Attosecond time-domain measurement of core-excitonic decay in magnesium oxide

TL;DR

This study uses attosecond transient reflectivity spectroscopy to measure the ultrafast decay of core-excitons in magnesium oxide, revealing strong exciton-phonon coupling effects on their coherence lifetimes.

Contribution

It provides the first time-domain measurements of core-excitonic decay in magnesium oxide and links short lifetimes to exciton-phonon interactions.

Findings

Coherence lifetimes of 2.3 and 1.6 femtoseconds for the first two core-excitons.

Optical pulses shift bright core-exciton energies and induce dark exciton features.

Strong exciton-phonon coupling causes ultrafast exciton decay.

Abstract

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 $\text{L}_{2,3}$ 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 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 features arising from dark core-excitons. We report coherence lifetimes for the first two core-excitons of $2.3 \pm 0.2$ and $1.6 \pm 0.5$ 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.