RAMAN-SCATTERING FROM SMALL SPHERICAL-PARTICLES

Raman coupling coefficients are calculated for the acoustic vibrations of a small dielectric sphere. The mean components of the strain tense r are calculated for the symmetric and quadrupolar Raman-active vibrat ional modes, within a continuum approximation which considers a vibrat ing homogeneous sphere. The Raman coupling coefficient depends on the crystalline structure and on the microscopic scattering mechanism. For cubic Bravais lattices and for a dipole-induced dipole scattering mec hanism, the coupling coefficient of the symmetric vibrations vanishes. The Raman intensity of the inner modes is found to be small with resp ect to that of surface modes. The scaling law, which gives the Raman c oupling coefficient as a function of the particle size, has been deriv ed. The coupling of the sphere with a surrounding elastic medium has b een considered and found to cause shift and broadening of the lines. T hese effects can alter significantly the estimated mean value and dist ribution of particle sizes.