Relaxation dynamics of magic clusters

Abstract

Using femtosecond time-resolved two-photon photoemission spectroscopy, relaxation rates of the optically excited states in ${\mathrm{Al}}_{13}^{\ensuremath{-}}$, ${\mathrm{Si}}_{4}^{\ensuremath{-}}$, and ${\mathrm{Au}}_{6}^{\ensuremath{-}}$ clusters are estimated to be $250\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, and about $1\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, respectively. These clusters show closed electronic shell structures and large gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital in the anionic $({\mathrm{Al}}_{13}^{\ensuremath{-}})$ or neutral (${\mathrm{Si}}_{4}$ and ${\mathrm{Au}}_{6}$) states. Thus the different behaviors of these clusters for the relaxation dynamics cannot be rationalized within a relaxation mechanism dominated by Auger-like electron-electron coupling. We suggest that for these small clusters, the major decay mechanism might be electron-phonon coupling.