The erstand second-order Raman spectra of diamond were studied using the 4880' and 5145 A lines of an Ar ion laser and the 6328A. 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 cm '. Polarization studies were carried out on both the erstand second-order spectra. From such studies it was established that the 1332cm ' Raman line is the zone-center optical phonon with P &"+& (F2,) 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. '

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We have theoretically investigated, by ab initio techniques, the phonon properties of several semiconductors with chalcopyrite structure. Comparison with experiments has led us to distinguish between materials with d electrons in the valence band (e.g., CuGaS 2 , AgGaS 2 ) and those without d electrons (e.g., ZnSnAs 2 ). The former exhibit a rather peculiar nonmonotonic temperature dependence of the energy gap which, so far, has resisted cogent theoretical description. We analyze this nonmonotonic temperature dependence by fitting two Bose-Einstein oscillators with weights of opposite sign leading to an increase at low temperatures and a decrease at higher temperatures and find that the energy of the former correlates well with characteristic peaks in the phonon density of states associated with low-energy vibrations of the d-electron elements. We hope that this work will encourage theoretical investigations of the electron-phonon interaction in this direction, especially of the current ab initio type.

The Brillouin spectrum of diamond excited with either the Ar+ or .He-Ne laser radiation is measured with a triple-passed piezoelectrically scanned Fabry-Perot interferometer. The polarization features and the selection rules have been verified for a number of scattering geometries, From the measured frequency shifts of the Brillouin components, the following values are obtained for the elastic moduli: c && = 10.764~0.002 ' c i2 --1.252 + 0.023, and c 44 --5.774 + 0.014 in units of 10" dyn/cm'. The relative intensities of the observed Brillouin components for a variety of scattering geometries are consistent with the following elasto-optic contants: p 44 ---0.172 and p»-p» ---0.292 determined by Denning et al. and p» + 2p~2 = 0.1640 obtained by Schneider, From a comparison of the Brillouin spectrum and the Raman sp:ctrum associated with the zone-center optical phonon, observed under identical conditions, we obtaIin a value for the single independent component characterizing the Raman tensor per unit cell, viz. ,~a~= 4.4~0.3 A'.

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