Infrared spectroscopy of the NO₃ radical from 2000 to 3000 cm⁻¹

The present paper reports high-resolution spectroscopic study of the ¹⁴NO₃ radical in the 2000–3000 cm⁻¹ region, where eight E′-A₂′ bands from the ground state are observed. Three bands at 2206, 2246, and 2377 cm⁻¹ are analyzed for the first time, and assigned to the ν₁ + 3ν₄, 2ν₂ + 2ν₄, and ν₃ + 3ν₄ bands, respectively. Bands at 2024, 2155, 2518, and 2585 cm⁻¹ are reassigned to the ν₁ + ν₃, 2ν₃, ν₁ + ν₃ + ν₄, and 2ν₃ + ν₄ bands, respectively, by adopting the new ν₃ vibrational frequency of 1055 cm⁻¹ lower than the previous ν₃ = 1492 cm⁻¹. The band at 2902 cm⁻¹ is observed for the first time and assigned to the ν₁ + ν₃ + 2ν₄ band which is the ν₁ combined band with the 1927 cm⁻¹ band. Band intensities observed in the 2000–3000 cm⁻¹ region are attributed to the intensity borrowing from the B̃²E^′–X̃²A₂^′ electronic transition through the vibronic interaction. Although the ν₃ fundamental band has not been observed due to the cancelation of vibrational intensity and borrowed intensity, the 2ν₃ band becomes stronger than ν₃ by a factor of more than 50. Perturbation effects are recognized for the bands observed except for the 2206 cm⁻¹ and 2377 cm⁻¹ bands, and are analyzed by taking into account the Coriolis interaction in the most cases. However, the 2024 cm⁻¹ band is free from the Coriolis interaction, and the v₁–v₃ interaction is incorporated in the analysis, leading to the 2ν₁ frequency of 2008.8 cm⁻¹, which is close to the energy value of 2010 cm⁻¹ observed by a laser induced fluorescence study.