The first- and second-order Raman spectra of diamond were studied using the 4880 Å and 5145 Å lines of an Ar ion laser and the 6328 Å line of a He-Ne laser. The spectra were recorded at room, liquid-nitrogen, and liquid-helium temperatures. In addition to the second-order spectrum previously reported by Krishnan, a new weaker second-order spectrum was observed in the range 1600-2100 ${\mathrm{cm}}^{-1\mathrm{}}$. Polarization studies were carried out on both the first- and second-order spectra. From such studies it was established that the 1332-${\mathrm{cm}}^{-1\mathrm{}}$ Raman line is the zone-center optical phonon with ${\Gamma }^{\mathrm{}(25+)\mathrm{}}$ ($F_{2g}$) symmetry. The prominent features in both the second-order Raman spectra reported here and the second-order infrared spectra are interpreted in terms of the critical points of the phonon dispersion curves established from neutron spectroscopy and on the basis of space-group selection rules.

• Received 30 September 1969

DOI:https://doi.org/10.1103/PhysRevB.1.1687