Multiple-Quantum Transitions in Nuclear Magnetic Resonance

Abstract

The theory of Bloch and Wangsness for nuclear magnetic resonance signals is applied to multiple-quantum transitions. In most of the NMR experiments, the energy level schemes are only slightly different from an equally spaced Zeeman pattern, so that a simultaneous absorption of several radiation quanta with the same frequency can take place. It is found that the multiplicity, or the number of quanta absorbed in the transition, is most easily determined through the specific dependence of the multiple quantum signals on the rf field amplitude. The dependence of the signals on various relaxation parameters is developed and is found to provide information about relaxation processes which is not derivable from ordinary single-quantum transitions.A method of enhancing multiple transitions by audio-modulating the radio-frequency field is described. This is helpful in cases where the frequency deviations from an equally spaced Zeeman pattern are so large that a direct multiple transition is too weak to be observed.