Photochemical activation of carbon dioxide in Mg⁺(CO₂)(H₂O)_0,1

We combine multi-reference ab initio calculations with UV-VIS action spectroscopy to study photochemical activation of CO₂ on a singly charged magnesium ion, [MgCO₂(H₂O)_0,1]⁺, as a model system for the metal/ligand interactions relevant in CO₂ photochemistry. For the non-hydrated species, two separated Mg⁺ 3s-3p bands are observed within 5.0 eV. The low-energy band splits upon hydration with one water molecule. [Mg(CO₂)]⁺ decomposes highly state-selectively, predominantly via multiphoton processes. Within the low-energy band, CO₂ is exclusively lost within the excited state manifold. For the high-energy band, an additional pathway becomes accessible: the CO₂ ligand is activated via a charge transfer, with photochemistry taking place on the CO₂ ^- moiety eventually leading to a loss of CO after absorption of a second photon. Upon hydration, already excitation into the first and second excited state leads to CO₂ activation in the excited state minimum; however, CO₂ predominantly evaporates upon fluorescence or absorption of another photon.