Small Nanoparticles and Surface Damping

At small diameters (d 30 nm] the absorption/extinction spectra are dominated by the dipole resonance (/ = 1) and the incident electric field causes the conduction electrons to oscillate coherently along the Eo -polarization direction. Moreover the scattering, which is a radiative process and requires coupling between electric and magnetic fields, is negligible. Thus the extinction efficiency is very close to the absorption efficiency, as it can observed in the spectra reported in Fig. 3.1. 

In all the spectra shown in Fig. 3.1 we can observe a feature that is inherent to the material properties and independent from the particle geometry at about 320 nm the efficiencies present a 

Moreover, in considering the effects of the size in the optical response of a metallic nanoparticle, we must put in evidence that in the case of particles with dimensions comparable or smaller than the mean free path of its oscillating electrons (i.e. for gold and silver particles of radius o 10 nm) the dielectric function of the particles becomes strongly size-dependent and the additional surface damping must be considered for a correct treatment of the resonances intensity. 

For large diameters (d 50 nm) the scattering contribution increases and the role of the surface-damping correction decreases, see Fig. 3.1c,d. 

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