Giant Plasmon-Exciton Coupling in Small Plasmonic Nanoparticles from an <i>Ab Initio</i> GW-BSE Approach

Plasmonic nanocatalysts have emerged as highly tunable photocatalytic systems for driving nonequilibrium chemistry. However, the underlying microscopic mechanisms are poorly understood, since prevailing models wash out many-body interactions or atomistic details. Here, we address this gap by studying a prototypical small plasmonic nanoparticle within a first-principles GW plus Bethe-Salpeter equation approach. Despite their metallic composition, we find that electronic correlations qualitatively change the electronic and optical properties of this system. The optical response is dominated by plexcitons─plasmons hybridized with strongly bound (>2 eV) electron-hole pairs─showing that the established understanding of nanoparticles underpinned by free electron models is qualitatively incorrect for small nanoparticles. Additionally, we develop a quantitative metric of plasmonicity based on the excited-state wavefunctions and find that one dopant atom perturbs both the low-energy excitons and plasmonic states. Our results suggest that excitonic effects may influence optically driven chemical reactions in small metallic nanoparticles.