A (FINAL ?) INVESTIGATION INTO THE MECHANISM OF THE ANTIPHASE STRUCTURE (APS) OF TRANSIENT ESR SPECTRA.
Anatole A. Neufeld and J. Boiden Pedersen.
Institut for Fysik og Kemi, Syddansk Universitet, DK-5230 Odense M, Denmark;
Abteilung Spektroskopie und photochemische Kinetik, Max-Planck-Institut für biophysikalische Chemie, Am Fassberg, D-37077 Göttingen, Germany
The experimental observation of an anti-phase structure in ESR spectra of short lived radical pairs in micelles  and biradicals  is by far the observation of magnetic field effects that has initiated the largest number of mechanism for its explanation. Although the initial models could qualitatively reproduce some of the features of the anti-phase it was unlikely that they could quantitatively explain the observations. Rather late, it was shown that none of these models were able to give rise to the effect , and thus the underlying physical mechanism remained unknown. All the initial models required a spatial region with a small exchange interaction. By performing accurate calculations with and without such a region, it was shown that APS was independent of this region. Regrettably, the first proposed mechanism  is still occasionally used to interpret experimentally observed APS spectra, probably because of its elegantly simplicity. However, the parameters determined by this method has no physical significance.
We have previously shown that APS can be understood in terms of the three polarization contributions . The longitudinal (z) component gives rise to CIDEP spectra, i.e. Lorentzian lines with varying intensity from E to A. The transversal components (x and y) give rise to APS, i.e. to a dispersive line shape, of opposite phases, where the main contribution comes from the x-component, the direction of the mw field in the rotating system. Both effects are created by the same physical mechanism, i.e. a two step coherent reencounter mechanism. In order to further prove the correctness of this mechanism, we have performed a series of calculations of the time development of the APS polarization. This is a much stronger test of the mechanism and shows several interesting behaviours, e.g. a two frequency dependence and that APS is created only at short times during the lifetime af the radical pair in the micelle. We have benefited from the impressive experimental works of Tarasov , who has adopted a different mechanism.
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Emneord: transient ESR spectra, antiphase structure, mechanism
|Period||10. Jul 2007|
|Event title||10th International Symposium on Magnetic Field and Spin Effects in Chemistry and Related Phenomena: null|
- transient ESR spectra, antiphase structure, mechanism