Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2021 / Paper 2A / Section B / 22 / a / iii

The approximate mirror relationship follows from the Franck-Condon principle when the ground and excited electronic states have similar vibrational frequencies and comparable potential shapes displaced along the active coordinates. Absorption samples $S_0,v\simeq 0\to S_1,v^{\prime }$ overlaps, while relaxed emission samples $S_1,v^{\prime }\simeq 0\to S_0,v$ overlaps. For displaced harmonic potentials, the corresponding overlap intensities form similar progressions on opposite sides of the transition between the lowest vibrational levels of the two electronic states.

The symmetry is approximate and is most naturally compared on an energy or wavenumber axis. A wavelength axis is nonlinear in photon energy, and solvent response, frequency changes, differing transition strengths and instrumental corrections can spoil an exact mirror image.