Berlin 2001 – wissenschaftliches Programm

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MO: Molekülphysik

MO 1: Posters Monday: Clusters, Fullerenes and Nanotubes

MO 1.17: Poster

Montag, 2. April 2001, 12:30–15:00, AT3

Caloric Curves and Phase Transitions of Sodium Clusters — •Jörn Donges, Thomas Hippler, Martin Schmidt, Bernd v. Issendorff, and Hellmut Haberland — Fakultät für Physik, Universität Freiburg, H.Herderstr. 3, D-79104 Freiburg, Germany

We present collected experimental works on caloric curves (temperature vs. internal energy) of mass selected sodium clusters in a temperature range of 20 to 500K whereby we can treat the gained caloric curves in both a canonical and a microcanonical way.

Steps in the caloric curve indicate the melting regions. We observe significant size dependent variations of the melting temperatures. In this context we managed to account for negative heat capacities for Na₁₄₇⁺. Both the low temperature region and the high temperature region are explored. Nearby ca. 50K we observe a flattening of the caloric curve which is much more pronounced as in the bulk case. In the high temperature region a liquid gas phase transition could be modelled.

In the experiment we use a gas aggregation source where the clusters are ionized by a hollow cathode discharge. Afterwards they are thermalized in a helium heat bath. The internal energies of the mass selected clusters are projected onto a photo fragmentation spectrum which scales in a characteristic way with the temperature.