The External Heavy Atom Effect on S-T Transitions

In this subsection we describe a more specific type of application that now is possible within the framework of the response methodology, namely the computation and interpretation of heavy atoms effects on S-T absorption spectra. In ref. [150] a new interpretation for the cause of the external heavy atom effect on S-T transitions was thus proposed basing on response theory results for X + C2H4, X = F, Cl, Br, HCl, Ar as model systems. It was found that this effect should be interpreted as an increased SOC due to back-charge-transfer from the heavy atom, which contrasted the more conventional interpretations in terms of hydrocarbon charge penetration into the heavy atom or in terms of an exchange induced effect. 

The internal and external heavy atom effects, IHA and EHA, have attracted a considerable attention in the community of molecular spectroscopists. This is part of an old problem of understanding environmental effects from solvents or solid matrices on S-T absorption or on phosphorescence of solute molecules. For higher temperature studies the triplet decay is quenched either by collision or by vibrational interaction with the matrix or the solvent. The molecules subject to studies in this respect have mostly been aromatic molecules perturbed by molecular oxygen, nitric oxide or other paramagnetic molecules, molecules either with heavy atoms and/or forming charge transfer complexes. 

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