Electronic Spectra and the Franck-Condon Principle
Electronic transitions in molecules produce band systems in the visible and ultraviolet whose vibrational structure is governed by the Franck-Condon principle.
Definition
Electronic spectra are the band systems produced when a molecule changes electronic state, typically in the visible or ultraviolet; the Franck-Condon principle states that because electronic transitions are fast compared with nuclear motion, they occur vertically on the potential-energy diagram and favour final vibrational levels whose wavefunctions best overlap the initial one.
Scope
This topic covers molecular electronic spectroscopy: transitions between electronic states accompanied by changes in vibrational and rotational quantum numbers, the resulting vibronic band systems, and the Franck-Condon principle that predicts which vibrational components are most intense. It treats absorption and emission (fluorescence and phosphorescence), the role of potential-energy-surface geometry, and how electronic spectra reveal excited-state structure.
Core questions
- Why do molecular electronic transitions appear as bands rather than single lines?
- What does the Franck-Condon principle say about transition intensities?
- How does the geometry change between electronic states shape the band envelope?
- How do absorption, fluorescence, and phosphorescence differ?
Key concepts
- Vibronic transitions
- Band systems and progressions
- Franck-Condon principle and factors
- Vertical transitions
- Fluorescence and phosphorescence
- Excited-state geometry
Key theories
- Vibronic band structure
- An electronic transition is accompanied by changes in vibrational and rotational quantum numbers, so a single electronic transition appears as a system of bands, each band a vibrational component carrying rotational fine structure.
- Franck-Condon principle
- Because electrons rearrange much faster than nuclei move, transitions are vertical and the intensity of each vibrational component is proportional to the squared overlap (Franck-Condon factor) of the initial and final vibrational wavefunctions.
Clinical relevance
Electronic spectra and Franck-Condon analysis underpin ultraviolet–visible spectroscopy and fluorescence used throughout chemistry and biology, including fluorescent labelling and imaging, the characterization of dyes and photovoltaic materials, and the remote identification of electronically excited species in flames and the upper atmosphere.
History
Franck proposed in 1925 that nuclei stay essentially fixed during an electronic transition, and Condon gave the idea quantitative quantum-mechanical form in 1926–1928 through the overlap integrals now called Franck-Condon factors. The principle became central to interpreting molecular band spectra and excited-state dynamics.
Key figures
- James Franck
- Edward Condon
- Gerhard Herzberg
Related topics
Seminal works
- condon1928
- herzberg1950
Frequently asked questions
- Why are electronic transitions drawn as vertical lines?
- On a potential-energy diagram with nuclear separation on the horizontal axis, the Franck-Condon principle says nuclei barely move during the fast electronic transition, so the transition is represented by a vertical line at the initial nuclear geometry.
- What is a Franck-Condon factor?
- It is the square of the overlap integral between the vibrational wavefunctions of the initial and final electronic states. These factors determine the relative intensities of the vibrational components within an electronic band system.