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Nonexponential photon-echo decays are observed in ruby, Er³⁺:YLiF₄, and Er³⁺:LaF₃ with samples of low ion concentrations in high (10--50 kG) magnetic fields. At the highest fields, photon-echo decays are field independent and their temporal forms are identical for all three systems. This behavior is attributed to optical dephasing arising from electron-spin nuclear-spin superhyperfine interactions in the presence of a frozen core.
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Ganem et al. (1991) studied this question.
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