Superatom State-Resolved Dynamics of the Au₂₅(SC₈H₉)₁₈^- Cluster from Two-Dimensional Electronic Spectroscopy

Superatom state-resolved dynamics of the Au₂₅(SC₈H₉)₁₈^- monolayer-protected cluster (MPC) were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The electronic ground state of the Au₂₅(SC₈H₉)₁₈^- MPC is described by an eight-electron P-like superatom orbital. Hot electron relaxation (200 ± 15 fs) within the superatom D manifold of lowest-unoccupied molecular orbitals was resolved from hot hole relaxation (290 ± 20 fs) in the superatom P states by using 2DES in a partially collinear pump-probe geometry. Electronic relaxation dynamics mediated by specific superatom states were distinguished by examining the time-dependent cross-peak amplitudes for specific excitation and detection photon energy combinations. Quantification of the time-dependent amplitudes and energy positions of cross peaks in the 2.21/1.85 eV (excitation/detection) region confirmed that an apparent energetic blue shift observed for transient bleach signals results from rapid hot electron relaxation in the superatom D states. The combination of structurally precise MPCs and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices.