Ultrafast Relaxation Dynamics of Rod-Shaped 25-Atom Gold Nanoclusters

We report a femtosecond spectroscopic investigation on the electronic structure and relaxation dynamics of a rod-shaped, 25-atom (Au<sub>25</sub>) nanocluster capped by organic ligands. Broadband femtosecond transient absorption spectra of the cluster show overlapped excited state absorption and ground state bleach signals. Two lifetimes (i.e., 0.8 ps fast component and a 2.4 μs long component) are identified, with the 0.8 ps component attributed to the fast internal conversion process from LUMO+<i>n</i> to LUMO and the long component to electron relaxation to the ground state. The rod shape of the cluster induces a strong anisotropic response in the transient absorption spectra, from which we deduce that the transition moment is oriented with the long axis of the prolate-shaped cluster. In addition, coherent phonon emission at 26 cm<sup>−1</sup> was observed and results in the modulation of the excited state absorption transition energy.