Department Optics and Beamlines

Femtoslicing HR: A new branchline for high spectral resolution fs-XAS and fs-XMCD at BESSY II+

After two decades of successful experiments with 100 fs [FWHM] X-ray pulses with variable polarization of 400 - 1400 eV at the UE56/1 ZPM at BESSY II, we are working on an upgrade, which will allow experiments with about a factor 10 higher energy resolution (E/ΔE > 4000) while maintaining the high time resolution of 100 fs [FWHM] and the variable (elliptical) polarization. For this purpose, a novel branch of the UE56/1 beamline is set up at a horizontal deflection angle of 2°. The unavoidable temporal fs x-ray pulse elongation due to diffractive optics is circumvented by performing pump-probe experiments in an intermediate focus in white (pink) light prior to monochromatization employing one out of an array of up to 11 blazed lithographically produced VLS gratings.

This experimental scheme allows for  time resolved XAS / XMCD measurements in transmission geometry with 100 fs resolution. The detector, approx. 37 m downstream  the source point, is a TimePix detector in combination with a gateable MCP stack. Furthermore, a future increase of the repetition rate (now 6 kHz) is planned, supported by a new filling pattern, making use of sequential generation of 100 fs X-ray pulses from multiple electron bunches of the BESSY II storage ring.

The main science cases for this new beamline are investigations of fs magnetization dynamics in thin films, where laser-induced magnetic and electronic dynamics can be investigated by XAS and XMCD in transmission geometry for previously unresolvable spectral changes at core-hole-lifetime limited spectral resolution. In the present work we show the conceptual design of the entire experiment, the optical layout, raytracing and the performance of the optics and detectors, as well as first results from the commissioning.

© 04/25 K.H.