Attosecond Time-Domain Measurement of Core-Level-Exciton Decay in Magnesium Oxide
Romain Géneaux, Christopher J. Kaplan, Lun Yue, Andrew D. Ross, Jens E. Bækhøj, Peter M. Kraus, Hung-Tzu Chang, Alexander Guggenmos, Mi-Ying Huang, Michael Zürch, Kenneth J. Schäfer, Daniel M. Neumark, Mette B. Gaarde, Stephen R. Leone
Abstract
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 L_{2,3} 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.