Inner transformation

This article deals with the internal transformation in spectroscopy. For the type of decay, see internal conversion .

Jabłoński diagram

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. ${\displaystyle S_{1}\rightarrow S_{0}}$	Phosphorescence e.g. B. ${\displaystyle T_{1}\rightarrow S_{0}}$
	radiationless	Oscillation or vibronic relaxation * z. B. and ${\displaystyle S_{0}^{*}\rightarrow S_{0}}$ ${\displaystyle T_{1}^{*}\rightarrow T_{1}}$
Energy constant		Inner transformation z. B. ${\displaystyle S_{1}\rightarrow S_{0}^{*}}$	Intersystem Crossing e.g. B. and ${\displaystyle S_{1}\rightarrow T_{1}^{*}}$ ${\displaystyle T_{1}\rightarrow S_{0}^{*}}$

) * 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

• Fast internal conversion in DNA

Individual evidence

1. ↑ ^{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 .