Analysis of the red and green optical absorption spectrum of gas phase ammonia

Highlights: • Visible wavekength spectra of ammonia recorded at Kitt Peak in 1980 analysed. • Analysis uses combination differences and two variational linelists. • 234 and 107 lines assigned in the red and green regions respectively. Room temperature NH₃ absorption spectra recorded at the Kitt Peak National Solar Observatory in 1980 are analyzed. The spectra cover two regions in the visible: 15,200 – 15,700 cm${}^{-1}$ and 17,950 – 18,250 cm${}^{-1}$. These high overtone rotation-vibration spectra are analyzed using both combination differences and variational line lists. Two variational line lists were computed using the TROVE nuclear motion program: one is based on an ab initio potential energy surface (PES) while the other used a semi-empirical PES. Ab initio dipole moment surfaces are used in both cases. 95 energy levels with $J=1-7$ are determined from analysis of the experimental spectrum in the 5ν_NH (red) region and 46 for 6ν_NH (green) region. These levels span four vibrational bands in each of the two regions, associated with stretching overtones.