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-pulse excitation sequence, ${{{θ}}}₁-τ-{{{θ}}}₂$. In general, the experimental spectral shapes reproduce qualitatively those calculated for an inhomogeneous system of noninteracting spins. However, we found that in certain ranges of the sequence parameters (θ₁, θ₂, and {τ}) 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 whenever the excitation sequence induces a transverse oscillating moment of the strictly resonant packets. The spectral narrowing is a transient effect, in the sense that the spectral widths tend to recover their theoretical values for long interpulse distances {τ}. We suggest a tentative interpretation which relates the narrowing to the coherence properties of the local field generated by precessional motion of the echo-active spins. {} 1996 The American Physical Society.