Detection of radical pairs in glassy systems

Abstract

Exposure of frozen aqueous solutions of hydrogen peroxide to near UV light at 77 K gave electron paramagnetic resonance (EPR) spectra characteristic of HO^˙₂ radicals. No ˙OH radicals were detected for [H₂O₂] > 0.3 mol dm^–3, and no features for radical pairs (triplet states) were present at g= 2. However, a well defined signal at g= 4, characteristic of pairs of HO^˙₂ radicals was observed. These results are interpreted in terms of attack of ˙OH radicals on neighbouring H₂O₂ molecules, to give HO^˙₂ radicals which are trapped in pairs over a wide range of separations. Hence distant pairs with unresolved zero-field splitting are detected as normal HO^˙₂ radicals at g= 2, whilst close pairs are detected at g= 4 via the ΔM_s= 2 transition for which the zero-field splitting is isotropic (M_s is the magnetic spin quantum number). It is suggested that these results might be typical for many solid-state photochemical and radiolysis studies in which pair trapping occurs over a range of separations and, hence, it is important to study the g= 4 region of the spectra.