Centered index under an "reaction arrow"

[Samuel Hammer]

Hi
does anyone know how to center an index under an reaction arrow ?
TIA
Sam
-- 
Samuel Hammer   
Institut f�r Umweltphysik der Universit�t Heidelberg
Im Neuenheimer Feld 229
Germany

Tel  ++ 49 6221 546529
Fax ++ 49 6221 546405