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 mu m 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 (NH3)-N-14 and (NH3)-N-15 recorded from 6400 to 6808 cm(-1) with a tunable diode laser spectrometer, we have exploited isotopic shift labeling to identify lines of the nu(3) + 2 nu(4) asymmetric stretch-bend combination band for both species. Line positions, inversion and l-doublet splittings, and nu(3) + 2 nu(4) 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 pm spectral window for environmental monitoring and remote sensing. An interesting coincidence is that the P-p(6, 6) inversion doublet for each of the nu(3) + 2 nu(4), nu(1) + nu(3), v,l + 2 nu(4) and nu(3) + nu(4) bands turns out to have been involved in previous (NH3)-N-14 monitoring studies, most notably for the much-studied nu(1) + nu(3) feature at 1.53 17 mu m or 6528.77 cm(-1) which we now find to be a close blend of a P-p(6, 6) component with the known P-p(5, 3)a line. (C) 2008 Elsevier Inc. All rights reserved.