A methodology combining high-acceptance angle-resolved photoelectron time-of-flight spectroscopy and ghost imaging is presented for enhancing the spectral characterization of x-ray free-electron laser pulses. This noninvasive high-resolution x-ray diagnostic improves energy resolution from ~1 to 0.5 eV, benefiting ultrafast x-ray spectroscopy by serving as a transparent beamsplitter for applications like transient absorption spectroscopy and covariance-based x-ray nonlinear spectroscopies.

Kai Li, Joakim Laksman, Tommaso Mazza, Gilles Doumy, Dimitris Koulentianos, Alessandra Picchiotti, Svitozar Serkez, Nina Rohringer, Markus Ilchen, Michael Meyer, Linda Young