Primary steps of an electron-proton reaction in aqueous electrolyte solutions

We report infrared and visible femtosecond spectroscopic data on primary st eps of an electron-proton reaction in aqueous concentrated solutions ([H2O] /[HCl] = 5 and 7, [D2O]/[DCl] = 7). After an initial electron photodetachme nt 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 invol ves 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 ch aracteristic time of 550 +/- 30 fs at 294 K. A H+/Li+ substitution does not modify this IR electronic dynamics. Near-IR spectroscopic investigations p rovide direct evidence that a specific pathway participates to an ultrafast electron-proton reaction. The elementary process whose the frequency rate is 1.18 x 10(12) s(-1) involves a transient nIR state ({Cl ... e(-) ... H+} (aq)). This three-body complex is localized similar to 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 attachm ent 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 intermolec ular vibrational modes that originate from a stretching of hydrogen-bridge OH ... O. These modes would assist a complete electron attachment on the hy drated proton. The effects of a HID isotope substitution on the ultrafast e lectron-proton reaction emphasize the prevailing role of solvent molecules coordinated to protonated hydrates. (C) 2000 Elsevier Science B.V. All righ ts reserved.