Sentence:  Molecular hydrogen is by the way a very difficult species to form in the gas phase by radiative association of H atoms, mainly because the <WSD>protomolecule</WSD> formed in this way is vibrationally highly excited and cannot get rid of the energy excess to enter inside the potential well and reach stability in a time short enough to avoid breaking. 
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.