Electronic Excited-State Dynamics of Au₂₅ and Au₃₈ Studied by Ab Initio Transient Absorption Spectroscopy

Transient absorption experiments have previously been carried out to study the excited-state dynamics of atomically precise gold nanoparticles such as Au₂₅ and Au₃₈. Different mechanisms have been proposed to explain the excited-state dynamics on various time scales, ranging from femtoseconds to nanoseconds. In this work, we perform ab initio transient absorption simulations to investigate the excited-state dynamics of Au₂₅ and Au₃₈, which enables state-specific resolution. For Au₂₅, pump energies corresponding to peaks in its optical absorption spectrum were applied to excite the nanocluster. By a detailed analysis of orbitals, we separated the contributions to excited-state absorption that arise from electrons and holes. For Au₂₅, the lowest-energy sub-100 fs dynamics arise from transitions among d electrons in gold, whereas higher-energy dynamics originate from transitions between superatomic dynamics. This ab initio state-resolved dynamics process agrees well with the rapid ligand-independent decay in the experiment. For Au₃₈, different energies and directions of the pump laser are applied to induce absorption polarized in different directions. The higher-energy dynamics in Au₃₈ tend to arise from transitions between superatomic orbitals, whereas the lowest-energy excited-state absorption (below 0.5 eV) originates from hole dynamics among d-band orbitals. Because of symmetry and polarizability, the induced dipole moment in anisotropic Au₃₈ remains in the direction of the pump for our simulation time scale, whereas the more isotropic Au₂₅ nanocluster is better able to transfer the energy from the pump to the other two polarization directions.