Garasevich S. Peculiarities of Raman scattering in tetragonal gyrotropic crystals.

Thesis contains results of experimental investigations of first order spatial dispersion effects in Raman spectra of gyrotropic crystals (point group 422). The splitting of low frequency E-doublets in Raman spectra of gyrotropic uniaxial tetragonal ZnP2, CdP2, crystals has been measured for different directions of phonon wave vector q. It was revealed that dependence of splitting on the angle between q and optical axis c is completely opposed to well known angular dependence of LO-TO splitting of double degenerated mode in uniaxial crystals. In both cases we found no splitting when wave vector q is in XY-plane and the symmetrical splittings for other directions of q which reaches the largest value when q is parallel to axis c. This splitting may be treated as entirely caused by first order of wave vector magnitude q spatial dispersion (gyrotropy). This conclusion is in accordance with observation of strong Raman circular dichroism for components of these doublets and is confirmed by IR measurements. The form of the index surface of the gyration tensor for this modes was determined. We studied the influence of isotope substitution on E-doublets in ZnP2 and CdP2 crystals that are split due to spatial dispersion and shows strong Raman circular dichroism. We observed only isotopic shifts of these doublets without changes of their splitting, Raman circular dichroism and linewidth. This confirms a collective character of motions in these modes.