We recently demonstrated that N-2 reduction by nitrogenase involves the obligatory release of one H-2 per N-2 reduced. These studies focus on the E-4(4H) "Janus intermediate", which has accumulated four reducing equivalents as two [Fe-H-Fe] bridging hydrides. E-4(4H) is poised to bind and reduce N-2 through reductive elimination (re) of the two hydrides as H-2, coupled to the binding/reduction of N-2. To obtain atomic-level details of the re activation process, we carried out in situ 450 nm photolysis of E-4(4H) in an EPR cavity at temperatures below 20 K. ENDOR and EPR measurements show that photolysis generates a new FeMo-co state, denoted E-4(2H)*, through the photoinduced re of the two bridging hydrides of E-4(4H) as H-2. During cryoannealing at temperatures above 175 K, E-4(2H)* reverts to E-4(4H) through the oxidative addition (oa) of the H-2. The photolysis quantum yield is temperature invariant at liquid helium temperatures and shows a rather large kinetic isotope effect, KIE = 10. These observations imply that photoinduced release of H-2 involves a barrier to the, combination of the two nascent H atoms, in contrast to a barrierless process for monometallic inorganic complexes, and further suggest that H-2 formation involves nuclear tunneling through that barrier. The oa recombination of E-4(2H)* with the liberated H-2 offers compelling evidence for the Janus intermediate as the point at which H-2 is necessarily lost during N-2 reduction; this mechanistically coupled loss must be gated by N-2 addition that drives the re/oa equilibrium toward reductive elimination of H-2 with N-2 binding/reduction.