An electromagnetic wave may induce an oscillating electric moment in a molecule
(possibly leading to absorption if the oscillation frequency is equal to the light
The amplitude of this moment is the transition momentbetween the initial (i) and final
states (here assumed to be non-degenerate):
is the electric dipole moment
operator, a vector
operator that is the sum of the position vectors of all charged particles weighted
their charge. The transition moment
is a vector in the molecular framework, characterized both by its direction and its
- The absorption probability for linearly polarized light is proportional
to the cosine square of the angle between the electric vector of the electromagnetic
light absorption will be maximized if they are
parallel, and no absorption will occur if they are perpendicular.
- It is frequently said that a transition is polarized along the direction of
its transition moment and this direction is called the polarization direction of
- In the case of a doubly degenerate final state f, each of the two components
at the same energy has a transition moment and the two moments define a plane.
The transition is then said to be polarized in that plane, which also defines
its polarization direction(s). This is typically the case for some of the transitions
in highly symmetrical molecules.
- In the case of a vibronic transition, where both the initial and the final states
may be characterized by (different) electronic and vibrational states, the
Franck–Condon principle is often applied. This approximation separates
electronic and nuclear descriptions and allows the transition moment to be
written as a product of a purely electronic transition moment and an overlap
integral between the two vibrational wavefunctions involved.
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 434
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook