Effect is energy absorption! You have to be more specific in your query.

Dear Abin Philip

Very interesting question. The answer is simple: In conventional molecules we talk about electronic excitation from ground to xyz (depending on HOMO-LUMO) excitation state. In case of nanoparticles we talk of same electronic excitation itself, but this time it is from valence band to conduction band. Depending upon what kind of nanoparticles you choose, you can tune the absorbance, based on change in band gap (distance between valence and conduction band), and that's precisely why one can also calculate the band gap of any material using uv-vis absorbance spectra by using certain formulas. For instance, semiconductors in bulk form has moderate band gap as compared to metals which don't have (so they conduct) a band gap in bulk form. However, while both are broken down to nano-regimes the band gap increases with a significant blue shift in the absorbance of both. That is, as energy (band gap, energy needed to push the electron by incoming photon) increases the wavelength decreases. You may wanna read more on UFMO— upper filled molecular orbitals; LVMO — lower valence molecular orbitals. Do take a look at:

Article Superior Resonant Nanocavities Engineering on Photonic Cryst...

Good luck!

Seemesh Bhaskar has gone a long way to answering the question.

It should be added that, whereas transitions between Molecular orbitals or semiconductor bands, as a result of the absorption of a photon, are usually considered to involve single electrons, whereas a plasmon is a collective excitation of electrons, which in nanoparticles of metallic elements, is localised on the surface. Thus, light of appropriate wavelength/frequency/energy excites a surface plasmon resonance.