Sentence:  The rate of the gas phase exchange reaction appears to be proportional to the first power of the absorbed light intensity indicating that the <WSD>radical</WSD> intermediates are removed at the walls or by reaction with an impurity rather than by bimolecular radical combination reactions . 
Meaning of the word marked with <WSD> tags:  (chemistry) two or more atoms bound together as a single unit and forming part of a molecule