Surface Modes in Small Particles

The extinction curves for magnesium oxide particles (Fig. 11.2) and aluminum particles (Fig. 11.4) show the dominance of surface modes. The strong extinction by MgO particles near 0.07 eV( - 17 ju.m) is a surface mode associated with lattice vibrations. Even more striking is the extinction feature in aluminum that dominates the ultraviolet region near 8 eV no corresponding feature exists in the bulk solid. Magnesium oxide and aluminum particles will be treated in more detail, both theoretically and experimentally, in this chapter. 

In Sections 12.1 and 12.2 we discuss the theory of surface modes in spherical and nonspherical particles, respectively in Sections 12.3 and 12.4 comparisons between theory and experiment are given, first for insulators and then for metals and metal-like materials. 

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The thermal properties of AU55 are treated in Sect. 3, using especially the results of MES measurements [24,25,42]. These are discussed in connection with the concept of bulk versus surface modes in small particles. An explanation of the temperature dependence of the MES [42] absorption intensities and the Cv results [25] on the basis of a model using the site coordination and the center-of-mass motion are briefly reviewed. The consequences of the Mossbauer results for surface Debye temperatures and for the melting temperature of small gold particles are also discussed. 

Following (9.27) we discussed the physical interpretation of the plasma frequency for a simple metal and introduced the concept of a plasmon, a quantized plasma oscillation. It may help our understanding of the physics of surface modes in small particles and the terminology sometimes encountered in their description if we expand that discussion. 

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