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

Abstract

The optical absorption of small, open sodium cluster ions ( ${\mathrm{Na}}_{n}^{\phantom{\rule{0ex}{0ex}}+},n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4,7,\mathrm{and}11$) exhibit an unexpectedly large temperature dependence. Clearly separated absorption lines observed for cold clusters $(T\ensuremath{\sim}35$ K) are interpreted as transitions between electronic states of the ${\mathrm{Na}}_{n}^{\phantom{\rule{0ex}{0ex}}+}$ molecule. At the highest temperature $(T>380$ K), however, the clusters are liquid, and broad absorption peaks are observed whose energy positions can be explained for $n>7$ by the model of nearly free electrons oscillating in a spheroidal container.