We are excited to see Lars’ first paper on “Saturable absorption of free-electron laser radiation by graphite near the carbon K-edge” published. In this experiment, we study in a single experiment supported by state-of-the-art numerical simulations how intense femtosecond X-ray free-electron laser pulses induce two different nonlinear responses, saturable absorption (SA) and two-photon absorption (TPA), depending on intensity. We find that SA and TPA are competing processes in the X-ray regime and the relative transition strengths determine at which intensities TPA becomes dominant over SA which is a material-specific property. Our results reveal the competing contributions of distinct nonlinear material responses to spectroscopic signals measured in the X-ray regime, demonstrating an approach of general utility for interpreting FEL spectroscopies. This opens new routes for dynamically probing properties of matter.

This work was done in collaboration with researchers from the Saykally group at UCB, LBNL, U of Texas Rio Grande Valley, UC San Diego, and FERMI. The experiments were conducted at the free-electron laser facility FERMI in Italy.

Our open access paper was now published in the Journal of Physical Chemistry Letters and was selected as Editor’s pick:
https://pubs.acs.org/doi/10.1021/acs.jpclett.2c01020