Analysis of the halo background in femtosecond slicing experiments

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 estimate

Difference in spin state and covalence between La<SUB>1&#8722;x</SUB>Sr<SUB>x</SUB>CoO<SUB>3</SUB> and La<SUB>2&#8722;x</SUB>Sr<SUB>x</SUB>Li<SUB>0.5</SUB>Co<SUB>0.5</SUB>O<SUB>4</SUB>

We present a comparative study of the spin states and electronic properties of La1&#8722;xSrxCoO3 and La2&#8722;xSrxLi0.5Co0.5O4 using X-ray absorption near-edge structure spectroscopy at both the O-K and Co-L2,3 thresholds. In the La2&#8722;xSrxLi0.5Co0.5O4 system the CoO6 octahedra are isolated, the holes induced by Sr doping are trapped in the isolated Co(IV)O6 octahedra, and a low-spin state is found for the Co ions, which does not change upon Sr doping. In the La1&#8722;xSrxCoO3 system, the interconnected CoO6 octahedra, with a 180&#176; Co-O-Co bond angle, give rise to a transition from low-spin to intermediate-spin state with a ferromagnetic alignment of the Co spins. The double-exchange, ferromagnetic coupling between Co ions mediated by the 180&#176; bond angle is responsible for suppressing the low spin-state. We find that the branching ratio of spectral intensities at the L2 and L3 thresholds in the Co-L2,3 X-ray absorption spectra is sensitive to the spin state of the Co ions allowing its direct spectroscopic determination