Temperature dependence of the optical response of sodium cluster ions Na+n, with 4≤n≤16

The temperature dependence of the optical response of small sodium cluster ions, Na-n(+) (n = 4,..,,16), was measured. At low temperature (35 to 100 K) individual lines are seen, whose peak positions agree with ab initio calculations if the geometric positions of the atoms in the cluster are taken into account. The peaks broaden slowly with increasing temperature and coalesce, until at high temperature only one, two, or three broad peaks remain, whose positions are for n greater than or equal to 7 in good agreement with the nearly free-electron model. Although the detailed shape of the optical spectra changes strongly with temperature, its moments are nearly temperature independent. No pronounced effect due to the solid to liquid phase transition has been observed for these small cluster sizes. [S0163-1829(99)13715-9].