Solar-driven hydrogen production from the abundant and cheap electron source water is a promising way to produce renewable energy. Plants and cyanobacteria have developed a water splitting enzyme which is able to oxidize water into molecular oxygen, protons and electrons using visible light energy within the membrane protein photosystem II. The heart of the enzyme is a Mn4CaO5 cluster at which water oxidation takes place following four sequential light-induced steps. Reactions at the Mn4CaO5 cluster consist of concerted electron and proton transfer, and form intermediate states that minimize the activation energy necessary for the water oxidation process. Photosystem II is thus a paradigm for engineering bio-inspired solar energy converting applications. A recent high-resolution three-dimensional structure of photosystem II gave a precise arrangement of the Mn-Ca cluster necessary for water oxidation. In addition, the combination of theoretical catalytic models with experimental data from numerous state-of-the-art spectroscopic techniques have given a possible view of Mn oxidation states during water oxidation, of water fixation steps, have revealed the importance of a set of amino acids in the catalytic mechanism, and given hints on proton transfer reactions involving extended hydrogen bonding networks. Despite these remarkable progresses in recent years, key questions remain opened. They concern the position of reactive molecules, the formation mechanisms of the oxygen molecule itself, and relaxation processes at the Mn4CaO5 cluster involving spin-state transitions and concerted electron and proton transfer. The PS2FIR project will contribute to answer these questions. We will gather the complementary expertise of three research teams: Team 1, R. Hienerwadel & C. Berthomieu, UMR 7265; Team 2, A. Boussac UMR 9198; and Team 3, J.B. Brubach & P. Roy, Synchrotron SOLEIL, to probe light- and near infared (NIR)- induced transitions and spin conversions at the Mn4CaO5 cluster, using state-of-the-art far-infrared FTIR difference spectroscopy. Vibrational modes in the far-infrared down to 10 cm-1 will allow probing the valence state of the Mn ions, cluster conformation, and Mn-O/Ca-O interactions. Of particular interest will be the identification of libration and connectivity modes of water molecules associated to the cluster below 300 cm-1 during the reaction cycle. To overcome the challenge of exploiting small-bands in the Far-infrared domain, setups will be optimized to probe different samples in parallel and to optimize NIR-induced spin-state transitions by controlled temperature jumps. We will also benefit from the brilliance of the synchrotron AILES beamline at SOLEIL. Highly resistant photosystem II from Thermosynechoccocus elongatus prepared to precisely select within heterogeneous oxidation or spin states of photosystem II will allow to decipher the molecular origin of different Mn4CaO5 cluster conformations, and ultimately to contribute to our understanding of water oxidation and O-O bond formation.