Two-photon excitation in nuclear quadrupole resonance

We introduce a novel magnetic resonance experiment in which nuclear quadrupole resonance (NQR) is observed following excitation at 1/2 the resonant frequency. Such two-photon excitation has recently been demonstrated in nuclear magnetic resonance, and may also have practical application to NQR. We show analytically that for axially symmetric quadrupole interactions (η=0), two-photon NQR in zero magnetic field produces observable magnetization only perpendicular to the rf excitation coil, necessitating a second coil for detection. If η≠0 or an external magnetic field is applied, magnetization observable in the excitation coil is produced. We demonstrate the phenomenon experimentally by exciting the 35Cl resonance in a single crystal of sodium chlorate in the presence of a weak magnetic field.