Femtosecond Laser-Induced Size Reduction of Aqueous Gold Nanoparticles: In Situ and Pump-Probe Spectroscopy Investigations Revealing Coulomb Explosion

In situ extinction spectroscopy and transient absorption spectroscopy of the femtosecond laser-induced fragmentation of 60 nm diameter aqueous gold nanoparticles were performed. The E threshold laser fluences of fragmentation determined by in situ spectroscopy and transmission electron microscopy, (7.3 +/- 1.5) mJ.cm(-2) for excitation at 400 nm and (3.6 +/- 0.5) mJ.cm(-2) at 532 nm, agreed well with the values of 6.0-7.4 and 3.4-4.1 mJ.cm(-2) calculated by our simulation based on the two-temperature and liquid drop models. The transient absorption study revealed that real-time observation of fragmentation is possible at picosecond time scales. When monitored at 490 nm, at which the effect of fast relaxation dynamics is minimal, excitation at 400 nm afforded a reduced extinction signal of the localized surface plasmon resonance (LSPR) band of gold nanoparticles at laser fluences greater than or equal to (6.1 +/- 1) mJ.cm(-2). The reduction can be ascribed to nanoparticle fragmentation because the intensity (I) of the LSPR band depends on particle radius (R), I proportional to R-3. The signal reduction occurred not instantaneously but gradually within 100 ps, suggesting separation of initial densely packed small clusters during the observation period. The onset of the size reduction was laser-fluence-dependent, and it occurred earlier at higher fluences. This fluence dependence was explained well within the framework of our model: fragmentation occurs for liquid rather than solid gold, and the onset suggests the initiation of particle melting. The present result demonstrated that femtosecond laser-induced fragmentation is dominated by the Coulomb explosion mechanism, discussed many times without experimental verification. We believe we can provide information long needed in the field.