Analysis of electron spin stimulated echo modulation in randomly oriented solids: 15N modulation of bacteriochlorophyll a radical cation

An improved and more reliable method for the analysis of the electron spin stimulated echo modulation in randomly oriented solids is presented. In this method the uncertainty in the decay function of the spin echo modulation envelope is largely eliminated by dividing modulation traces of various tau in the three pulse spin echo measurements. A nonlinear least squares fit procedure is then used to determine the hyperfine couplings. We have applied this method to determine the isotropic and the anisotropic hyperfine coupling constants of the 15N nuclei in the bacteriochlorophyll a radical cation. In our model the hyperfine tensors of four nitrogen nuclei are assumed to be axially symmetric and parallel to each other. The results are consistent with the reported values obtained by the electron nuclear double resonance technique