Flash photolysis of intermediates in hydrated electron reactions

Abstract

A triple flash photolysis apparatus is described. Hydrated electrons (e[sup - / sub aq]) produced by U.V. flash photolysis of some common inorganic anions (OH⁻, SO[sup = / sub 4]) were monitored at 633 nm. This species was identified by its absorption spectrum and by the effect of well known electron scavengers. In the absence of an electron scavenger, e[sup - / sub aq] decayed largely by a second order process to form a comparatively long lived (τ~ms) intermediate, X. Results indicate that X is probably a dimeric hydrated electron (e[sup 2-/ sub 2])[sub aq] or a product therefrom and can be photodissociated in the 300 - 400 nm region to regenerate hydrated electrons. Metal ions Ag⁺ and Tl⁺ react with e[sup - / sub aq] to produce the respective short lived metal atoms (hyper-reduced states) which also regenerate e[sup - / sub aq] by a second flash. M⁰ + hν (λ~ 300 nm) → M⁺ + e[sup - / sub aq] Ag⁰ has an absorption spectrum with λ[sub max] at 315 nm and an oscillator strength of 0.63. This band is assigned as charge-transfer-to-solvent (CTTS). Acetone, which reacts rapidly with e[sup - / sub aq], produces a transient, presumably (CH₃)₂CO)⁻, at pH 11. The absorption spectrum of this transient was recorded, (λ[sub max] = 255 nm, e[sub max] ≈ 1.3 x 10⁴ M⁻¹ cm⁻¹) and its photo-max max ^ lysis in this region was again shown to regenerate e[sup - / sub aq]. Hence, it seems that the absorption spectra of many such transients may be CTTS bands in that they produce e[sup - / sub aq] upon photolysis. The decay of e[sup - / sub aq] was followed in the presence of some inorganic gases, e.g., SF₆, N₂O and Xe. The possibility of intermediate formation in those reactions are discussed. The carbonate radical ion, CO[sup - / sub 3], was produced during the flash photolysis of an N₂O saturated alkaline (pH 11) solution containing CO₂ and was studied by its absorption spectrum and decay kinetics. It was demonstrated that HCOO⁻ ions, methanol or 2-propanol, commonly used OH scavengers, yield e[sup - / sub aq] upon flash photolysis.