Question

5. What simple assumption is behind the Franck‐Condon Principle? Why do we use a concept of “vertical transitions” in understanding electronic spectra?

Answer 1

Franck-Condon Principle is based on the simple assumption that electronic transitions occur much faster than nuclear vibrations. In other words, electronic transitions can essentially be considered to be instantaneous when compared with the time scale of nuclear vibrations. This assumption is fairly correct and can be attributed to the fact that nuclei are very large compared to electrons and the electronic transition will take place faster than the nuclei can respond.

The transition of electrons involved is a vertical transition. We use vertical transitions since the internuclear distances do not change (as nuclei are assumed to be fixed in position during excitation). Hence, if we have a molecule initially in the ground state (both electronically and vibrationally), then the transition can only be represented by a vertical line.