Temperature Dependence of the Optical Response of Small, Open Shell Sodium Clusters

The optical absorption of small, open sodium cluster ions ( ${\mathrm{Na}}_n^{+},n=4,7,\mathrm{and}11$) exhibit an unexpectedly large temperature dependence. Clearly separated absorption lines observed for cold clusters $(T\sim 35$ K) are interpreted as transitions between electronic states of the ${\mathrm{Na}}_n^{+}$ molecule. At the highest temperature $(T>\mathrm{}380\mathrm{}$ K), however, the clusters are liquid, and broad absorption peaks are observed whose energy positions can be explained for $n>\mathrm{}7\mathrm{}$ by the model of nearly free electrons oscillating in a spheroidal container.