Femtosecond Relaxation Dynamics of Au25L18− Monolayer-Protected Clusters

Abstract

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 reflec...