Continuous-wave EPR spectrum of in at room temperature. Each line in the triplet signal is labeled with the corresponding projection of the nuclear spin. Measurement parameters: microwave frequency, ; microwave power, ; modulation amplitude, ; and modulation frequency, .
Electron spin relaxation times ( and ) of in , measured using the central line. The ratio is maintained over the full temperature range for which the solvent remains liquid.
Electron spin relaxation times ( and ) of in , measured using the high field line. ESEEM is used to resolve the individual decay rates of the inner and outer coherences [see Eq. (3)]. Dashed curves show corresponding data taken for the central line, for comparison.
The temperature dependence of of is linear in Arrhenius coordinates, consistent with the Orbach relaxation mechanism. An energy gap can be extracted. Because we cannot make a low-temperature approximation in this case, the standard Orbach plot of vs must be adjusted to include the constant of proportionality, [see Eq. (8)]. The plot is then recursively fit to fine-tune and obtain the slope, . is given in microseconds.
Temperature dependence of and times for in . For comparison, dashed lines show linear fits to the corresponding data for in (from Fig. 3).
Comparison of times for in solution with the model described in the text. The curves labeled “ZFS” are derived from Eqs. (4)–(7). The “total” fit to is achieved by combining the relaxation rate from the fluctuating ZFS model, with an intrinsic decay taken to be of . The only free parameter in the model was a constant ZFS parameter, . The contribution of the ZFS model to and both is shown to be negligible (top panel).
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