Physics - Molecular Theory of the Living Cell - page 17

Fig. 2.3 A schematic representation of the Franck-Condon principle (Reproduced from http://en.
wikipedia.org/wiki/Franck-Condon_principle ). The upward arrow indicates the most favored
vibronic (i.e., both vibrational and electronic) transition predicted by the Franck-Condon princi-
ple. The downward arrow indicates electron transfer from the electronic excited state, E 1 , to the
ground electronic state, E 0 . See text for more detail

electronic state (see the vertical upward arrow in Fig. 2.3 ) which rapidly decays to
the ground vibrational level, v 0 ¼ 0, from which electron transfer is most likely to
occur to the excited vibrational level of the ground electronic state, i.e., v 00 ¼ 2 (see

the downward arrow), with the concomitant emission of the photon or fluorescence.
A year later, Born and Oppenheimer justified what later became known as the
Franck-Condon principle in terms of the large mass difference between the electron
and average nuclei in a molecule (Born and Oppenheimer 1927). The proton is 1,836
times as massive as the electron.
The Born-Oppenheimer approximation is also known as the “adiabatic path-
way” meaning that there is a complete separation between nuclear and electronic
motions within atoms. Although this approximation has been found to be generally
valid in atomic and molecular spectroscopy and in chemical reactions, there are also
well-established exceptions, which are referred to as “nonadiabatic pathways,” or
“non-Born-Oppenheimer coupling” (Bowman 2008; Garand et al. 2008).