ARTICLE pubs.acs.org/JPCC
Ultrafast Relaxation Dynamics of Rod-Shaped 25-Atom Gold Nanoclusters Matthew Y. Sfeir,*,† Huifeng Qian,‡ Katsuyuki Nobusada,§ and Rongchao Jin‡ †
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States § Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan ‡
bS Supporting Information ABSTRACT: We report a femtosecond spectroscopic investigation on the electronic structure and relaxation dynamics of a rod-shaped, 25-atom (Au25) 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 LUMOþn 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 cm1 was observed and results in the modulation of the excited state absorption transition energy.
’ INTRODUCTION The size- and shape-dependent properties of nanoparticles constitute a central topic of current nanoscience research. Spectroscopic studies, including steady state and time-resolved optical spectroscopies, provide critical information on the electronic structure and relaxation dynamics of nanoparticles. For metal nanoparticles, a large quantity of previous work has focused on plasmonic nanoparticles (typically 5100 nm).19 In these materials, the observed ultrafast dynamics are dominated by plasmon recovery and electronphonon interactions.6,1012 Photoexcitation with a pulsed laser generates a nonequilibrium distribution of electron kinetic energies, which thermalize into an elevated FermiDirac electronic temperature on the femtosecond (fs) scale (typically
