Vibronic transition
Encyclopedia
A vibronic transition denotes the simultaneous change of vibrational and electronic quantum number
in a molecule
. According to the separability of electronic and nuclear motion in the Born-Oppenheimer approximation
, the vibrational transition and electronic transition may be described separately. The selection rule
for electronic transitions is given by Koopmans' theorem
, while that for vibrational transitions is described by the Franck-Condon principle
.
Most processes leading to the absorption and emission of visible light, are due to vibronic transitions. This may be contrasted to pure electronic transitions which occur in atom
s and lead to sharp monochromatic lines (e.g. in a sodium vapor lamp
) or pure vibrational transitions which only absorb or emit infrared light.
Quantum number
Quantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities. This is distinguished from classical mechanics where the values can range continuously...
in a molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
. According to the separability of electronic and nuclear motion in the Born-Oppenheimer approximation
Born-Oppenheimer approximation
In quantum chemistry, the computation of the energy and wavefunction of an average-size molecule is a formidable task that is alleviated by the Born–Oppenheimer approximation, named after Max Born and J. Robert Oppenheimer. For instance the benzene molecule consists of 12 nuclei and 42...
, the vibrational transition and electronic transition may be described separately. The selection rule
Selection rule
In physics and chemistry a selection rule, or transition rule, formally constrains the possible transitions of a system from one state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions...
for electronic transitions is given by Koopmans' theorem
Koopmans' theorem
Koopmans' theorem states that in closed-shell Hartree-Fock theory, the first ionization energy of a molecular system is equal to the negative of the orbital energy of the highest occupied molecular orbital...
, while that for vibrational transitions is described by the Franck-Condon principle
Franck-Condon principle
The Franck–Condon principle is a rule in spectroscopy and quantum chemistry that explains the intensity of vibronic transitions. Vibronic transitions are the simultaneous changes in electronic and vibrational energy levels of a molecule due to the absorption or emission of a photon of the...
.
Most processes leading to the absorption and emission of visible light, are due to vibronic transitions. This may be contrasted to pure electronic transitions which occur in atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s and lead to sharp monochromatic lines (e.g. in a sodium vapor lamp
Sodium vapor lamp
A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure...
) or pure vibrational transitions which only absorb or emit infrared light.
External links
- Definition of "vibronic transition" in the IUPAC Compendium of Chemical Terminology