Excitation of ionic solids with extreme ultraviolet pulses creates localized core-excitons, which in some cases couple strongly to the lattice. Here, core-excitonic states of magnesium oxide are studied in the time domain at the Mg L2,3edge with attosecond transient reflectivity spectroscopy.Attosecond pulses trigger the excitation of these short-lived quasi particles, whose decay is perturbed by time-delayed near infrared optical pulses. Combined with a few-state theoretical model, this reveals that the optical pulse shifts the energy of bright core-exciton states as well as induces featuresarising from dark core-excitons. We report coherence lifetimes for the first two core-excitons of 2.3±0.2 and 1.6±0.5 femtoseconds and show that these short lifetimes are primarily a consequence of strong exciton-phonon coupling, disclosing the drastic influence of structural effects in this ultrafast relaxation process.
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