MULTIPLE-PHONON RAMAN PROCESSES IN ULTRAMICROSCOPIC SCATTERING CENTRES OF LIF SING6E CRYSTALS

Stokes-Ramanspectra of ultramicroscopic scattering centres in LiF single crystals excited by a focussed 2 mW He-Ne-gaslaser radiating at 632.8 nm reveal a striking difference in their structure to those formerly obtained with a 20 mW laser. Ten more lines appear and the spectrum changes its shape. One set of lines very closely corresponds to the theoretically predictable positions of those of the two-phonon processes of the second order Ramaneffect in the ideal crystal lattice. Other lines are due to three-phonon processes with zero momentum transfer. This is corroborated by ultrasonic measurements of de Klerk and Klemens and structural disparities in the spectra found at different power levels. © 1968 Springer-Verlag.