Temporal characterization of the new time-delay-compensating monochromator beamline at FLASH and first user experiments

The new pulse-length preserving double monochromator beamline FL23 at FLASH went into user operation beginning of 2024. The beamline is designed to work in the soft X-ray regime covering the spectral range between 1.3 nm and 20 nm with a spectral resolving power of approximately 2000 [1]. The double grating design guarantees an ultrafast beamline response, thereby maximizing the time-resolution for dynamical studies. Besides a flexible microfocus provided by a bendable Kirkpatrick-Baez mirror system, a femtosecond optical laser synchronized to the FEL enables experiments in a pump-probe scheme. In this contribution, the results of a spectral encoding campaign to characterize the ultrafast beamline response are shown, as well as first user experiments on 1) a liquid jet sample using time-resolved X-ray absorption spectroscopy at the Nitrogen K-edge and 2) the demonstration of soft X-ray magnetic circular dichroism measurements at the L-edges of the important transition metals Fe, Co, and Ni, employing the new variable polarization afterburner undulator [2] at FLASH2. [1] L. Poletto, F. Frassetto, G. Brenner, M. Kuhlmann and E. Plönjes, Double-grating monochromatic beamline with