Direct detection of two-quantum coherence

Abstract

A theoretical analysis of the oscillating voltage at twice the Larmor frequency (2ω0), due to quadrupolar polarization of 14N nuclei, predicts a signal at least ten orders of magnitude smaller than the magnetic-induction signal produced by the same sample. However, nonsecular parts of the quadrupolar interaction are predicted to give rise to observable magnetic-induction signals at 2ω0 that provide a monitor of "two-quantum coherence." The theoretical prediction is verified using a single crystal of NaNO2 at room temperature. The corresponding signal from a crystalline powder is zero. Some limitations and possible extensions of the experiment are proposed.