Ground-state geometries and optical properties ofNa8−xLix(x=0–8)clusters

Abstract

We have investigated equilibrium geometries of mixed clusters ${\mathrm{Na}}_{8\ensuremath{-}x}{\mathrm{Li}}_{x}$ $(x=0--8)$ using ab initio molecular dynamics within the framework of density functional theory. The resulting cluster geometries show evolution from ${C}_{3v}$ symmetry for ${\mathrm{Li}}_{8}$ to ${D}_{2d}$ symmetry for ${\mathrm{Na}}_{8}$ via ${T}_{d}$ symmetry $({\mathrm{Li}}_{4}{\mathrm{Na}}_{4}).$ We also present and discuss the absorption spectra obtained using the time-dependent local density approximation formalism. The calculated absorption spectra of all ${\mathrm{Na}}_{8\ensuremath{-}x}{\mathrm{Li}}_{x}$ clusters demonstrate the presence of a strong plasmon peak located approximately at the same energy $(\ensuremath{\approx}2.7 \mathrm{eV}).$ Our calculations predict nearly the same oscillator strengths for the main plasmon peaks of ${\mathrm{Li}}_{8}$ and ${\mathrm{Na}}_{8}.$ Since the polarizabilities of ${\mathrm{Li}}_{8}$ $(9.7 {\mathrm{\AA{}}}^{3})$ and ${\mathrm{Na}}_{8}$ $(14.02 {\mathrm{\AA{}}}^{3})$ are significantally different, the above observation indicates the significance of nonplasmon contributions in the optical spectrum of ${\mathrm{Li}}_{8}$ and implies that the simple jellium approach and the plasmon pole approximation are not valid for ${\mathrm{Li}}_{8}.$