Nuclear Magnetic Double Resonance Spectrum of Ammonia

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Abstract
If the magnitude of the second rf field is small, proton nuclear magnetic double resonance spectra of ammonia, recorded by sweeping the magnetic field, are in good agreement with a calculation based on a treatment of double resonance which neglects relaxation effects. To calculate the width of spectral lines it is assumed that one relaxation mechanism is dominant, and that linewidths are small compared to the separation of a given multiplet. With these assumptions, a linewidth can be associated with the lifetimes of states giving rise to that line. The calculation of field‐sweep double resonance spectra is discussed in detail, and calculated spectra are displayed by photographing traces formed on the on‐line oscilloscope of an IBM 7090 digital computer. For larger magnitudes of the second rf field, the treatment neglecting relaxation fails to describe adequately the observed field‐sweep double resonance spectra. A limited calculation based on the density matrix treatment of double resonance indicates that the effect of the second rf field on line shape is significant. The density matrix theory presented here has been extended to include exactly overlapping lines, but neither degenerate energy levels nor partially overlapping lines are considered.