SHORT-LIVED CHARGE-TRANSFER-TO-SOLVENT-STATES AND MULTIPLE ELECTRONIC RELAXATIONS FOLLOWING FEMTOSECOND EXCITATION OF AQUEOUS CHLORIDE-ION

Early charge transfer processes triggered by the photoexcitation of an aqueous sodium chloride solution (molar ratio H2O/NaCl = 55) at 294 K have been investigated by using femtosecond absorption UV-IR spectroscopy. The initial UV energy deposition proceeding by one- (4 eV) and/or two-photon (2 x 4 eV) absorption results in the formation of multiple short-lived electronic states which have been discriminated within the spectral range 360-1250 nm (3.44-0.99 eV). Two well-separated spectral signatures involving UV and infrared transitions have been discriminated and assigned to different excited CTTS states (charge transfer to solvent) as recently suggested by quantum simulations of an aqueous halide I- [Sheu and Rossky, Chem. Phys. Letters 202 (1993) 186; 213 (1993) 233]. A short-lived ultraviolet component appearing within the UV pump pulse and exhibiting a monoexponential relaxation time of 190 +/- 20 fs would correspond to a low excited CTTS state of the chloride ion (CTTS*). The other ultrashort-lived band peaking above 0.99 eV (1250 nm) and characterized by a high deactivation rate (similar to 2 X 10(13) s(-1)) is tentatively assigned to a high excited CTTS state (CTTS**) triggered by a two-photon absorption process (8 eV). This transient state precedes the appearance of a well-defined infrared component peaking around 1250 nm and due to the (p-like) excited hydrated electron (e(prehyd)(-)). The relaxation of this infrared electron occurs with a time constant of 300 fs and leads to the formation of the ground state of the hydrated electron (e(hyd)(-)). Near-infrared spectroscopic investigations performed in the energy range 1.51-1.24 eV (820-1000 nm) have permitted to dearly identify the existence of additional absorption bands peaking around 880 nm. It is the first time that near-infrared bands are directly observed in an aqueous solution of halide ions. These spectral contributions are assigned to inhomogeneous populations of electron-atom pairs ({e(-):Cl}(A,B)(nH2O)). The involved photochemical channel can compete with the electron hydration channel for which a pre-hydrated state (e(prehyd)(-)) has been identified. The existence of these near-infrared states would be due to local solvent effects which assist or impede an electron localization outside the first hydration shells of the atomic core (Cl). The electronic population absorbing in the near infrared exhibits a dual behavior whose characteristic times are 330 fs ({e(-):Cl}(A)(nH2O)) and 750 fs ({e(-):Cl}(B)(nH2O)) respectively. The fastest relaxation channel due to {e(-):Cl}(A)(nH2O) is interpreted in the framework of an ultrafast electron-atom reaction within the solvation shells of the chlorine atom. The ultrafast deactivation of the {e(-):Cl}(A)(nH2O) population represents a specific electron-atom reaction which does not contribute to the early geminate recombination between the ground state of the hydrated electron and the chlorine atom. The slower deactivation channel (1.29 X 10(12) s(-1)) would be due to an electronic state ({e(-):Cl}(B)(nH2O)) whose interconversion with a ground state of a hydrated electron has been identified in the present study. This electron photodetachment pathway leads to a delayed formation of hydrated electrons (e(hyd)(-1)) and can be seen as a specific solvent cage effect in the vicinity of the counterion (Na+). The direct characterization of short-lived semi-ionized states by near-infrared spectroscopy provides new informations on solvent cage effects during ultrafast electron transfer reactions in ionic solutions. These complex photochemical data obtained with aqueous sodium chloride are discussed at the microscopic level considering recent quantum theories on semi-ionized or metastable states in ionic solutions.