XAFS and X-MCD spectroscopies with undulator gap scan.

The first experimental applications of the undulator gap-scan technique in X-ray absorption spectroscopy are reported. The key advantage of this method is that during EXAFS scans the undulator is permanently tuned to the maximum of its emission peak in order to maximize the photon statistics. In X-MCD or spin-polarized EXAFS studies with a helical undulator of the Helios type, the polarization rate can also be kept almost constant over a wide energy range.