Rodiaf. Phys. C/tern.Vol. 34, No. I, pp. S-14, 1989 Inr. J. Radiat. Appl. Instrum. Part C Printed in Great Britain. All rights reserved 01465724/89 $3.00 + 0.00 Copyright 0 1989 Pergamon Press plc ELECTRON REACTIVITY IN AQUEOUS MEDIA: A FEMTOSECOND INVESTIGATION OF THE PRIMARY SPECIES Y. GAUDIJEL,S. POMMERET, A. MIGUS and A. ANTONETTI Laboratoire d’Optique Appliquke, INSERM U275, Ecole Polytechnique-ENSTA, Batterie de I’Yvette, 91120 Palaiseau, France Abstract-The electron is universally considered as a primary species in the radiolysis and photolysis of aqueous media. The existence of this simplest aqueous radical implicates fascinating questions concerning the coupling of excess electrons with polar liquids whose chemical and physical properties may have a determinant influence on reactivity in chemistry and biology. The recent experimental results available from the femtosecond spectroscopy of water or ionic aqueous solutions provide unique experimental data for the elucidation of detailed mechanisms of mimary processes involved in electron reactivity with an essentially protic solvent. I. INTRODUCTION The primary species initiated by interactions of ioniz- ing energy with liquids are subject to numerous studies and investigations in various domains such as radiation chemistry, photochemistry and photo- physics. During the last decade, the investigation of the early events involved in the primary species formation have been greatly enhanced by experi- mental and theoretical analysis of energy deposition during radiolysis or photolysis of polar (water, alco- hols) and nonpolar liquids (hydrocarbons) (see for example, Rentzepis et al., 1973; Baxendale et al., 1973; Baxendale, 1977; Chase and Hunt, 1975; Jonah et al., 1977; Huppert et al., 1981; Kenney-Wallace and Jonah, 1982; Lewis and Jonah, 1987). case of aqueous solutions at ambient temperature, the elucidation of the primary processes (electron thermalization, localization and solvation) has always been limited by the instrumental resolution (Fig. 1). The excess electron is a primordial intermediate of radiation chemistry of liquids. This is the reason why numerous researches using pulse radiolysis of polar fluids at the nanosecond or picosecond time scale have been extensively performed to generate excess and to study the reactions of this unstable species following energy deposition in condensed matter. Although its evolution through physical and chemical stages remains only imperfectly understood, the excess electron has been used in polar liquids as a microprobe of the local structure and for the dynamics of the molecular reorganization induced by the presence of a local field or charge. The time scale of physical and chemical events involved in the absorption of energy by water mol- ecules extends from lo-% (formation of excited or ionized state of water) to lo-“s (thermal orientation of water molecules in liquid water) and to lo-‘s (formation of molecular products in the spur and diffusion out of the spur). During the interaction of ionizing radiation with the aqueous medium, the absorption of energy initiates the processes of ioniz- ation and energy exchange between excess electrons and the molecules of the solvent follows. Ionization occurs on the time scale of an electronic transition (lo-%). After the initial ionization event, radiolysis of water molecules leads, in less than 10-‘2s to the formation of solvated electron, H30+, OH and dissociation products of H20* in the spur (Fig. 1). The subsequent energy exchange inside the spur leads to complex couplings between elementary charge (electron, proton) and polar molecules (Buxton, 1987; Klassen, 1987). Radiation chemistry is also a powerful method of studying the reaction of electrons in aqueous medium. However, since the discovery of the hydrated electron (Hart and Boag, 1962), there have &en numerous unanswered issues about the mechanism involved in the electron-water molecule interaction because this technique provides at best a ps resolution (Chase and Hunt, 1975; Baxendale, 1977; Jonah et al., 1977). For instance in the specific In the specific case of the electron, these energy exchanges result in the formation of several inter- mediates states including quasi-free or dry electron (e,,), thermalized electron (e;), localized electron (e,;) and finally the solvated electron (e,,): D hv D + + eqi +e,; +e,&+e;, . (1) Although the exact nature of these different states is currently subject to debate, the hydrated electron (e,,) continues to be of prime importance in chem- istry and biology because it is the most important 5