SPIN HAMILTONIAN OF 2 EQUIVALENT NUCLEI - APPLICATION OF I2- CENTERS

The second-order hyperfine splittings observed in the ESR spectrum of an unpaired electron spin interacting with two equivalent nuclei depend upon the relative magnitude of the quadrupole interaction HQ and the second-order effect Hhf2 of the hyperfine interaction Hhf. If the first-order effect of HQ is much smaller than Hhf2 (case A), then a perturbation analysis of the spin Hamiltonian can be conveniently carried out in the coupled |I1I2IMI representation, and for the allowed ESR transitions the I1+I2+1 values of I are good quantum numbers. If, however, HQ is much larger than Hhf2 (case B), then the perturbation analysis must be carried out in the |I1I2, M12+M22, MI representation, and M12+M22 replaces I as a good quantum number. The second-order hyperfine patterns are qualitatively and quantitatively quite different for the two cases. As a specific example, the ESR spectra of the VK-type I2- center in KI, and the VK- and VF-type I2- centers in Pb++-doped KCl: KI and KBr: KI are discussed. The ESR analysis is limited to the situation where the magnetic field is parallel to the internuclear axis (θ=0°), and it is shown that the I2- spectra are very well described by the case B perturbation solution. The θ=0°spectrum shows weakly allowed transitions from which the quadrupole term can be accurately determined. In these transitions M1 and M2 change by one unit but in opposite senses, so that MI remains unchanged. A short discussion of the formation and decay of the I2- centers is also given. © 1968 The American Physical Society.