s a t u r a t i o n photothermique l a d e t e c t i o n par deflexion photothermique e t 2 1'excita;ion 1ocalis;e. On t r a i t e l a s a t u r a t i o n photoacoustique comme cas s p e c i a l . Abstract-The concept of photothermal s a t u r a t i o n i s generalized t o include photothermal d e f l e c t i o n d e t e c t i o n and l o c a l i z e d e x c i t a t i o n . Photoacoustic s a t u r a t i o n i s t r e a t e d a s a s p e c i a l case. = 1 , o r t o t a l l y unsaturated

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... When a thermally homogeneous sample i s used, and one-dimensional a n a l y s i s i s permitted by sample geometry, t h i s simple c r i t e r i o n i s adequate. This dependence of s a t u r a t i o n on j u s t two parameters i s a consequence of t h e thermo-acoustic coupling mechanism i n photoacoustic c e l l s where the a c o u s t i c wavelength i n t h e c e l l f l u i d i s much l a r g e r than a f l u i d thermal d i f f u s i o n l e n g t h a t t y p i c a l photoacoustic modulation frequencies. Under t h e s e circumstances, the photoacoustic s i g n a l i s p r o p o r t i o n a l t o the sample s u r f a c e temperature i n t e g r a t e d over a d i s t a n c e comparable t o a n a c o u s t i c wavelength. Thus photoacoustic d e t e c t i o n cannot observe s p a t i a l v a r i a t i o n s i n t h e s u r f a c e temperature s i n c e t y p i c a l a c o u s t i c wavelengths a r e much l a r g e r than t h e sample dimensions. From the viewpoint of s a t u r a t i o n , t h i s means t h a t the photoacoustic s i g n a l i s i n s e n s i t i v e t o t h e shape of the temperature p r o f i l e on t h e sample surface. Any t r a n s v e r s e heat flow i n t h e sample which merely r e d i s t r i b u t e s t h e s u r f a c e temperature leaving t h e average s u r f a c e temperature constant has no e f f e c t on t h e photoacoustic s i g n a l . A s a consequence, t h e photoacoustic s i g n a l depends only upon how deep i n t h e sample heat i s generated ( a s measured by p ) , and how f a r h e a t can t r a v e l i n a half cycle of t h e modulation frequency ( a s measured by 6). When h e a t generated i n t h e sample can reach t h e s u r f a c e i n a half c y c l e , the sample i s s a t u r a t e d . Article published online by EDP Sciences and available at http://dx.