D o c t o r o f P h i l o s o p h y o f t h e U n i v e r s i t y o f L n d i t s d e t u n i n g ( A ) f r o m t h e a t o m i c t r a n s i t i o n f r e q u e n c y (wQ ) . T h e l a s e r f i e l d i s a s s u m e d t o b e m o n o c h r o m a t i c a n d l i n e a r l y p o l a r i z e d . I t i s c o n s t a n t o v e r t h e c o l l i s i o n d u r a t i o n a n d i t s f r e q u e n c y i s s l i g h t l y d e t u n e d f r o m t h e a t o m i c t r a n s i t i o n f r e q u e n c y . T h e a t o m s a r e a s s u m e d t o f o l l o w c l a s s i c a l r e c t i l i n e a r p a t h s a n d o n l y u n d e r g o b i n a r y c o l l i s i o n s . T h e r e s o n a n c e i n t e r a c t i o n i s t a k e n t o b e t h e f i r s t o r d e r d i p o l e -d i p o l e i n t e r a c t i o n a c t i n g w i t h i n t h e g r o u n d a n d r e s o n a n c e l e v e l s o f a p a i r o f a t o m s w h o s er e s p e c t i v e a n g u l a r m o m e n t a a r e (Fg= 0) a n d (Fe = l ) . Q u a l i t a t i v e e x p l a n a t i o n s a r e o f f e r e d f o r t h e r e s u l t s o b t a i n e d . T h e y i n v o l v e s p l i t t i n g t h e H a m i l t o n i a n i n t o p a r t s t h a t r e p r e s e n t t h e f r e e e v o l u t i o n o f t h e s y s t e m a tt h e R a b i f r e q u e n c y , t h e " n o r m a l " r e s o n a n c e i n t e r a c t i o n , a n d t h e " p e r t u r b a t i v e " i n t e r a c t i o n r e s p o n s i b l e f o r o p e n i n g n e w s c a t t e r i n g c h a n n e l s d e p e n d i n g u p o n t h e r e l a t i v e s t r e n g t h s o f t h e r e s o n a n c e i n t e r a c t i o n ( R ) , t h e g e n e r a l i s e d R a b i f r e q u e n c y (O') a n d t h e r a t i o ( Q / A ) . T h e s e e x p l a n a t i o n s s a t i s f a c t o r i l y d e s c r i b e t h e m a j o r i t y o f t h e r e s u l t s o b t a i n e d . O t h e r p o s s i b l e e x p l a n a t i o n s a r e g i v e n w h e r e t h e s i m p l e p e r t u r b a t i o n t h e o r y i s i n a d e q u a t e .3 ACKNOWLEDGMENTS

No abstract

The authors present an investigation of the modification caused by an intense laser field to the rates of mJ changing collisions for Hg perturbed by argon. They have studied experimentally the competition between radiative and collisional coupling between mJ states with a strong laser field tuned to the Hg J=1 to J=1 transition at lambda 435.8 nm. Population is pumped into the strongly dressed manifold from the ground state with a weak UV 'probe' laser detuned slightly from the lambda 253.7 nm intercombination line. The fluorescence observed at lambda 404.7 nm depends critically on whether the excitation proceeds via collisional redistribution or resonance through Stark shifted levels. The authors present data that clearly exhibit the effects of this competition. The data show in particular how one can produce excitation to transient Stark shifted levels in the presence of collisions. This will be critical in future experiments on collisions in strong fields.

Theory relevant to the treatment of resonance collisions between two-level dressed atoms has been extended to the three-level case in a self-contained calculation of field-dependent collision rates. With the laser field tuned close to the~e, )~~ez ) transition, dressing of the upper two-level system is achieved. We solve coupled equations of motion for two-atom states that describe one atom in the ground state and the other in one of two dressed states. Dressed-state collision rates are calculated within the impact approximation using S-matrix theory. The coupled equations of motion are simplified in the limit of intense field. We have applied the theory to describe approximately collisions between Ne atoms in fields intense enough to dress the collision complex. PACS number(s): 34.50.Rk, 32.70.n, 32.80.t