Electron spin manifolds

The angles cpa and cpp define the axis of quantization for the a and electron-spin manifolds, respectively, and are... 

Figures 1 and 3 show that although the modulations of the three-pulse, or stimulated echo are less intense than those of its two-pulse counterpart, the resolution is much higher and the spectrum is simplified because combination peaks only enter into the data through the presence of multiple ESEEM-active nuclei. Equation (8) shows that for an S = 1 /2, 7 = 1/2 spin system, judicious selection of the r-value can control the ESEEM amplitudes of the hyperfine frequencies from a and electron spin manifolds allowing them to be optimized or suppressed. For weakly coupled protons, where the modulation frequencies from both electron spin manifolds are centered at the proton Larmor frequency, x can be set at an integer multiple of the proton Earmor frequency to suppress the contributions of this family of coupled nuclei from the three-pulse ESEEM spectrum. It is common for three-pulse ESEEM data to be collected at several r-values, including integer multiples of the proton Larmor period, to accentuate the other low frequency modulations present in the data and to make sure that ESEEM components were not missed because of T-suppression.

Equation (29) also applies to the case of IIN. However, naturally occurring iodine is monoisotopic with / = and the iodine nucleus can have any one of the six spin states m, = j, f, +f. The spin Hamiltonian that results is shown in matrix form in Table 4, where again the y axis is chosen as the quantizing axis for the nuclear spin. As before, r) terms mix states with Affj = 2 in the same electron spin manifold, whereas the hyperfine terms connect states in different electron spin manifolds with Am, = 0 or Am, = 1. The pro ton hyperfine terms (not included in Table 4) are relatively unimportant in this case, primarily because the — Ty energy separation is much larger. Thus, in spite of the large iodine hyperfine coupling, the simultaneous electron spin/proton spin transitions are weak and not observed at low power in triplet IIN. 

Fig. 4. Schematic energy level diagram for the lowest triplet state of I IN in zero held, showing the splitting of the and T. electron spin manifolds due to quadrupole and hyperfine interactions (y = 0 i) /9). Arrows denote the two forbidden transitions observed at 76.5 and 205.3 MHz (Kothandaraman et al., 1975).

Figure 9. HYSCORE spectra of MCRbps (see schematic for structure), (a) X-band (9.7 GHz) spectrum at 20 K, with signals assigned to Hy and Hri. (b) Q-band (35.3 GHz) spectrum at 20 K. The position of the principal values, determined from the full set of HYSCORE and ENDOR spectra, are indicated. The intense signal on the diagonal around (-5,5) MHz is due to an incomplete transfer of nuclear coherences between the two electron spin manifolds by the non-ideal n pulse. Modified with permission from [38]. Copyright 2006, Wiley-VCH.

The residual hyperfine spliding can be eliminated with the pulse sequence shown in Figure 13b. In contrast to the previous pulse sequence, the nuclear coherence during die decoupling pulses evolves now in bodi electron spin manifolds. It... 

These considerations present an experimenter with a choice as to what EPR transition is beder to use for pulsed ENDOR measurements. From the standpoint of minimizing the cfi distortions in the ENDOR spectra and simplifying their analysis, die best EPR transitions to use are those involving the electron spin manifolds with mz = 5/2. However, the width of ENDOR lines from mz = 5/2 manifolds in a disordered system is five times greater, and die amplitude is correspondingly smaller, than those of the lines from the mz = 1/2 manifolds. On the other hand, unless an extreme electron spin polarization is present, the most intense transition in the EPR spectrum is that between mz = 1/2 and -1/2 (see Fig. 5). Therefore, the... 

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