What can we learn from Raman spectra of nanopowders

The phenomenon of line broadening for optical vibrational eigenmodes in nanoparticles is incorporated into the microscopic theory of Raman scattering in nanopowders of nonpolar crystals. Particular broadening mechanism investigated in details is the phonon scattering by weak delta-correlated spatially randomized variations of atomic masses. Analytical calculations reveal parametrically different behavior of the linewidth in regimes of separated and overlapped levels. The dependence of linewidths on the particle mean size, shape, variance of the size distribution function, strength of disorder, as well as on phonon quantum numbers is determined for both regimes, and crossover scales for mean size and disorder strength are obtained. Analytical results are strongly supported by numerical calculations. The possibility to extract the aforementioned parameters (excluding quantum numbers) from experimental data on the Raman peak shape and position is demonstrated and illustrated with the use of concrete example.