Thermal relaxation and brillouin scattering in liquids

Wea k co upling of inte rnal degrees of free dom of molec ul es to the tran slational degrees of fr eedom of a fluid res ults in additiona l mod es of motion for de nsity fluctuation s. These ne w modes affect th e s pec tral dis tribution of light scattered by de ns it y flu c tuation s so that th e LandauPlacze k ratio is not sati s fi e d. Th e case of the rmal re lax at ion with a s in gle relaxati on tim e is work ed out in de tail. Formulas for the s pec tral di stribution of the sc atte re d li ght , for the ratio of th e inte ns iti es of the ce ntra l (Rayl e igh) to th e Brillouin co mpon e nts and for the phonon velocity are de riv e d a nd appli ed to carbon di s ulfid e and ca rbon tetrac hloride. Th e data for carbo n te trach lorid e are s hown to be in co ns is te nt with the s in gle relaxa ti on tim e mod el fo r th e rm al re laxa ti on. Ke y Word s: Brillouin scatt e rin g, de ns ity flu ctuati ons in liquids, li ght scatt e rin g in liquid s, Rayl e igh sc att e rin g, s pec tra l di s tributi on of scatt ered li ght , the rmal re laxati o n, vulum e viscos it y.