New Methods of Nuclear Quadrupole Resonance *

Direct NQR signals from the light nuclei H, B, N use to be weak; their detection is difficult. For these light nuclei nuclear quadrupole double resonance (DNQR) methods are preferable in which the sensitivity is enhanced due to the thermal contact of two spin-systems and indirect detection. The higher the initial magnetic field B0, the better is the DNQR sensitivity. Apart from this, the DNQR sensitivity is dependent on the efficiency and duration of thermal contact between the quadrupole and magnetic systems. We have shown [1] that, if the sample in the applied zero magnetic field is saturated simultaneously by two NQR frequencies, the proton magnetization is destroyed much more effectively. The two-frequency saturation of a quadrupole system leads to increased DNQR sensitivity, thus allowing the assignment of lines to the equivalent nuclei in multiple spectra [2]. For the v_-line in ^ N 1 ! ! DNQR spectra which have the lowest intensity, the two-frequency saturation considerably increases the intensity. If the sample in the applied zero magnetic field is irradiated by the rf field coP ± coQ, the thermal contact is realized due to the solid effect. coQ is the NQR frequency and coP is the proton NMR frequency in the local magnetic field. It follows from the solid effect analysis [3], that the most intensive line is the coQ + coP one. In some cases two frequency irradiation also increases the solid effect satellite intensities. The efficiency of thermal contact between the proton and quadrupole systems may be increased by the selective magnetic field technique, see Figure 1 and [4,