Sentence:  It would appear that it should be possible to determine unique mechanisms for the thermal and photochemical reactions in both the liquid and gas phases and to determine values for <WSD>activation</WSD> energies of some of the intermediate reactions of atoms and free radicals , as well as information on the heat of dissociation of the carbon halogen bond . 
Meaning of the word marked with <WSD> tags:  the energy that an atomic system must acquire before a process (such as an emission or reaction) can occur