Using the photoinduced resonance shift in Fe and Ni as time reference for ultrafast experiments at low flux soft x-ray sources.

We study the optical-pump induced ultrafast transient change of x-ray absorption at absorption resonances of the transition metals Ni and Fe in the FeNi alloy. We find the effect for both elements to occur simultaneously on a femtosecond timescale. This effect may hence be used as a handy cross correlation scheme, providing a time-zero reference for ultrafast optical-pump soft x-ray-probe measurement. The method benefits from a relatively simple experimental setup as the sample itself acts as time-reference tool. In particular, this technique works with low flux ultrafast soft x-ray sources. The measurements are compared to the cross correlation method introduced in an earlier publication.