1ST-ORDER SPATIAL-DISPERSION EFFECTS IN THE RAMAN-SPECTRUM OF NACLO3

In cubic crystals the infrared-active triply degenerate modes split in to an LO and a doubly degenerate TO mode. For crystals with T symmetry , first-order spatial dispersion splits the polar TO modes into a set of two circularly polarized T+, and T- modes whose frequencies have on ly a small separation. Using incoherent high-resolution Raman spectros copy (a tandem Fabry-Perot Raman double monochromator of limiting reso lution 0.005 cm-1), we have measured the splitting of the low-energy T O lattice modes in the optically active material sodium chlorate. Expe riments were performed in various scattering geometries to separate th e optical Raman-active modes of different symmetries (A, E, LO and TO modes). Furthermore, the difference in the Raman response of the T+ an d T- modes to left or right circularly polarized incident light was de tected and analysed. We measured for the three low-energy TO modes (77 , 98 and 125 cm-1 at T = 6 K) a splitting of 0.08, 0.08 and 0.105 cm-1 respectively in a right angle geometry and 0.11, 0.11 and 0.135 cm-1 respectively in a backscattering geometry (an increase in agreement wi th a shift proportional to the wave vector length).