COHERENT ASPECTS OF THE LOCAL DIPOLAR FIELD IN ECHO DYNAMICS - A SPECTRAL NARROWING EFFECT

We have measured the spectral content of the echo radiation emitted by a two-level spin system ([AlO4](0) centers in quartz), after a two-pu lse excitation sequence, theta(1)-tau-theta(2). In general, the experi mental spectral shapes reproduce qualitatively those calculated for an inhomogeneous system of noninteracting spins. However, we found that in certain ranges of the sequence parameters (theta(1), theta(2) and t au) the widths of the experimental spectra are less than expected by a factor that amounts up to 0.5. The observed narrowing effect involves the spin packets located in a narrow spectral bandwidth (of the order of the Rabi frequency) around the excitation frequency and occurs whe never the excitation sequence induces a transverse oscillating moment of the strictly resonant packets. The spectral narrowing is a-transien t effect, in the sense that the spectral widths tend to recover their theoretical values for long interpulse distances tau. We suggest a ten tative interpretation which relates the narrowing to the coherence pro perties of the local field generated by precessional motion of the ech o-active spins.