Sentence:  As pointed out many years ago (see, for example, Duley and Williams, 1984), molecular hydrogen cannot form efficiently in the gas phase in the ISM, when H atoms are neutral and in the ground state, since three-body reactions are necessary to take out the excess energy quickly enough before the <WSD>protomolecule</WSD> falls apart. 
Meaning of the word marked with <WSD> tags:  A collection of unstably bound atoms that has the potential to form a molecule if the structure can shed sufficient energy.