in excited states, including state-specific lifetimes and predissociation dynamics. Despite these advances, our understanding of the predissociation dynamics of C₂ remains limited; thus, several future prospects are proposed, and implications for the precise controlled synthesis of carbon materials are also mentioned.