text: The analysis of vibronic spectra of diatomic molecules provides information concerning both the ground electronic state and the excited electronic state. Data for the ground state can also be obtained by vibrational or pure rotational spectroscopy, but data for the excited state can only be obtained from the analysis of vibronic spectra. For example, the bond length in the excited state may be derived from the value of the rotational constant B′.
  In addition to stable diatomic molecules, vibronic spectroscopy has been used to study unstable species, including CH, NH, hydroxyl radical, OH, and cyano radical, CN. The Swan bands in hydrocarbon flame spectra are a progression in the C-C stretching vibration of the dicarbon radical, C for the  electronic transition. Vibronic bands for 9 other electronic transitions of C have been observed in the infrared and ultraviolet regions.