  Raman spectroscopic investigations are carried out on ZnO nanoparticles for
various photon energies. Intensities of E1-LO and E2 modes exhibit large
changes as the excitation energy varied from 2.41 to 3.815 eV, signifying
substantially large contribution of Frohlich interaction to the Raman
polarizability as compared to deformation potential close to the resonance.
Relative strength of these two mechanisms is estimated for the first time in
nanoparticles and compared with those in the bulk.
