On the Mössbauer hyperfine spectra of paramagnetic clusters

The hyperfine interaction of a system containing two weakly coupled paramagnetic ions is investigated. It is shown that even a very weak interaction cannot be broken by a large magnetic field, if the Zeeman interaction of the two paramagnetic centers is identical. Small differences in the g-values, however, yield immediately a quite different hyperfine pattern which corresponds to two isolated paramagnetic centers in the case of a large external magnetic field. Mössbauer experiments on reduced bacterial ferredoxin where one molecule contains two iron clusters with S=1/2 each are used to analyze the interaction of two paramagnetic centers.