  Photosynthetic water oxidation is a fundamental process that sustains the
biosphere. A Mn$_{4}$Ca cluster embedded in the photosystem II protein
environment is responsible for the production of atmospheric oxygen. Here,
time-resolved x-ray emission spectroscopy (XES) was used to observe the process
of oxygen formation in real time. These experiments reveal that the oxygen
evolution step, initiated by three sequential laser flashes, is accompanied by
rapid (within 50 $\mu$s) changes to the Mn K$\beta$ XES spectrum. However, no
oxidation of the Mn$_{4}$Ca core above the all Mn$^{\text{IV}}$ state was
detected to precede O-O bond formation. A new mechanism featuring
Mn$^{\text{IV}}$=O formation in the S$_{3}$ state is proposed to explain the
spectroscopic results. This chemical formulation is consistent with the unique
reactivity of the S$_{3}$ state and explains facilitation of the following
S$_{3}$ to S$_{0}$ transition, resolving in part the kinetic limitations
associated with O-O bond formation. In the proposed mechanism, O-O bond
formation precedes transfer of the final (4$^{\text{th}}$) electron from the
Mn$_{4}$Ca cluster, in agreement with experiment.
