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

General Physics and Astronomy
Phys. Rev. Lett.
High-Harmonic Generation and EUV Science

Géneaux, R, Kaplan, C.J, Yue, L, Ross, A.D, Bækhøj, E, Kraus, P.M, … Leone, S.R. (2020). Attosecond Time-Domain Measurement of Core-Level-Exciton Decay in Magnesium Oxide. Phys. Rev. Lett., 124(20), 207401: 1–207401: 6. doi:10.1103/physrevlett.124.207401