How to Probe\nHydrated Dielectrons Experimentally: <i>Ab Initio</i> Simulations\nof the Absorption Spectra of Aqueous\nDielectrons, Electron Pairs, and Hydride

In the radiation chemistry of water, two hydrated electrons\n(<i>e</i><sub>hyd</sub><sup>−</sup>) can react to form H<sub>2</sub> and OH<sup>–</sup>. Experiments\nand simulations suggest that this reaction occurs through a mechanism\ninvolving colocalization of two <i>e</i><sub>hyd</sub><sup>−</sup>’s into the same\nsolvent cavity, forming a hydrated dielectron ((ehyd)22−) intermediate, with aqueous hydride (H<sup>–</sup>) as a subintermediate. However, there has been no\ndirect experimental observation of either (ehyd)22− or H<sup>–</sup>. Here, we present\nTD-DFT-based predictions for the absorption spectrum of open-shell-singlet\nand triplet <i>e</i><sub>hyd</sub><sup>‑</sup> pairs, (ehyd)22−, and H<sup>–</sup>. We find that\nrelative to <i>e</i><sub>hyd</sub><sup>−</sup>, triplet and open-shell singlet electron\npairs show spectral shifts to the blue and red, respectively. Additionally,\nwe find that (ehyd)22− absorbs even further to the red, and that\nH<sup>–</sup> has a charge-transfer-to-solvent-like transition\nat wavelengths several eV to the blue, providing a direct experimental\nhandle with which to probe these species. We propose a three-pulse\ntransient absorption experiment that should allow detection of (ehyd)22− and H<sup>–</sup>.