New VISTA on ammonia in the 1.5 μm region: Assignments for the ν3 + 2ν4 bands of 14NH3 and 15NH3 by isotopic shift labeling

Further insight into the manifold of vibrational combination and overtone bands in the 1.5 μm infrared spectrum of ammonia has been obtained using a vibrational isotopic shift technique for assignment (VISTA), a simple stratagem for classifying spectral lines into their respective absorption bands from their fingerprint isotopic shifts. In a comparison of spectra of 14NH3 and 15NH3 recorded from 6400 to 6808 cm−1 with a tunable diode laser spectrometer, we have exploited isotopic shift labeling to identify lines of the ν3 + 2ν4 asymmetric stretch-bend combination band for both species. Line positions, inversion and l-doublet splittings, and ν3 + 2ν4 parameter estimates from isolated-state fits are reported for the two isotopologues. This band accounts for the majority of strong ammonia lines with unreported assignments in the important 1.5 μm spectral window for environmental monitoring and remote sensing. An interesting coincidence is that the pP(6, 6) inversion doublet for each of the ν3 + 2ν4, ν1 + ν3, ν1 + 2ν4 and ν3 + ν4 bands turns out to have been involved in previous 14NH3 monitoring studies, most notably for the much-studied ν1 + ν3 feature at 1.5317 μm or 6528.77 cm−1 which we now find to be a close blend of a pP(6, 6) component with the known pP(5, 3)a line.