The ammonia absorption spectrum revisited between 5650 and 6350 cm⁻¹

The ammonia spectrum is revisited in the important 1.6 µm atmospheric transparency window between 5650 and 6350 cm−1 on the basis of recently recorded high-resolution Fourier transform spectra. These spectra offer an improved frequency sampling and better traceability of the measurement conditions compared to previously studied Kitt Peak spectra (Cacciani <i>et al.</i> J. Quant Spectrosc Radiat Transf 2021; 258:107334. https://doi.org/10.1016/j.jqsrt.2020.107334). Overall, 4812 14NH3 lines were measured in the 5650–6350 cm−1 region corresponding to 4866 transitions including 2066 new ones compared with our previous study, included in the HITRAN2020 database. The energy of 1023 upper state levels (including 107 new ones) was derived from 2608 assigned transitions. Independently, ammonia spectra recorded in a supersonic jet expansion using a tunable extended cavity diode laser allow us to check the absolute transition energies in the 5980–6080 cm−1 range. Accurate jet spectra calibration was achieved using CH4 reference line positions known with kHz accuracy. The position agreement with the FTS data is within the 4×10−4 cm−1 uncertainty of the positions measured in the jet expansion. A recommended line list of 5620 transitions is provided for 14NH3 in the considered 5650–6350 cm−1 region.本研究针对5650~6350 cm⁻¹的重要1.6 μm大气透过窗口内的氨光谱开展重新分析，所用数据来自新近录制的高分辨率傅里叶变换（Fourier Transform）光谱。相较于此前基于基特峰（Kitt Peak）光谱开展的研究（Cacciani等，《量子光谱与辐射传输学报》，2021；258：107334，https://doi.org/10.1016/j.jqsrt.2020.107334），本次光谱数据的频率采样精度更优，且测量条件的可追溯性更强。总体而言，本研究在5650~6350 cm⁻¹波段内共测得4812条¹⁴NH₃谱线，对应4866个跃迁，其中相较于收录于HITRAN2020数据库的此前研究，新增了2066个跃迁；通过2608个已指认的跃迁，推导得到1023个上态能级的能量值，其中包含107个新增能级。此外，本研究借助可调谐外延腔二极管激光器（tunable extended cavity diode laser）采集了超音速喷流膨胀环境下的氨光谱，以此验证5980~6080 cm⁻¹波段内跃迁的绝对能量；本次喷流光谱的高精度校准，通过采用精度可达千赫兹级的甲烷（CH₄）参考谱线位置完成，本研究所得结果与傅里叶变换光谱数据的谱线位置偏差，处于喷流膨胀测量所得谱线位置的4×10⁻⁴ cm⁻¹不确定度范围内。本研究为目标波段5650~6350 cm⁻¹内的¹⁴NH₃提供了包含5620个跃迁的推荐谱线表。