ESR Study of NO3 and NO2 Produced on uv Irradiation of HNO3 Solutions at 77°K

Abstract

The paramagnetic species produced on uv photolysis of nitric acid at 77°K were studied using electron spin resonance. The primary photochemical process HNO₃→OH+NO₂ is followed by OH+HNO₃→H₂O+NO₃. Three species are observed. One species is a triplet and is characteristic of the NO₂ spectrum. The second is a singlet in H¹⁴NO₃ (g=2.018) and a nonfully resolved doublet in H¹⁵NO₃. On the basis of its blue color, absorption spectrum, photodecomposition on irradiation in its wave bands, competitive elimination on addition of OH radical scavengers, and hyperfine structure it is identified as the NO₃ radical. The third species (g=2.0075), is produced as a product of the photodecomposition of NO₃, and is tentatively assigned to NO.