Darmstadt 2008 – wissenschaftliches Programm

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A: Fachverband Atomphysik

A 7: Metal clusters

A 7.2: Vortrag

Dienstag, 11. März 2008, 11:30–11:45, 3D

Melting temperatures and latent heats of oxidized sodium clusters — •Christian Hock, Samuel Straßburg, Christof Bartels, Hellmut Haberland, and Bernd v.Issendorff — Fakultät für Mathematik und Physik,Universität Freiburg, Stefan-Meier-Str. 19, 79104 Freiburg

Melting points and latent heats of oxidized Na cluster cations (Na_nO₂)⁺ in the size range n=135…191 have been measured. The results are compared to earlier measurements performed on pure Na clusters [1]. The cluster ions are produced in a sodium evaporation source and oxidized by the addition of O₂ after exiting the aggregation tube. Then they are thermalized in a heat bath (helium 0.7 mbar) with controllable temperature (6 K…350 K).
A significant lowering of the melting points and the latent heats is observed. The variation of the melting temperature with cluster size for oxidized sodium clusters is remarkably similar to the one of pure sodium cluster. Furthermore there seems to be a correlation between the melting points of a Na_n⁺ cluster and the corresponding (Na_n+2O₂)⁺ cluster; the difference of their melting temperatures is always around 16 K. The latent heat is always reduced and the variation with cluster size is similar to the one of pure sodium clusters.
For a selected cluster size of n = 144 sodium atoms additional caloric curves have been measured for higher degrees of oxidation for 4 and 6 additional oxygen atoms. As expected, both the melting points and the latent heats are reduced further.
[1] H. Haberland et al., Phys. Rev. Lett. 94, 035701 (2005)