Femtosecond Relaxation Dynamics of Au₂₅L₁₈⁻ Monolayer-Protected Clusters

Excited-state relaxation dynamics of a gold cluster, the anion Au25(SCH2CH2Ph)18−, with a known crystal structure are measured with femtosecond laser spectroscopies. The cluster consists of an icosahedral Au13 core bonded to six Au2(SCH2CH2Ph)3 semirings. Pump−probe experiments excite Au13 core electronic transitions and then monitor relaxation of the system as it reaches quasi-equilibrium in lower-energy fluorescing state(s) localized on the semiring moieties. The measurements show that an extremely rapid (<200 fs) internal conversion process takes place within the multilevel electronic structure of the Au13 core, whereas core to semiring relaxation requires 1.2 ps. Photoinduced optical anisotropy persists for up to 1 ps after excitation of the lowest-energy Au13 core-localized transition, which, as suggested by the “superatom” model, distinguishes the optical response from that of a system with spherical symmetry (i.e., jellium sphere). Detection of an 80 cm−1 vibration localized to the Au13 core reflects the strong vibronic coupling of a delocalized Au−Au bond-stretching vibration analogous to the radial breathing modes of larger Au nanoparticles. Overall, the results give strong support to the superatom model for electronic structure of the cluster. Observation of the 80 cm−1 vibration highlights the contrast in mechanical properties between small clusters and larger nanoparticles.