Term: predissociation https://doi.org/10.1351/goldbook.P04808 Definition: Dissociation occurring by tunnelling from a 'bound' to an 'unbound' vibronic state. In an absorption spectrum of a molecular entity, the appearance of a diffuse band region within a series of sharp bands is called predissociation, since irradiation with frequencies within the diffuse region leads to effective dissociation. The energy of the band is smaller than that of the dissociation continuum of the bound state. Related Terms: 1) dissociation (http://doi.org/10.1351/goldbook.D01801-1). 2) tunnelling (http://doi.org/10.1351/goldbook.T06531). 3) absorption spectrum (http://doi.org/10.1351/goldbook.A00043). 4) irradiation (http://doi.org/10.1351/goldbook.I03255). 5) dissociation (http://doi.org/10.1351/goldbook.D01801-2). Source: PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2266 (https://doi.org/10.1351/pac199668122223) Citation: 'predissociation' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.P04808 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. 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.