Spectroscopic evidence of positive clusters in Ag colloids obtained by laser ablation in aqueous solutions

We have found evidence of positive cluster formation during the laser ablation process of a silver target in aqueous solutions. In particular, by employing in situ shot-by-shot UV-vis spectroscopy in the early stages of the ablation, we observed a weak and unstable absorption band around 266 nm and a more stable one around 290 nm, which could be assigned to charged clusters like Ag 3 2+ and Ag 4 2+ , respectively. Surface-enhanced Raman scattering experiments performed with a test molecule adsorbed on a silver colloid obtained in pure water were compatible with the presence of Ag 4 2+ active sites on the surface of the Ag nanoparticles.