Multiphonon Raman and infrared spectra of isotopically controlled diamond

Two-phonon spectra of 12C1-x13Cx diamonds, 0≤x≤1, are measured with a sensitive charge-coupled device-based Raman and a Fourier transform infrared spectrometer. Exploiting the infrared activity and the polarization characteristics of the Raman spectra recorded for a variety of scattering geometries, the two-phonon features are interpreted in terms of the critical points revealed in the phonon dispersion curves determined by inelastic neutron scattering. The occurrence of identical spectroscopic features in both 13C and natural diamond, consistent with virtual crystal approximation, proved invaluable in arriving at assignments for them. Space group selection rules for two-phonon combinations/overtones and the wave-vector conservation, all employed in a self-consistent fashion, yield the frequencies of the critical points with improved precision. copyright 1998 The American Physical Society