ENDOR intensity

Relative signs of hyperfine and quadrupole coupling constants. In an EPR spectrum with resolved hfs the relative signs of hf and quadrupole coupling constants may be determined from the dependence of the ENDOR intensities on the nuclear quantum number mi of the saturated EPR line . For two magnetically equivalent 1 = 1 nuclei, the signs can be obtained from similar, though more complex, intensity patterns . ... 

Powder ENDOR lines are usually broadened by the anisotropy of the hyperfine couplings. The parameters of well resolved spectra can be extracted by a visual analysis analogous to that applied in ESR. The principle is indicated in Fig. 3.25 for an 5 = V2 species with anisotropic H hyperfine structure, where the hyperfine coupling tensor of axial symmetry is analysed under the assumption that 0 < A < Aj. < 2 vh- The lines for electronic quantum numbers ms = V2 and -Vi, centered at the nuclear frequency vh 14.4 MHz at X-band, are separated by distances equal to the principal values of the hyperfine coupling tensor as indicated in the figure. Absorption-like peaks separated by A in the 1st derivative spectrum occur due to the step-wise increase of the amplitude in the absorption spectrum, like in powder ESR spectra (Section 3.4.1). The difference in amplitude commonly observed between the ms = /2 branches is caused by the hyperfine enhancement effect on the ENDOR intensities first explained by Whiffen [45a]. The effect of hyperfine enhancement is apparent in Figs. 3.25 and 3.26. 

The principal values and even the orientation of the principal axes of the Na hyperfine coupling tensor with respect to axes of the g tensor could be determined from Mims and Davies pulsed ENDOR spectra, refer to Section 2.3.3 in Chapter 2. The values Axx( Na) = Ayy( Na) = 6.3 and Azz( Na) = 10.9 MHz were obtained by simulation taking angular selection into account. The so-called hyperfine enhancement of ENDOR intensities due to the interaction between the radio frequency field and the electron spin could lead to pronounced differences in the ENDOR intensities between signals from different rris electron spin states in experiments at conventional MW frequencies such as in X-band, but also at the W-band. The Na (I = 3/2) nuclear quadrupole tensor is almost coaxial to the A tensor, 2zz = 0.48 MHz, Qyy = -0.07 MHz, and Qxx = -0.41 MHz. Simulation of orientation-selective ENDOR spectra as described in [26, 33] serves to refine the principal values of the hyperfine coupling tensors estimated from experiment. In... 

From the ENDOR spectrum of I in frozen RC solution (100 K) at least 16 hyperfine splittings were obtained (see Fig. 2). Except for an effect on the overall ENDOR intensity, long (2 min) or brief... 

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