Alkali clusters
(Figure adapted from M.Brack in Scientific American, p.50, Vol.277 No.6, December 1997 ) 
The figure shows the optical response of Na 11 + as a function of the frequency of the exciting laser and for different cluster temperatures. At the lowest temperature (35 K) six narrow lines are observed which broaden with increasing temperature and become two broad lines when the cluster is molten. The peaks at high temperature are in good agreement with predictions from the Nearly Free Electron Gas Model, which predicts that this cluster has the shape of an American football. The two peaks are collective oscillations of all the valence electrons parallel to the short and long axis of the football. 
1) The temperature dependence of all spectra of Na n + , with 3 <= n <= 16 are given in: 
a) Ch. Ellert, M. Schmidt, Ch. Schmitt, Th. Reiners, and H.Haberland, Temperature dependence of the optical response of small, open shell sodium clusters, Phys. Rev. Lett. 75, 1731 (1995) 
b) M. Schmidt, Ch. Ellert, W. Kronmüller, and H. Haberland, Temperature Dependence of the Optical Response of Sodium Cluster Ions, Na n + , with 3 <= n <= 16, Phys. Rev. B 1999 
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2) The size dependence, and a moment analysis for 3 <= n <= 64 can be found in: 
a) Th. Reiners, Ch. Ellert, M. Schmidt and H. Haberland, Size dependence of the optical response of spherical sodium clusters, Phys. Rev. Lett. 27, 1558 (1995) 
b) M.Schmidt, and H. Haberland, Optical Spectra and their Moments for Sodium Clusters, Na n + , with 3 <= n <= 64, European Physical Journal D 1999 
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3) The charge dependence of three spherical clusters is given in: 
Ch. Schmitt, Ch. Ellert, M. Schmidt, and H. Haberland, Doubly charged sodium clusters: plasmon response and photo production, Z. Phys. D42, 145 (1997) 
4) The data of 1 to 3 are summarized in: 
H. Haberland, Optical and Thermal Properties of Sodium Clusters, in Metal Clusters , W. Ekardt, ed. John Wiley and Sons Ltd 
5) There exist close analogies between metal clusters and Nuclear Physics, as both nuclei and hot metal clusters can be treated as finite Fermion systems: 
H. Haberland, Metal clusters and nuclei, some similarities and differences, Nuclear Physics A 1999 
6) For the negatively charged, spherical cluster Na 91 - , the optical response as well as the branching ratio for atom with respect to electron emission has been measured: 
Th. Reiners and H. Haberland, Plasmon Enhanced Electron and Atom Emission from a Spherical Sodium Cluster: Na 91 - , Phys.Rev. Lett. 77, 2440 (1996) 
7) Li-clusters: 
Ch. Ellert, M. Schmidt, Ch. Schmitt, and H. Haberland, Reduced oscillator strength in lithium atoms, clusters, and bulk, Phys. Rev. B 1999 
8) Cs-clusters: 
Ch. Ellert, M. Schmidt, Ch. Schmitt, and H. Haberland, Optical response of small, closed shell Cs clusters ions, Surface Science Reviews and Letters 3, 515 (1996)