Optical Properties and Electronic Energy Relaxation of Metallic Au₁₄₄(SR)₆₀ Nanoclusters

Abstract

Electronic energy relaxation of Au₁₄₄(SR)₆₀^q ligand-protected nanoclusters, where SR = SC₆H₁₃ and <i>q</i> = −1, 0, +1, and +2, was examined using femtosecond time-resolved transient absorption spectroscopy. The observed differential transient spectra contained three distinct components: (1) transient bleaches at 525 and 600 nm, (2) broad visible excited-state absorption (ESA), and (3) stimulated emission (SE) at 670 nm. The bleach recovery kinetics depended upon the excitation pulse energy and were thus attributed to electron–phonon coupling typical of metallic nanostructures. The prominent bleach at 525 nm was assigned to a core-localized plasmon resonance (CLPR). ESA decay kinetics were oxidation-state dependent and could be described using a metal-sphere charging model. The dynamics, emission energy, and intensity of the SE peak exhibited dielectric-dependent responses indicative of Superatom charge transfer states. On the basis of these data, the Au₁₄₄(SR)₆₀ system is the smallest-known nanocluster to exhibit quantifiable electron dynamics and optical properties characteristic of metals