Primary steps of an electron–proton reaction in aqueous electrolyte solutions Original Research Article

We report infrared and visible femtosecond spectroscopic data on primary steps of an electron–proton reaction in aqueous concentrated solutions ([H2O]/[HCl]=5 and 7, [D2O]/[DCl]=7). After an initial electron photodetachment triggered by a two-photon UV excitation of aqueous chloride ion, a first electronic channel appears with a time constant of 130±10 fs and involves a IR p-like state ({e−IR}p→s). This transient IR state exhibits a deactivation process toward the hydrated electron ground state with a characteristic time of 550±30 fs at 294 K. A H+/Li+ substitution does not modify this IR electronic dynamics. Near-IR spectroscopic investigations provide direct evidence that a specific pathway participates to an ultrafast electron–proton reaction. The elementary process whose the frequency rate is 1.18×1012 s−1 involves a transient nIR state ({Cl⋯e−⋯H+}aq). This three-body complex is localized ∼1 eV below the level of {e−IR}p→s. We conclude that the 4s-like character of nIR {Cl⋯e−⋯H+}aq would be more favorable for an efficient electron attachment on the hydrated proton than a 2p-like state IR prehydrated electron. A low frequency band (270–560 cm−1:0.0334–0.0694 eV) characterizing a short-lived three-body complex {Cl⋯e−⋯H+}aq is assigned to intermolecular vibrational modes that originate from a stretching of hydrogen-bridge OH⋯O. These modes would assist a complete electron attachment on the hydrated proton. The effects of a H/D isotope substitution on the ultrafast electron–proton reaction emphasize the prevailing role of solvent molecules coordinated to protonated hydrates.