Stationary effects, mary-spectroscopy

Degeneracy in zero field is common for any pairs with hyperfine couplings and results in the strongest line [13,14]. The position of intersection points in nonzero fields depends on the particular set of hfi parameters [15]. For instance, for an even number n 4 of equivalent nuclei with spin 1/2 the first intersection occurs always in the field H - 3A. The corresponding resonances have been observed in many systems [12]. The next weaker line was observed for tetramethylethylene radical cation [12] and for hexafluorobenzene and perfluorocyclobutane radical anions [16, 17] in experiments on photoionization. It is predicted [18] that resonances can also occur in the fields substantially exceeding the hyperfine ones if there is a difference in radical g-factors. The narrow MARY spectra have so far been recorded only in systems with known ESR spectra in order to demonstrate the phenomenon itself. However, actually, the method can be used to obtain parameters of ESR spectra and identify the unknown short-lived pairs. 

A particular property of the line in zero magnetic field is that its width is determined by the decay rate of the coherent pair state. This decay can be due to either spin relaxation or chemical transformation of pair partners. If the rate of spin relaxation in zero field is not large, as is expected for organic radical ions, the analysis of the lines allows direct determination of the rate of the chemical decay of pair partners. 

The broadening of the MARY lines was also analyzed to study the degenerate electron exchange in the irradiated alkane solutions involving cw-decalin radical cation [21] and hexafluorobenzene radical anion [20, 21]. It was found that for cis-decalin radical cation, the rate of charge transfer was determined by the number of diffusion collisions, while for hexafluorobenzene radical anion, it was lower and reached the diffusion-controlled limit only at high viscosity. 

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