Franck–Condon principle

Franck–Condon Principle

The Franck–Condon principle is a fundamental rule in the field of quantum mechanics and spectroscopy that explains the intensity distribution of vibrational spectra in molecular electronic transitions. Named after the physicists James Franck and Edward Condon, the principle provides a quantum mechanical description of the electronic transitions in molecules, particularly focusing on the overlap between vibrational wave functions of the initial and final states.

Overview

The Franck–Condon principle is based on the assumption that electronic transitions in molecules occur so rapidly that the nuclei of the atoms involved do not have time to move significantly during the process. As a result, the transition between electronic states can be considered vertical on a potential energy diagram, which plots the potential energy of a molecule as a function of the positions of its nuclei. This concept is often visualized through Franck–Condon diagrams, which illustrate the potential energy curves of the initial and final electronic states and the vertical transitions between them.

Theoretical Background

In quantum mechanics, the probability of a transition between two states is proportional to the square of the overlap integral of their wave functions. According to the Franck–Condon principle, the intensity of a spectral line in an electronic transition is proportional to the square of the overlap (or Franck–Condon factor) between the vibrational wave functions of the initial and final electronic states. This means that transitions are most probable between vibrational levels where the wave functions have significant overlap.

Vibrational Progressions

A direct consequence of the Franck–Condon principle is the appearance of vibrational progressions in the spectra of molecules undergoing electronic transitions. These progressions consist of a series of spectral lines corresponding to transitions between different vibrational levels of the initial and final electronic states. The relative intensities of these lines reflect the Franck–Condon factors and provide information about the molecular structure and the changes in the equilibrium positions of the nuclei during the transition.

Applications

The Franck–Condon principle has wide applications in various fields of science, including physical chemistry, molecular physics, and material science. It is essential for understanding the spectra of molecules in the gas phase and in solution, and it plays a crucial role in the study of molecular dynamics, photochemistry, and the development of photonic and electronic devices based on molecular materials.

Limitations

While the Franck–Condon principle provides a robust framework for understanding electronic transitions in molecules, it has its limitations. It does not account for the effects of nuclear motion during the electronic transition, which can be significant in some cases. Moreover, it assumes that the molecular Hamiltonian can be separated into electronic and nuclear parts, which is not always valid.

See Also

• Quantum mechanics
• Spectroscopy
• Molecular electronic transitions
• Vibrational spectroscopy

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