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 radical intermediates are removed at the walls or by reaction with an impurity rather than by bimolecular <WSD>radical</WSD> 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