Birth of the Hydrated Electron via Charge-Transfer-to-Solvent Excitation of Aqueous Iodide.

A primary means to generate hydrated electrons in laboratory experiments is excitation to the charge-transfer-to-solvent (CTTS) state of a solute such as I - (aq), but this initial step in the genesis of e - (aq) has never been simulated directly using ab initio molecular dynamics. We report the first such simulations, combining ground- and excited-state simulations of I - (aq) with a detailed analysis of fluctuations in the Coulomb potential experienced by the nascent solvated electron. What emerges is a two-step picture of the evolution of e - (aq) starting from the CTTS state: I - (aq) + hν → I -* (aq) → I • (aq) + e - (aq). Notably, the equilibrated ground state of e - (aq) evolves from I -* (aq) without any nonadiabatic transitions, simply as a result of solvent reorganization. The methodology used here should be applicable to other photochemical electron transfer processes in solution, an important class of problems directly relevant to photocatalysis and energy transfer.