Non-radiative energy dissipation

Bratt CE, Arvidsson P-0, Carlsson M, Akerlund H-E (1995) Regulation of violaxanthin de-epoxidase activity by pH and ascorbate concentration. Photosynth Res 45 169-175 Brugnoli E, Cona A and Lauteri M (1994) Xanthophyll cycle components and capacity for non-radiative energy dissipation in sun and shade leaves of Ugustrum ovalifolium exposed to conditions limiting photosynthesis. Photosynth Res 41 451-463... 

A regulatory mechanism to dissipate excess of excitation energy during high light exposure has been proposed to partially protect leaves from photoinhibition of PS2 photochemistry <9, 10). This mechanism is the non-radiative energy dissipation of excess excitation energy where the xanthophylls cycle may play an Important role (11). 

Dissipation of non radiative energy with a rate constant k. ... 

Fig. 21. a) Radiative and non-radiative processes dissipate energy competitively to bridge an energy difference or gap. The non-radiative processes require the emission of a number of phonons thus the non-radiative probability decreases as the number of phonons increases, b) Non-radiative decay rates plotted against the energy gap to the next lowest level for the excited states of various trivalent lanthanides in different hosts. The experimental gap law, eq. (10), is seen to apply. Hash marks denote the span of radiative decay rates encountered in these systems, thus if the non-radiative probability is below the hashed region fluorescence will be observed. After Riseberg and Weber (1976) and Imbusch and Kopelman (1981). 

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

Over the course of fluorescence, which accompanies energy relaxation, the molecule can keep part of the energy it received in the form of vibrational energy of the ground state. This excess vibrational energy is dissipated by collisions or other non-radiative processes called vibrational relaxation. The emission of lower energy photons is also possible and gives rise to fluorescence in the mid infrared. 

Consider, now, how the energy is dissipated. In the absence of B, A may lose its energy either as fluorescence emission or in some non-radiative process such as interaction with the solvent. On the convention used by Forster and Weller the rate coefficients or probabilities for these two processes are denoted by and sec S respectively. The lifetime Tq of the exdted spedes is then ( f+nquantum yield < >o of the fluorescence process is f/( f+nthird method of energy dissipation from A is by reaction with B for this a pseudo-first order rate coeffident k2C sec may be assigned. The lifetime of A (x) is now given by... 

---