Femtosecond time resolved soft X-ray scattering, diffraction, XMCD, Reflectometry at UE56-1-ZPM
Experimental station for fs time resolved XMCD and XAS
The FEMTOSPEX XMCD/XAS experimental station was set up in 2004 for the proof-of-principle Femtoslicing experiments and has since then been a workhorse for both in-house research and user operation at the HZB. It has been built particularly for time-resolved XAS and XMCD measurements in transmission, coping with the reduced photon flux in fs time resolved experiments. The main scientific focus is on ultrafast magnetization dynamics and time-resolved X-ray absorption spectroscopy.
The experimental setup for laser pump – X-ray probe on magnetic samples consists of a measurement chamber housing the magnet and transmission sample, and the detector chamber with a fast avalanche photodiode (APD). Laser and X-ray beams enter through the beamline flange under an angle of 1.5° (almost collinear). An Al foil mounted between the chambers prevents laser light to enter the detection chamber with the APD.
Within the last two decades resonant soft X-ray diffraction (RSXD) has emerged as a highly efficient experimental technique. It allows probing nanoscale ordering phenomena in solid state materials, like electronic order, charge or orbital order, as well as magnetic order. In particular, RSXD is one of the few methods that can probe antiferromagnetic order. For these reasons time-resolved pump -probe RSXD is ideally suited to study the dynamics of photo-induced phase transitions in complex materials when it is combined with ultra-short photon pulses.
A special case of resonant x-ray diffraction is the spectroscopic measurement of the specular sample surface (single Bragg plane) reflection at grazing incidence angles (<12°). The advantage of x-ray reflection spectroscopy (XRS) over XAS in transmission geometry is that it lifts the strong constrain of having to use thin films of a few tens of nanometers thickness as a sample. This allows access to dynamics in crystalline bulk samples and films or nanostructures grown on thicker substrates.
Since 2008 the RXD and more recently XRS have been made available for ultrafast studies at the FemtoSpeX Slicing Facility. A dedicated two circle UHV diffractometer has been set up for diffraction (reflection) geometries within the horizontal plane. By cryogenic cooling sample temperatures down to 6K can be reached. Avalanche photodiodes (APDs) are used for gated photon pulse detection. The angular acceptance of the diffractometer is set by vertical detector entrance slits of variable size. The APDs are screened from light of the pump-laser by Al membranes (Luxel Corporation) and a light tight housing. Low noise amplification (ca. 60dB by Hamamatsu and Kuhne preamplifiers) allows besides analog pulse detection for time-correlated single-photon pulse counting. Generally signals as low as ~5 photons/sec from the sample can be detected. This corresponds to a diffraction (reflection) efficiency of >5e-5.