Vibronic spectroscopy
（重定向自Vibronic transition）
![The Morse potential (blue) and harmonic oscillator potential (green). The potential at infinite internuclear distance is the dissociation energy for pure vibrational spectra. For vibronic spectra there are two potential curves (see Figure at right), and the dissociation limit is the upper state energy at infinite distance.](/Images/godic/202502/19/Morse-potential1912.png")
![](/Images/godic/202502/19/Franck-Condon-diagram1912.png")
![Fortrat diagram created with B′=0.8, B′′=1, showing displacement of rotational lines from the vibrational line position (at 0 cm−1). Centrifugal distortion is ignored in this diagram.](/Images/godic/202502/19/Fortrat_diagram1912.png")
![Spectrum of the blue flame from a butane torch showing excited molecular radical band emission and Swan bands due to C2.](/Images/godic/202502/19/Spectrum_of_blue_flame1912.png")
Vibronic spectra involve simultaneous changes in the vibrational and electronic energy states of a molecule. In the gas phase vibronic transitions are accompanied by changes in rotational energy also. Vibronic spectra of diatomic molecules have been analysed in detail; emission spectra are more complicated than absorption spectra. The intensity of allowed vibronic transitions is governed by the Franck–Condon principle. Vibronic spectroscopy may provide information, such as bond-length, on electronic excited states of stable molecules. It has also been applied to the study of unstable molecules such as dicarbon, C2, in discharges, flames and astronomical objects.