{"term":{"id":"06460","title":"transition moment","longtitle":"IUPAC Gold Book - transition moment","doi":"10.1351\/goldbook.T06460","code":"T06460","status":"current","synonym":"<em>synonyms<\/em>: transition dipole moment, electronic transition moment","definitions":[{"id":1,"text":"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 frequency). The amplitude of this moment is the transition moment between the initial (i) and final (f) states (here assumed to be non-degenerate): \\[M_{\\rm{if}} = \\, < \\rm{f} \\mid M_{\\rm{op}}\\mid \\rm{i} >\\] where \\(M_{\\rm{op}}\\) is the electric dipole moment operator, a vector operator that is the sum of the position vectors of all charged particles weighted with their charge. The transition moment \\(M_{\\rm{if}} \\) is a vector in the molecular framework, characterized both by its direction and its probability.","notes":{"1":"The absorption probability for linearly polarized light is proportional to the cosine square of the angle between the electric vector of the electromagnetic wave and \\(M_{\\rm{if}} \\); light absorption will be maximized if they are parallel, and no absorption will occur if they are perpendicular.","2":"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 the transition.","3":"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.","4":"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."},"links":[{"term":"Franck–Condon principle","url":"https:\/\/goldbook.iupac.org\/terms\/view\/F02510"},{"term":"angle","url":"https:\/\/goldbook.iupac.org\/terms\/view\/A00346"},{"term":"electric dipole moment","url":"https:\/\/goldbook.iupac.org\/terms\/view\/E01929"},{"term":"overlap integral","url":"https:\/\/goldbook.iupac.org\/terms\/view\/O04357"},{"term":"polarization","url":"https:\/\/goldbook.iupac.org\/terms\/view\/P04712"},{"term":"probability","url":"https:\/\/goldbook.iupac.org\/terms\/view\/P04855"},{"term":"vibronic transition","url":"https:\/\/goldbook.iupac.org\/terms\/view\/V06618"}],"sources":["PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 434 (https:\/\/doi.org\/10.1351\/pac200779030293)"]}],"altoutputs":{"html":"https:\/\/goldbook.iupac.org\/terms\/view\/T06460\/html","xml":"https:\/\/goldbook.iupac.org\/terms\/view\/T06460\/xml","plain":"https:\/\/goldbook.iupac.org\/terms\/view\/T06460\/plain"},"citation":"Citation: 'transition moment' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351\/goldbook.T06460","license":"The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/) for individual terms.","collection":"If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org .","disclaimer":"The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.","accessed":"2026-05-14T18:34:04+00:00"}}