SHORT-LIVED CHARGE-TRANSFER-TO-SOLVENT-STATES AND MULTIPLE ELECTRONICRELAXATIONS 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 spectros copy. The initial UV energy deposition proceeding by one- (4 eV) and/o r two-photon (2 x 4 eV) absorption results in the formation of multipl e short-lived electronic states which have been discriminated within t he spectral range 360-1250 nm (3.44-0.99 eV). Two well-separated spect ral signatures involving UV and infrared transitions have been discrim inated 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 U V 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 (12 50 nm) and characterized by a high deactivation rate (similar to 2 X 1 0(13) s(-1)) is tentatively assigned to a high excited CTTS state (CTT S*) triggered by a two-photon absorption process (8 eV). This transie nt state precedes the appearance of a well-defined infrared component peaking around 1250 nm and due to the (p-like) excited hydrated electr on (e(prehyd)(-)). The relaxation of this infrared electron occurs wit h a time constant of 300 fs and leads to the formation of the ground s tate of the hydrated electron (e(hyd)(-)). Near-infrared spectroscopic investigations performed in the energy range 1.51-1.24 eV (820-1000 n m) have permitted to dearly identify the existence of additional absor ption bands peaking around 880 nm. It is the first time that near-infr ared bands are directly observed in an aqueous solution of halide ions . These spectral contributions are assigned to inhomogeneous populatio ns of electron-atom pairs ({e(-):Cl}(A,B)(nH2O)). The involved photoch emical channel can compete with the electron hydration channel for whi ch a pre-hydrated state (e(prehyd)(-)) has been identified. The existe nce of these near-infrared states would be due to local solvent effect s which assist or impede an electron localization outside the first hy dration shells of the atomic core (Cl). The electronic population abso rbing in the near infrared exhibits a dual behavior whose characterist ic 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)(nH 2O) is interpreted in the framework of an ultrafast electron-atom reac tion within the solvation shells of the chlorine atom. The ultrafast d eactivation of the {e(-):Cl}(A)(nH2O) population represents a specific electron-atom reaction which does not contribute to the early geminat e 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 inte rconversion with a ground state of a hydrated electron has been identi fied 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 counteri on (Na+). The direct characterization of short-lived semi-ionized stat es by near-infrared spectroscopy provides new informations on solvent cage effects during ultrafast electron transfer reactions in ionic sol utions. These complex photochemical data obtained with aqueous sodium chloride are discussed at the microscopic level considering recent qua ntum theories on semi-ionized or metastable states in ionic solutions.