ESR Study of NO2 and NO3 in Irradiated Lead Nitrate

Lead nitrate crystals have been irradiated with electrons at room temperature and their electron spin resonance spectra examined at 77°K to reveal three paramagnetic species. One species shows no hyperfine structure and has an isotropic g tensor with a g value at the free spin value: little can be said of its nature. The others are located, by their spectral anisotropy, at nitrate sites in the crystal with the axes of their axially symmetric g tensors parallel to the nitrate triad axes. One of these species is interpreted as NO3: it shows no nitrogen hyperfine structure but a slight interaction with 207Pb. The other is identified as NO2 rotating in the plane of the nitrate site. Theoretical predictions are in accord with these assignments. Various conclusions are drawn for the radiation chemistry. Other species than the NO2— and O2, revealed by chemical analysis, are significant products. The particular lattice determines the nature of these products, their stability and, in the case of NO2, its mechanism of formation. The annealing kinetics of NO2 and NO3 differ from that of the NO2—: Mechanisms previously postulated for the latter are almost certainly too simple.