Excess dephasing in photon-echo experiments arising from excitation-induced electronic level shifts

Working on the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_0^{}{}_{}{}^{}$${\mathrm{-}}^5$${\mathrm{D}}_0$ transition of ${\mathrm{Eu}}^{3+}$ in ${\mathrm{Y}}_2$${\mathrm{O}}_3$, we provide the first detailed characterization of excitation-induced shifts in optical transition frequencies and demonstrate their exaggeration of nominally homogeneous dephasing rates. The shifts (found to vary in rough proportion to the number of ${\mathrm{Eu}}^{3+}$ ions excited and to be as small as one part in ${10}^{12}$) were identified through their unique signature in specially designed photon-echo experiments. The effects reported may constitute a rather general complication in optical measurements of homogeneous relaxation rates in solids.