Sub-50 fs temporal resolution in an FEL-optical laser pump-probe experiment at FLASH2

High temporal resolution is essential for ultra -fast pump -probe experiments. Arrival time jitter and drift measurements, as well as their control, become critical especially when combining XUV or X-ray free -electron lasers (FELs) with optical lasers due to the large scale of such facilities and their distinct pulse generation processes. This paper presents the application of a laser pulse arrival time monitor that actively corrects the arrival time of an optical laser relative to the FEL's main optical clock. Combined with post -analysis single pulse jitter correction this new approach improves the temporal resolution for pump -probe experiments significantly. Benchmark measurements on photo -ionization of xenon atoms performed at FLASH beamline FL26, demonstrate a sub -50 fs FWHM overall temporal resolution.