The 14 μm bands of BF3–CO complexes: Isotope-dependent chaotic fine structure

Infrared absorption spectra of jet-cooled four isotopic 10,11BF3–12,13CO complexes are studied in the 14 μm region. The spectra of the 10BF3–12CO and 10BF3–13CO complexes are parallel bands with unresolved K-structure characteristic of a heavy symmetric top molecule. On the other hand, the spectra of the 11BF3–12CO and 11BF3–13CO complexes exhibit a complicated fine structure that cannot be interpreted by an ordinary semirigid molecular model. All of the observed bands are shifted to the red by 28–30 cm−1 from the ν3 band origin of uncomplexed BF3, indicating a considerable increase in bond energies by the vibrational excitation. Possible origins of the anomalous fine structure are discussed.