A b s t r a c t . -Time r e s o l v e d r e s u l t s on t h e r e l a x a t i o n o f m o l e c u l a r r o t a t i o n s i n l i q u i d s a r e r e p o r t e d . I n CS2, i n a d d i t i o n t o t h e 2 picosecond Debye r e l a x a t i o n time, a 330femtosecond s h o r t r e l a x a t i o n t i m e i s observed f o r t h e f i r s t t i m e . T h i s s h o r t r e l a x a t i o n t i m e i s i n t e r p r e t e d as due t o t h e r e l a x a t i o n o f t h e molecules w i t h i n t h e l i

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... b r a t i o n a l w e l l s i n the l i q u i d . A s i m i l a r s h o r t r e l a x a t i o n t i m e on t h e o r d e r o f 150 femtoseconds i s a l s o observed i n n e a t l i q u i d nitrobenzene. He r e p o r t i n t h i s paper t h e r e s u l t s o f t h e f i r s t t i m e -r e s o l v e d s t u d y o f t h e r e l a x a t i o n o f m o l e c u l a r r o t a t i o n s i n t h e l i a u i d s t a t e . The femtosecond l a s e r i n t e r f e r o m e t r i c techniaue r e p o r t e d r e c e n t l y [1] i s used. O r i e n t a t i o n a l r e l a x a t i o n o f molecules i n 1 i q u i d s has been a s u b j e c t o f c o n s i d e r a b l e i n t e r e s t f o r many years. Because t h e corresnondino r e l a x a t i o n times a r e g e n e r a l l y i n t h e p i c o -o r sub-picosecond r e g i o n , i t has n o t been p o s s i b l e t o s t u d y these e f f e c t s d i r e c t l y i n t h e t i m e domain. Most o f t h e r e l e v a n t i n f o r m a t i o n has come from Rayleiqh s c a t t e r i n q [2] i n t h e p a s t and more r e c e n t l y a l s o from spin-resonance s t u d i e s [3]. The problem i s complicated by t h e f a c t t h a t t h e r e l a x a t i o n process i s n o t governed by a s i n s l e e x p o n e n t i a l decay process. I t i s known t h a t t h e s p e c t r a l p r o f i l e o f Rayleiah s c a t t e r e d l i q h t o e n e r a l l y c o n s i s t s o f a narrow L o r e n t z i a n c e n t r a l comoonent and a broad wing c o n s i s t i n ? o f a r e l a t i v e l y f l a t r e g i o n merqing c o n t i n u o u s l y i n t o a t h i r d r e o i o n where t h e i n t e n s i t y drops o f f more s h a r p l y again w i t h the freauency s h i f t of t h e Rayleioh s c a t t e r e d l i g h t . The shape o f t h e low-frequency c e n t r a l component i s r e l a t e d t o t h e "lono-time" b e h a v i o r o f t h e o r i e n t a t i o n a l r e l a x a t i o n process. T h i s can be d e s c r i b e d as an a n i s t r o p y d i f f u s i o n process c h a r a c t e r i z e d by a Debye r e l a x a t i o n t i m e t y p i c a l l y i n t h e picosecond domain. The more remote Rayleigh-wing, o r larqe-freouency r e g i o n , i s r e l a t e d t o t h e sub-picosecond o r " s h o r t t i m e " behavior o f t h e o r i e n t a t i o n a l dynamics o f t h e molecules. Here, because t h e s c a t t e r e d i n t e n s i t y i s l o w and t h e l i n e shape i s d i f f i c u l t t o i n t e r n r e t , t h e correspondinq m o l e c u l a r dynamics a r e n o t known o r well-understood. *work supported b y t h e N a t i o n a l Science Foundation through t h e M a t e r i a l s Science Center o f C o r n e l l U n i v e r s i t y , I t h a c a , NY 14853, U.S.A. +Now a t B e l l L a b o r a t o r i e s , Murray H i l l , NJ, U.S.A. Article published online by EDP Sciences and available at http://dx.