TEMPERATURE-DEPENDENCE OF THE OPTICAL-RESPONSE OF SMALL, OPEN-SHELL SODIUM CLUSTERS

The optical absorption of small, open sodium cluster ions (N-n(+), n = 4, 7, and 11) exhibit an unexpectedly large temperature dependence. Clearly separated absorption lines observed for cold clusters (T similar to 35 K) are interpreted as transitions between electronic states of the Na-n(+) 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.