Abstract The s u r f a c e reflectometer, CRISP, on t h e ISIS pulsed neutron source i s described. Recent developments i n t h e a n a l y s i s of r e f 1 e c t i . v i . t ~p r o f i l e s i s discussed. Some recent examples of t h e a p p l i c a t i o n of t h e technique from t h e extensive s c i e n t i f i c programme i n s u r f a c e chemistry, s u r f a c e magnetism and s o l i d f i l m s a r e presented. RBsumd - On dBcrit l e rdflectomhtre d e surface,CRISP i n s t a l l 6 s u r l a source d e neutrons pulsBs I S I S e t l ' o n d i s c u t e l e s dBveloppements r b c e n t s dans l ' a n a l y s e d e s p r o f i l s de r d f l e c t i v i t b . 0 n prdsente dgalement quelques exemples rbcents d e l ' a p p l i c a t i o n d e c e t t e technique 5 d e s programmes s c i e n t i f i q u e s importants en chirnie de surface, magnstisme d e s u r f a c e e t dans l ' b t u d e d e s couches minces. 1
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INTRODUCTION
Most of the common o p t i c a l phenomena, including r e f r a c t i o n , r e f l e c t i o n and i n t e r f e r e n c e , have been observed with slow neutrons (1). Since t h e f i r s t measurements of t o t a l r e f l e c t i o n by Fermi and coworkers (2), it has been e x t e n s i v e l y used i n neutron p o l a r i s e r s (3) and neutron guides ( 4 ) . I n recent years, however, a t t e n t i o n has focused on t h e a p p l i c a t i o n of specular r e f l e c t i o n of neutrons t o study s u r f a c e and i n t e r f a c i a l problems. It was shown by Thomas and coworkers (5) t h a t neutron r e f l e c t i o n experiments give information about t h e neutron r e f r a c t i v e index p r o f i l e normal t o t h e surface, and t h a t a judicious use of hydrogen/deuterium c o n t r a s t can provide unique information f o r a range of problems i n surface chemistry. Due t o t h e magnetic d i p o l e i n t e r a c t i o n , magnetic m a t e r i a l s e x h i b i t a neutron s p i n dependent r e f r a c t i v e index ( t h i s i s t h e b a s i s of neutron s p i n p o l a r i s e r s using c r i t i c a l r e f l e c t i o n ) and Felcher (6) has shown t h a t t h e specular r e f l e c t i o n of s p i n p o l a r i s e d neutrons i s a p a r t i c u l a r l y s e n s i t i v e probe of s u r f a c e magnetism. The advent of dedicated spectrometers (7,8) has been accompanied by a r a p i d expansion i n t h e s c i e n t i f i c a p p l i c a t i o n of t h e technique t o s u r f a c e chemistry (9) , surface magnetism (10) and s o l i d f i l m s (11). I n t h i s p r e s e n t a t i o n one of t h e s e (7) new breed of dedicated reflectometers, CRISP ( s i t u a t e d on t h e ISIS pulsed neutron source) w i l l be described. Recent advances i n t h e adaption of m u l t i l a y e r o p t i c a l methods t o t h e modelling and i n t e r p r e t a t i o n of neutron r e f l e c t i v i t i e s i s discussed. Finally, some recent examples of s t u d i e s i n surface chemistry, s o l i d f i l m s and s u r f a c e magnetism from t h e extensive s c i e n t i f i c programme on CRISP w i l l be presented. 2
- THEORETICAL DEVELOPMENTS
It has been shown that. t h e i n t e n s i t y of t h e r e f l e c t e d and t r a n s m i t t e d neutrons follows t h e same laws a s electromagnetic r a d i a t i o n with t h e e l e c t r i c vector perpendicular t o t h e plane of incidence (12). Hence t h e r e f r a c t i v e index a t t h e boundary between two media i s defined i n t h e usual way as,
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989709
where k ,k are t h e neutron wave vectors inside and outside t h e medium. index i h c8mmonly written as
The refractive
where = Nb/2n, C = Naa/4nl N i s t h e atomic number density, b i s t h e bound coherent scattering length, a, 1s the adsorption cross section and 1 i s t h e neutron wavelength.
A
Accordingly neutron reflection data has been analysed using standard o p t i c a l equations (for example, Fresnels law) and methods developed f o r multilayer o p t i c s (13). In particular, t h e matrix method of Born and Wolf ( 1 4 ) provides a convenient framework t o calculate r e f l e c t i v i t i e s f o r systems t h a t can be t r e a t e d a s a s e r i e s of discreet layers. However, f o r multilayer systems where t h e interfaces a r e non-ideal (diffuse or roughened) then t h i s approach rapidly becomes numerically unwieldly. A suitable a l t e r n a t i v e i s the method of Abeles (15) which i n conventional optics defines a c h a r a c t e r i s t i c matrix per layer i n terms of Fresnel coefficients, and phase factors from the relationship between t h e e l e c t r i c vectors i n successive layers such t h a t the c h a r a c t e r i s t i c matrix per layer i s ,
where, rm, i s t h e Fresnel reflection coefficient a t t h e m-1,mth interface such t h a t
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and
(21111) nm,lsin~m-l,
pm = nm sine,.
Following Cowley and Ryan (16) it i s now possible t o include a roughened o r diffuse interface a t each boundary (without dividing it i n t o a s e r i e s of discreet layers) by introducing a Gaussian roughness factor of t h e form described by Nevot and Crock ( 1 7 ) such t h a t r