Transverse collective electron resonance

If metal particles become very small, reaching the nanometer-size scale, a color may occur. This is a typical phenomenon of nanometric metals. Actually, optical absorption may result in the ultraviolet or visible part of the spectrum, and this arises from a surface plasmon resonance. This is due to a collective electron plasma oscillation (plasmon) that is coupled to an external transverse electromagnetic field through the particle surface. It is possible to quantitatively relate the absorption coefficient to the wavelength of the exciting radiation by the Mie theory for spherical inclnsions in a dielectric matrix (34). Far-IR Inminescence is another optical phenomenon frequently observed with nanosized metals (46). 

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