Optical Spectroscopy of the Au₄ ⁺ Cluster: The Resolved Vibronic Structure Indicates an Unexpected Isomer

Knowledge of the geometric and electronic structure of gold clusters and nanoparticles is vital for understanding their catalytic and photochemical properties at the molecular level. In this study, we report the vibronic optical photodissociation spectrum of cold and mass-selected Au₄ ⁺ clusters measured at a resolution high enough to allow for comparison with Franck-Condon simulations of the excited state transitions based on time-dependent density functional theory calculations. The three vibrational frequencies identified for the lowest-lying optically accessible excited state at 2.17 eV stem from the Y-shaped isomer (C_2v ) and not from the rhombic isomer (D_2h ) considered to be the ground state structure of Au₄ ⁺ . This study demonstrates that an analysis of low-resolution electronic spectra by calculations of vertical transitions alone is not sufficient for a reliable isomer assignment of such metal clusters.