First analysis of the ν₂+ν₇ and ν₂+ν₉ and ν₂+ν₆ combination bands of HNO₃: evidence of perturbations due to large amplitude OH torsion in the 2¹9¹ excited state

This paper is the first of two back-to-back works, whose goal is to update the existing line positions and intensities spectroscopic parameters for the 5.8 µm region of nitric acid in the HITRAN or GEISA spectroscopic databases. This first study is devoted to the generation of first linelists for the ν₂+ν₉-ν₉, ν₂+ν₇-ν₇ and ν₂+ν₆-ν₆ hot bands in the 5.8 µm region, whereas the second study will concern the positions and intensities for the ν₂ band centred at 1709.567 cm⁻¹. High-resolution Fourier transform spectra were recorded in the 5.8 µm and in the 4.2–4.6 µm spectral ranges. The identification of the ν₂+ν₉, ν₂+ν₇ and ν₂+ν₆ bands of HNO₃ at 2165.0036, 2285.9657 and 2351.216 cm⁻¹, respectively, was performed, and several assignments were confirmed by searching for ν₂+ν₉-ν₉ and ν₂+ν₇-ν₇ transitions within those of ν₂ at 5.8 µm. Both ν₂+ν₆ and ν₂+ν₉ bands are highly perturbed. Surprisingly, both ν₂+ν₉ and ν₂+ν₉-ν₉ bands exhibit large amplitude torsional splittings of ∼0.043 cm⁻¹. This is presumed to be due to the existence of an anharmonic resonance that couples together the 2¹9¹ energy levels with those of a dark state involving high excitation in the ν₉ large amplitude OH torsional mode.本研究为两篇连续发表的配套论文的第一篇，旨在更新HITRAN与GEISA光谱数据库中硝酸在5.8 µm频段内的现有谱线位置与强度光谱参数。本首项研究聚焦于生成5.8 µm频段内ν₂+ν₉-ν₉、ν₂+ν₇-ν₇以及ν₂+ν₆-ν₆热谱带的首批谱线列表，而第二项研究将针对中心波数为1709.567 cm⁻¹的ν₂基频谱带的位置与强度展开。本研究在5.8 µm以及4.2~4.6 µm的光谱范围内采集了高分辨率傅里叶变换（Fourier Transform）光谱。研究完成了硝酸（HNO₃）分别在波数2165.0036 cm⁻¹、2285.9657 cm⁻¹以及2351.216 cm⁻¹处的ν₂+ν₉、ν₂+ν₇与ν₂+ν₆谱带的归属工作，并通过在5.8 µm频段的ν₂谱带中检索ν₂+ν₉-ν₉以及ν₂+ν₇-ν₇跃迁，验证了部分谱线的指认。ν₂+ν₆与ν₂+ν₉谱带均存在显著的微扰。令人意外的是，ν₂+ν₉与ν₂+ν₉-ν₉谱带均展现出约0.043 cm⁻¹的大振幅扭转分裂。推测这一现象源于存在一处非谐共振：其将2¹9¹能级与另一个暗态能级耦合在一起，该暗态对应ν₉大振幅OH扭转模式下的高激发态。