FTIR spectroscopy of NO₃: Perturbation analysis of the ν₃+ν₄ state

High-resolution Fourier transform infrared spectra of the ¹⁵NO₃ ν₃+ν₄ and ν₃+ν₄-ν₄ bands were observed in the 1472 and 1112 cm^-1 regions. Compared with the case of ¹⁴N species, large effects of perturbations were recognized in many rotational levels of the ¹⁵NO₃ ν₃+ν₄ state, and it was found that the ν₂+2ν₄ state is responsible for the perturbation. Although a direct Coriolis interaction (Δν₂ = 1, Δν₃(or Δν ₄)=1) is not present between these two vibrational levels, anharmonic terms including Φ₃₄₄ and Φ₄₄₄ mix ν₃+ν₄ and 3ν₄, ν₂+2ν ₄, and ν₂+2ν₄ mixes with ν₂+ν₄ to produce Coriolis interaction between ν₃+ν₄ and ν₂+2ν₄. An analysis gave the energy difference of 7.274 cm^-1 between two levels, and interaction parameters were determined. Similar perturbation analysis was applied for the ¹⁴N species, and the previous ^pP(N,K) assignment of the ν₃+ν₄ A′-ν₄ E′ band was changed for giving one A₂′ state. Spectral lines to another A₁′ state were not assigned because of weak intensity, which is explained by intensity anomaly through vibronic interaction, reflecting the transition moment of the B̃²E′- X̃²A₂′ electronic band.