The theoretical investigation of the multiple-pulse sequences in which the radio-frequency pulses are applied simultaneously at two transitions are presented. The time-independent solution of the quantum Liouville equation for the average Hamiltonian of inhomogeneous broadening in the sequence is obtained by the projection operator technique. This solution predicts the multiple echo signals on the third transition that is not irradiated by radio-frequency pulses.

A simple and effective technique for calculating the transient signals in three-level spin systems irradiated at one and two frequencies is described. Al1 possible signals are presented and discussed. The application of the two-frequency excitation technique for the investigation of the inhomogeneity of the crystalline lattiee is suggested.

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The application of multiple-pulse sequences consisting of two-transition composite pulses for remote nuclear quadrupole resonance detection of explosives and narcotics is discussed. The pulse sequence for obtaining these pulses is described and it is shown that these pulses are equivalent to the pulse applied at the third (nonirradiated) transition. The Ioss of the signal intensity in this case is absent.

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