Electron Spin Resonance in Two Salts containing Gadolinium

The electron spin resonance spectra of Gd 3+ have been measured in gadolinium hexa-antipyrene iodide (HAPI), lanthanum HAPI, and cadmium fluoride. In CdF 2 the Gd 3+ ions are sometimes in sites of cubic and sometimes in sites of tetragonal symmetry. In the latter the zero field splitting is remarkably small, being about 0.05 cm -1 . In the HAPI there is a large zero field splitting, about 0.98 cm -1 overall, and even in the undiluted salt the line width is only about 230 gauss, so that it may be suitable for a zero field maser. An Appendix gives a discussion of the general problem of rotation of the frame of reference in which the crystal field is described so that the spin Hamiltonian may be written in a form diagonal in the applied magnetic field. The coefficients have been tabulated for the polynomials involved in the general rotation matrix for the operator equivalents. The polynomials have been plotted as a function of angle. There is also a table of factors to relate some of the untabulated matrix elements of the operator equivalents to the Wigner 3- j symbols.