Instantaneous diffusion effect on spin-echo decay: Experimental investigation by spectral selective excitation

The influence of the instantaneous diffusion process on spin-echo decay of E_γ^' centers in gamma irradiated silica is experimentally probed by spectral selective excitation within their inhomogeneous resonance line. Our results evidence the different effectiveness of this dephasing mechanism on varying the resonance field, manifesting itself by a faster decay of the echo signal when generated by spin packets located in the central part of the spectrum. It is shown that the dependence of the instantaneous diffusion rate on the spectral position of echo-active spins reproduces the shape of the E_γ^' centers resonance spectrum. These features are discussed in the framework of theoretical models concerning the transversal spin relaxation in solid systems and point out the correlation between the instantaneous diffusion rate and the concentration of echo-active spins located within a spectral bandwidth of the order of the Rabi frequency.