Spin Temperature in Nuclear Double Resonance

A magnetic double resonance technique is described which allows the strong resonance of one nuclear species to be used to detect the much weaker resonance of a second species. Based on the proposal of Hartmann and Hahn, the new technique is experimentally simpler, and involves no critical adjustments of experimental parameters. It involves the method of adiabatic demagnetization in the rotating reference frame. The theoretical interpretation is an adaptation of Redfield's thermodynamic approach, performed in a specially chosen reference system. The technique and theory are verified quantitatively in lithium metal using the strong resonance of the 92.6% abundant ${\mathrm{Li}}^7$ at 15 Mc/sec to detect the much weaker resonance of the 7.4% abundant ${\mathrm{Li}}^6$ at 5.679 Mc/sec.