Rotational structure of ammonia N–H stretch overtones: Five and six quanta bands

Abstract

The five and six quanta bands of the N–H overtone bands of NH3, with origins near 15 450 and 18 110 cm−1, respectively, have been rotationally analyzed. Assignments of 318 and 105 lines in the two regions have been made on the basis of microwave-detected microwave-optical double resonance spectra and of photoacoustic absorption spectra. Both regions contain hybrid bands with parallel and perpendicular components which show Coriolis interactions between the optically bright components, and frequent, generally patternless, interactions with background dark vibrational states. These interactions cause almost all lines to be broken into several components. The five quanta region also shows a strong Fermi interaction, suggested due to 4νN–H+3νa2, which causes a negative inversion splitting in the upper states. The relative strengths of the parallel and perpendicular bands are (0.3:0.7) and (0.35:0.65) for the five and six quanta regions, in contrast to lower energies where parallel bands are weak or absent. The band structure can be qualitatively explained on the basis of a local mode description as the A and E states with five and with six quanta of stretch in one of the N–H bonds. Rotational constants of these bands can be well predicted from projections based on the single quanta excitations in contrast to the bands lower in energy.