Measurement of dipolar structure of 17O nuclear quadrupole resonance lines by three-frequency irradiation.

A modification of nuclear quadrupole double resonance with coupled multiplet is proposed which can be used for the measurement of the dipolar structure of the 17O nuclear quadrupole resonance lines in case of a strong 1H--17O dipolar interaction. The technique is based on magnetic field cycling between a high magnetic field and zero magnetic field and on the simultaneous application of three rf magnetic fields with the frequencies that are close to the three 17O NQR frequencies nu(5/2-1/2)>nu(5/2-3/2)nu(3/2-1/2) during the time spent in zero static magnetic field. When the sum of the two lower irradiation frequencies nu(1)+nu(2) is not equal to the highest irradiation frequency nu, the three-frequency irradiation increases the proton relaxation rate in zero magnetic field and consequently decreases the proton NMR signal at the end of the magnetic field cycle. The new technique is theoretically analyzed and compared to the single-frequency and two-frequency irradiation techniques. It is shown that the sensitivity of the new technique exceeds the sensitivity of the two-frequency irradiation technique. As a test of the new technique we measured the shape of the highest-frequency 17O NQR line in paraelectric KH2PO4.