Overtone-excited HN3(X̃ 1A′): Anharmonic resonance, homogeneous linewidths, and dissociation rates

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Abstract
High‐resolution spectra have been obtained for the predissociative N–H stretching overtone levels 5ν1 (15 120 cm−1) and 6ν1 (17 670 cm−1) of HN3, cooled in a free‐jet expansion. The spectral bandwidth (Doppler limited at 0.007 cm−1 full width at half‐maximum (FWHM) is sufficiently narrow to reveal the homogeneous linewidths of individual rovibrational transitions in the 6ν1 band, for which we previously measured the dissociation lifetime in time‐domain experiments. Two distinct manifestations of vibrational coupling characterize the spectra: (a) anharmonic mixing of the N–H stretch with other vibrational motions to give a complex spectrum of vibrational eigenstates and (b) homogeneous widths of the resultant states determined by the dissociation lifetime. The results are discussed with reference to previous studies of overtone spectroscopy and intramolecular mixing. Time‐domain measurements of dissociation rates are reported for four vibrational levels with zero‐order labels 5ν1 +νx. Over the range 15 100 cm−1–17 700 cm−1, the dissociation rate increases monotonically with vibrational energy, with no apparent ‘‘mode‐specific’’ variations. This is interpreted in terms of the highly mixed character of any particular vibrational eigenstate that is excited.