The NH stretch in pyrrole: A study of the fundamental (Δv=1) and third overtone (Δv=4) bands in the bulk gas and in a molecular beam

Abstract

We have examined the NH stretching vibration in pyrrole both in the infrared and visible regions of the spectrum. Three techniques were used—direct absorption spectroscopy, intracavity photoacoustic spectroscopy, and optothermal molecular beam spectroscopy. In the bulk gas, the Δv=1 transition is composed of a well‐resolved manifold of subbands. Molecular beam spectra of a number of those subbands reveal detailed structure due to single state‐to‐state transitions. An asymmetric rotor model was found to provide an adequate description of the spectra and a complete set of rotational parameters has been determined. Little evidence of rotational perturbations was observed in the Δv=1 spectrum. The bulk gas Δv=4 overtone band is composed of three vibrational transitions, whose rotational contours overlap. These transitions involve Fermi resonance among assignable NH and CH stretching excited states. Attempts to detect Δv=4 overtone transitions on the molecular beam failed, in spite of the large anticipated signal‐to‐noise ratio calculated by extrapolation from the Δv=1 data. This negative result is a likely consequence of extensive level mixing in the Δv=4 region.