Deciphering core-exciton dynamics in CaF$_2$ with attosecond spectroscopy

Abstract: Core excitons in solids have garnered increasing interest, yet their behavior and decay mechanisms are not fully understood. Here, we use attosecond extreme ultraviolet (XUV) transient absorption spectroscopy, performed with a broadband 25-45 eV sub-fs XUV pump pulse and a 500-1000 nm sub 5 fs near-infrared (NIR) supercontinuum probe pulse to monitor the excitation, dynamics, and decay of core excitons in CaF$2$ at the Ca$^{2+}$ M${2,3}$ edge. The XUV pulses are used to excite core excitons in CaF$_2$ based around the Ca$^{2+}$ and the polarization of the medium is subsequently perturbed by the time-delayed NIR pulses to measure the spectral changes and decays. A number of features are identified in the transient absorption spectrum, which suggest transfer between excitonic states, Stark shifts, and the emergence of light-induced states. We find that various core excitons identified exhibit coherence lifetimes spanning 3-7 fs. Furthermore, a NIR-intensity-dependent analysis finds a negative correlation with the coherence lifetime of various identified excitonic features, supporting a phonon-mediated mechanism as responsible for the core exciton decoherence. We present a computational band structure projection analysis strategy to estimate the orbital structure of the core excitons and determine which core excitonic transitions should be allowed by selection rules with the probe beam. This strategy is found to successfully describe the observed spectroscopic data. The outlined joint spectroscopic and computational investigation of core excitons is a powerful technique that explains the complex behavior of core excitons in solid-state materials.