Density fluctuations in fluids having an internal degree of freedom

Th e fr eclu e nc y s pectrum o f d e ns ity flu c tuati ons is c al c ula te d for a fluid whose mo lec ul es possess a n inte rna l d eg ree o f freedom whi c h is wea kl y co up le d to th e tra ns la ti o na l d egree of free d o m o f th e fluid.Irre ve rs ib le th e rmod ynamics is used to obtain a n equ a tion o f mo tio n for th e inte rn a l d egree of freed o m. Thi s equat ion plus th e lin e arize d h ydro d yna m ic e qu a ti ons a re so lve d fo r the frequ e ncy s pec• trum of d e ns it y flu c tu a tio ns .Th e res ults a re co mpared with a s imi la r ca lc ul a ti o n invo lvin g a fr equ e ncy d e pe nd e nt vo lum e viscos it y.T he res ults are id e nti ca l fo r s tru c tura l re la xa ti o n but th e re is a differe nce fu r th e rma l re laxation .Th e ori g in of th e diffe re nce is di sc ussed a nd th e mag nitude o f the diffe re nce is e xa min e d for CC I.I a nd fo r CS2• Key Word s: Bri ll ouin sca ll e rin g, de ns it y flu c tu a ti o ns in liquid s, Ray le igh sca ll e rin g, s pec tra l cli s tr ibution of sca ll e re d li g ht , s tru c tura l re lax ati o n , th e rm a l re laxa tiun, vo lum e vi scos it y.