External cavity tunable diode laser spectra of the ν1 + 2ν4 stretch-bend combination bands of 14NH3 and 15NH3

The ammonia ν1 + 2ν4 perpendicular stretch-bend combination band has been investigated in spectra of 14NH3 and 15NH3 recorded in the 6400-6800 cm−1 region with an external cavity tunable diode laser (ECTDL) spectrometer. For 14NH3, new assignments were determined initially by extrapolating from published low-J jet-cooled beam results up to transitions of higher J and K. Corresponding ν1 + 2ν4 transitions for the 15NH3 species were then found by identifying similar patterns of lines with a characteristic downshift of approximately 9.7 cm−1. Assignments were confirmed employing ground-state combination differences. Term values, a-s inversion splittings, l-doubling energies and parameter estimates from simple single-state fits are reported for the two ammonia species.