Electronic relaxation in Ag nanoclusters studied with time-resolved photoelectron spectroscopy

Abstract

The decay mechanism of excited electronic states in small Ag nanoclusters is studied using time-resolved photoelectron spectroscopy. The low density of states in these clusters inhibits Auger-like decay channels that are responsible for ultrafast relaxation of optically excited states in bulk Ag. Thus, much longer relaxation times are expected for small clusters showing large gaps between electronic states. In contrast to this expectation, lifetimes below 1 ps were observed for most of the ${\mathrm{Ag}}_{n}^{\ensuremath{-}}\phantom{\rule{0.3em}{0ex}}(n<22)$ studied here. The only exception is the magic ${\mathrm{Ag}}_{7}^{\ensuremath{-}}$ with a relaxation time of $3.8\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. The observed fast relaxations are discussed in view of their ability to undergo fast shape deformations. This approach can also explain the slow relaxation of the rigid ${\mathrm{Au}}_{n}^{\ensuremath{-}}$ nanoclusters and the extremely fast relaxation of the flexible magic ${\mathrm{Al}}_{13}^{\ensuremath{-}}$.