Analysis of the halo background in femtosecond slicing experiments

Abstract

The slicing facility FemtoSpeX at BESSY?II offers unique opportunities to study photo-induced dynamics on femtosecond time scales by means of X-ray magnetic circular dichroism, resonant and non-resonant X-ray diffraction, and X-ray absorption spectroscopy experiments in the soft X-ray regime. Besides femtosecond X-ray pulses, slicing sources inherently also produce a so-called 'halo' background with a different time structure, polarization and pointing. Here a detailed experimental characterization of the halo radiation is presented, and a method is demonstrated for its correct and unambiguous removal from femtosecond time-resolved data using a special laser triggering scheme as well as analytical models. Examples are given for time-resolved measurements with corresponding halo correction, and errors of the relevant physical quantities caused by either neglecting or by applying a simplified model to describe this background are estimated.Besides femtosecond X-ray pulses, synchrotron-based laser-slicing sources inherently produce a halo background with a different time structure, polarization and pointing direction. A detailed experimental characterization of this halo radiation at the FemtoSpeX facility at BESSY?II is presented, and a method for its correct and unambiguous removal from femtosecond time-resolved data using a dedicated laser triggering scheme as well as analytical models is demonstrated.