  X-ray free-electron lasers (XFELs) have been widely used for applications
such as X-ray crystallography and magnetic spin probes because of their
unprecedented performance. Recently, time-resolved X-ray magnetic circular
dichroism (XMCD) with ultrafast XFEL pulses made it possible to achieve an
instantaneous view of atomic de-excitation. However, owing to the narrow
bandwidth and coherence of XFEL, X-ray absorption spectroscopy (XAS) and XMCD
are time- and effort-consuming for both machine scientists and users of XFELs.
In this work, an efficient scheme using a broadband XFEL pulse and single-shot
X-ray spectrometer is proposed, in which the XAS and XMCD measurements can be
accomplished with the same machine condition. An evolutionary multiobjective
optimization algorithm is used to maximize the XFEL bandwidth offered by the
Shanghai soft X-ray FEL user facility without additional hardware. A numerical
example using MnO is demonstrated, showing that using approximately 1000
consecutive XFEL shots with a central photon energy of 650 eV and full
bandwidth of 4.4%, precise spectral measurements for XAS and XMCD can be
achieved. Additional considerations related to single-shot XAS and XMCD are
discussed.
