Intracavity laser absorption spectroscopy of HOCl overtones. I. The 3v1+2v2 band and numbers of vibrational states

The near infrared high‐resolution spectra of the a‐type transitions of the weak 3v1+2v2 combination band of transient HO35(37)Cl at 12 600 cm−1 has been recorded in an ultrasensitive titanium:sapphire intracavity laser absorption spectrometer (ICLAS). We report line assignments, new and refined anharmonicity parameters, and the spectroscopic constants for the excited rovibrational states of 3v1+2v2. The Fermi resonance perturbations in this five quanta region, where the internal energy of the molecule is already more than 2/3 of the dissociation energy E0, remain localized and they are the exception, while the extent of intermode mixing and thus intramolecular vibrational energy distribution (IVR) seems to be still small. A Dunham expansion is used for band origin predictions and representations of vibrational states N(E) of HO35Cl up to the dissociation threshold. The results are compared with harmonic and anharmonic numbers of states from a recently proposed stretch–bend coupling model.