Electron spin-lattice relaxation mechanisms of radiation produced trapped electrons and hydrogen atoms in aqueous and organic glassy matrices: modulation of electron nuclear dipolar interaction by tunnelling modes in a glassy matrix

Abstract

The spin lattice relaxation of trapped electrons in aqueous and organic glasses and trapped hydrogen atoms in phosphoric acid glass has been directly studied as a function of temperature by the saturation recovery method. Below 50 to 100 K, the major spin lattice relaxation mechanism involves modulation of the electron nuclear dipolar(END) interaction with nuclei in the radical's environment by tunnelling of those nuclei between two or more positions. This relaxation mechanism occurs with high efficiency and has a characteristic linear temperature dependence. The tunnelling nuclei around trapped electrons do not seem to involve the nearest neighbour nuclei which are oriented by the electron in the process of solvation. Instead the tunnelling nuclei typically appear to be next nearest neighbours to the trapped electron. The identities of the tunnelling nuclei have been deduced by isotopic substitution and are attributed to: Na in 10 mol dm^–3 NaOH aqueous glass, ethyl protons in ethanol glass, methyl protons in methanol glass and methyl protons in MTHF glass. For trapped hydrogen atoms in phosphoric acid, the phosphorus nuclei appear to be the effective tunnelling nuclei. Below ∼10 K the spin lattice relaxation is dominated by a temperature independent cross relaxation term for H atoms in phosphoric acid glass and for electrons in 10 mol dm^–3 NaOH aqueous glass, but not for electrons in organic glasses. This is compared with recent electron–electron double resonance studies of cross relaxation in these glasses. The spin lattice relaxation of O^– formed in 10 mol dm^–3 NaOH aqueous glass was also studied and found to be mainly dominated by a Raman process with an effective Debye temperature of about 100 K.