ENDOR of trapped hydrogen atoms in gamma-irradiated glassy acid matrices and in polycrystalline oxyanion matrices

Abstract

Trapped H atoms are produced in H2SO4/H2O and H3PO4/H2O glasses and in KHSO4/H2O and KH2PO4/H2O polycrystalline matrices by γ-irradiation at 77°K. High power proton ENDOR spectra of the H atoms have been obtained and analyzed. Prominent matrix ENDOR lines at the free proton frequency together with weak sidebands are observed. Two methods have been developed for increasing the resolution of weak sidebands due to isotropic hyperfine interaction. The two methods involve high irradiation dose and the addition of paramagnetic transition metal ions. The ENDOR line shapes are analyzed in terms of modulation of the electron-nuclear dipolar interaction by the relaxing electron. The isotropic hyperfine constant is about 0.75 G in all matrices and is assigned to the nearest neighbor matrix protons 1.8 Å distant. The center matrix ENDOR lines have been analyzed by computer simulation of the line shape to arrive at the spatial extent of the H atom wavefunction. In glassy matrices the wavefunction is nearly unperturbed from that of the free atom, while in polycrystalline matrices the extreme tail of the wavefunction shows significant distortion.