Calculations of multipulse sequence in NQR of spins 3/2

The general formalism of the interaction representation with respect to an operator which is its own inverse is developed and applied to pure NQR of s pins I = 3/2. Under the assumption of no relaxation and no dipolar coupling , it is shown that the calculation of the response to pure NQR multipulse s equences can be performed with the same concepts used in high field NMR, su ch as coherence pathways. All the tools and mathematical expressions to pre dict the time evolution of the signal created by a pure NQR multipulse sequ ence are presented explicitly. It takes into account the off-resonance irra diation as well as the angular dependence of the excitation and detection f or every value of the electric held gradient asymmetry parameter. Particula r attention is devoted to the powder average, which is performed via a prob ability function derived analytically for the first time, leading to a dras tic reduction of simulation times. The theory is illustrated by the study o f the optimization and excitation bandwidths of one- to three-pulse sequenc es and compared to experimental results on Chloranil. We show that the thre e-pulse "stimulated echo" sequence gives a more uniform excitation profile than the traditional two-pulse echo sequence for powder samples. Thus, the "stimulated echo" sequence could be useful to cover a large spectrum when t he experiment duration, or the signal to noise ratio, are not critical para meters. Analytical expressions for the nutation spectra obtained by one or two-pulse sequences are also derived for the first time. (C) 1999 Academic Press.