Giant resonances in metallic clusters

The second, and more important kind is the giant dipole resonance intrinsic to the delocalised closed shell of a metallic cluster. Such resonances have received a great deal of attention [684]. They occur at energies typically around 2-3 eV for alkali atoms, and have all the features characteristic of collective resonances. In particular, they exhaust the oscillator strength sum rule, and dominate the spectrum locally. 

They have certain specific properties as compared with the atomic case  

The fact that they tend to be fairly symmetrical (at least when they occur below the ionisation threshold) is related to their time characteristics from the lifetime widths and resonance energies, one can deduce that the giant resonances in metallic clusters are many-body oscillations undergoing several periods. Giant resonances in metallic clusters can truly be considered as plasmons, and relate quite clearly to surface plasmons in solids. 

In fig. 12.15, we show a typical resonance, as obtained for closed shell clusters. This similarity is believed to extend to their structure (although some experimental evidence indicates a more complex situation - see section 12.17). In particular, it is very interesting to consider departures from the purely spherical shape, when the shell has a number of electrons 

The fullerenes also possess a shell of delocalised electrons each C atom contributes four delocalised electrons, constrained to move on a sphere roughly one atom thick. This shell possesses a prominent giant resonance 

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