Ground-state geometries and optical properties of Na_8-xLi_x (x=0-8) clusters

Abstract

We have investigated equilibrium geometries of mixed clusters Na_8-xLi_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 Li₈ to D_2d symmetry for Na₈ via T_d symmetry (Li₄Na₄). We also present and discuss the absorption spectra obtained using the time-dependent local density approximation formalism. The calculated absorption spectra of all Na_8-xLi_x clusters demonstrate the presence of a strong plasmon peak located approximately at the same energy (≈2.7 eV). Our calculations predict nearly the same oscillator strengths for the main plasmon peaks of Li₈ and Na₈. Since the polarizabilities of Li₈ (9.7 ų) and Na₈ (14.02 ų) are significantally different, the above observation indicates the significance of nonplasmon contributions in the optical spectrum of Li₈ and implies that the simple jellium approach and the plasmon pole approximation are not valid for Li₈.