Dynamic Orientation of Nuclei by Forbidden Transitions in Paramagnetic Resonance

Abstract

Principally from the viewpoint of orienting radionuclei, this paper considers magnetically dilute paramagnetic ions in crystals for various cases in which there is a non-negligible radio-frequency transition probability for inducing a simultaneous flipping of an electron spin and a nuclear spin. These transitions, forbidden in zero order in high magnetic fields, may be provided by hyperfine interactions, and allow for direct dynamic nuclear orientation by applied rf fields. The transition probabilities are calculated for a general anisotropic spin Hamiltonian; thermal relaxation transitions are qualitatively discussed. The resulting steady-state dynamic nuclear polarization and alignment are calculated for the equalization of populations of pairs of levels by sufficient applied rf fields. The influence of various relaxation transitions is considered and it is noted that the nuclear orientation available through the forbidden transitions is considerably less sensitive to competing relaxation transitions than that obtained by saturation of the allowed transitions.The general predictions are found to be in qualitative agreement with the results at Berkeley of Abraham and Kedzie using radionuclei.The possibilities for dynamic alignment of radionuclei of diamagnetic atoms by forbidden transitions due to weak nuclear-electron dipolar coupling are also briefly discussed.