Size-Dependent Plasmon Lifetimes and Electron-Phonon Coupling Time Constants for Surface Bound Na Clusters

Abstract

In this paper we show that time-resolved analysis of second harmonic generation via femtosecond pump-probe studies can be used to extract simultaneously mean cluster radii, decay times ${\ensuremath{\tau}}_{\mathrm{sp}}$ of localized surface plasmon excitations, and electron-phonon coupling time constants ${\ensuremath{\tau}}_{\mathrm{ep}}$ in large ${\mathrm{Na}}_{n}$ $(n\ensuremath{\approx}{10}^{7})$ clusters, bound to insulator surfaces. From the dependence of plasmon lifetime on the mean cluster radius $(5\ensuremath{\le}{r}_{0}\ensuremath{\le}55\mathrm{nm})$ a maximum of ${\ensuremath{\tau}}_{\mathrm{sp}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10\mathrm{fs}$ at a mean radius of about 25 nm is observed. Additionally, for Na clusters with mean radii of 30 to 50 nm and bound to a lithium fluoride surface the electron-phonon coupling time constant is found to be ${\ensuremath{\tau}}_{\mathrm{ep}}\ensuremath{\approx}1.1\mathrm{ps}$.