Unspecified verticality of Franck-Condon transitions, absorption and emission spectra of cyanine dyes, and a classically inspired approximation.

The computed vertical energy, , from the equilibrium geometry of the initial electronic state is frequently considered as representative of the experimental excitation/emission energy, = / . Application of the quantum mechanical version of the Franck-Condon principle does not involve precise specification of nuclear positions before, after, or during an electronic transition. Moreover, the duration of an electronic transition is not experimentally accessible in spectra with resolved vibrational structure.