Ground-state geometries and optical properties of Na<SUB>8-x</SUB>Li<SUB>x</SUB> (x=0-8) clusters

We have investigated equilibrium geometries of mixed clusters Na8-xLix (x=0-8) using ab initio molecular dynamics within the framework of density functional theory. The resulting cluster geometries show evolution from C3v symmetry for Li8 to D2d symmetry for Na8 via Td 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 Na8-xLix clusters demonstrate the presence of a strong plasmon peak located approximately at the same energy (&#8776;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 &#197;3) and Na8 (14.02 &#197;3) 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