We consider a process, in which a pump pulse creates an exciton. A probe pulse excites an electron from a core orbital to the states, where an exciton is created. The resulting XAS spectra strongly depend on the shape of orbitals forming an exciton. This way, the probe pulse measures the structure of excitons with an orbital selectivity.

Our ab initio, BSE-based scheme to calculate XAS of photo-excited materials and reveal fingerprints of valence excitons:

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.110.235126

Our analytical analysis of attosecond XAS of Frenkel excitons:

https://www.mdpi.com/1420-3049/28/11/4502

We also develop an ab initio scheme to calculate attosecond XAS of laser-driven materials.