Inner transformation
Internal conversion ( engl. Internal conversion , "IC") is a term used in photochemistry and spectrometry . It refers to the isoenergetic and therefore radiation -less transition from the vibrational state of an electronically excited state (z. B. S 1 ) (z. B. the electronic in a highly excited vibrational state of the next lower lying electronic state ground state S 0 ) without changing the multiplicity (No Spin reversal).
If the multiplicity changes, on the other hand, it is an intersystem crossing :
Multiplicity constant | Multiplicity is changed (slowly, since "forbidden") |
||
---|---|---|---|
under energy delivery | with emission of radiation |
Fluorescence e.g. B. |
Phosphorescence e.g. B. |
radiationless |
Oscillation or vibronic relaxation * z. B. and |
||
Energy constant |
Inner transformation z. B. |
Intersystem Crossing e.g. B. and |
) * from the excited oscillation state S x * of a certain electronic state to the respective basic oscillation state S x
Internal transformations are most likely to occur near the intersection of two potential curves . If the second potential curve belongs to an unbound continuum state , dissociation occurs . Otherwise, the transition into a vibronically excited state occurs through internal transformation, which then, with the release of energy through collisions with surrounding particles ( vibration relaxation ), changes without radiation to the basic vibration state of the respective electronic state within 10-12 seconds.
If the starting level is a higher excited electronic state (e.g. S 4 ), internal transformations with subsequent oscillation relaxation take place one after the other until the lowest electronically excited state (S 1 ) is reached. Because large molecules more vibration modes have as small, is with them i the probability of such a transition. A. higher.
The adjacent Jabłoński scheme gives an overview of the possible transitions .
The melanin of the skin and the DNA use the internal transformation to convert UV radiation, which is harmful to the organism, into harmless heat. This ultra-fast internal transformation is responsible for the excellent UV protection of these large natural molecules.
Web link
Individual evidence
- ↑ a b c Dieter Wöhrle, Michael W. Tausch, Wolf-Dieter Stohrer: Photochemistry: Concepts, Methods, Experiments . Wiley-VCH, Weinheim 1998, ISBN 3-527-29545-3 , pp. 65-66 .