Transition intensities geometric calculations

For vibrationally resolved PE spectra, comparing Franck-Condon factors and PE peak intensities helps identify the geometric conformation and electronic state of the photoelectron carrier. According to the Franck Condon principle, the intensity of a vibrational peak in an electronically allowed transition is proportional to the absolute square of the overlap integral of the vibrational wave functions of the initial and final states. This overlap integral is known as the Franck Tondon factor. To calculate such factors, potential functions must be known for both the initial and final states and can be obtained by ab initio calculations. In many cases, a harmonic approximation has yielded adequate descriptions for low vibrational levels of potential wells. ... 

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