Raman line shapes of anharmonic phonons

Anharmonic effects in crystals typically result in the symmetric broadening of the single-phonon Lorentzian line shape. However, a simple model, based on consideration of a highly anharmonic potential, shows that anharmonicity can lead not only to asymmetric line shapes but also to line shapes composed of multiple subpeaks. Experimental Raman data on the soft ${\mathit{A}}_1$(TO) phonon in ferroelectric ${\mathrm{PbTiO}}_3$ indicates that this mode does indeed consist of a series of subpeaks whose intensities are accurately predicted (with no fitting parameters) by our model.