Nuclear double resonance

A new method of nuclear double resonance, which has increased sensitivity over conventional resonance techniques, is investigated both theoretically and experimentally. The technique, originally proposed by Hahn, is' based on the observation that two nuclear species, with gyromagnetic ratios γA and γB, in a solid will couple strongly to each other if they are simultaneously irradiated with alternating magnetic fields (H1)A and (H1)B at their respective resonance frequencies and if the magnitudes of the fields are adjusted such that γA(H1)A = γB(H1)B. If one of the nuclear species is abundant and has a strong resonance signal, the presence of a dilute nuclear system can be detected by observing its effect on the resonance of the abundant nuclear system. Double resonance is observed by preparing the two nuclear species in states which correspond to different spin temperatures in a suitably chosen reference frame. Thermal contact between the two species is then established, and the two spin systems come to equilibrium at a common spin temperature through cross-relaxation. A simple thermodynamic theory of the double resonance process is developed which rigorously includes the effects of the local fields. The theory is based on Redfield's hypothesis of a spin temperature in the rotating reference frame and extends Redfield's ideas to the case of a common spin temperature in two reference frames, simultaneously rotating at different frequencies. Experiments on the two isotopes of metallic lithium, Li7 (92.6 percent abundant) and Li6 (7.4 percent abundant), which demonstrate quantitative agreement with the theoretical formulas are described. The experimental results further demonstrate that the magnitudes of the two alternating fields are not critical and that the number of dilute nuclei in the solid can be determined. The experimental procedure and the apparatus are described in detail, and calculations of the cross-relaxation time in the doubly rotating reference frame are presented.published or submitted for publicationU of I Onlythesi