Negative Heat Capacity

Physicists at the University of Freiburg in Germany have performed an experiment in which clusters of sodium atoms respond to added energy by cooling down. The clusters, typically consisting of 147 atoms, are made by blowing cold helium gas over a surface of boiling sodium. This leads to formation of clusters in a process which is similar to cloud formation in nature. The clusters are swept by the helium gas into a cell, where they are cooled or heated to some temperature. Afterwards the clusters are sorted by size and irradiated by a laser.

The laser light can fragment the clusters and the Freiburg group has developed a method on how to read the energy (i.e. the energy before the laser light was absorbed) from the fragmentation pattern. Near the melting point of the cluster, the measured internal energy can actually decrease even as the temperature rises. This may sound counter-intuitive, but is in agreement with theory, and no law of thermodynamics is violated.

Negative heat capacity has been predicted to occur in such systems as stars and atomic nuclei in the act of fragmentation, but this is the first time the phenomenon has been observed experimentally in atom clusters. (Schmidt et al., Physical Review Letters, 12 February; contact Hellmut Haberland, 49-761-203-5726, haberland@physik.uni-freiburg.de)