Spatiotemporal stability of a femtosecond hard-x-ray undulator source studied by control of coherent optical phonons.

We report on the temporal and spatial stability of the first tunable femtosecond undulator hard-x-ray source for ultrafast diffraction and absorption experiments. The 2.5-1 Angstrom output radiation is driven by an initial 50 fs laser pulse employing the laser-electron slicing technique. By using x-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an x-ray pulse duration of 140+/-30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. Optical control of coherent lattice motion is demonstrated.

Observation of the x-ray magneto-optical Voigt effect.

The existence of the x-ray magneto-optical Voigt effect is demonstrated. By means of polarization analysis the Voigt rotation and ellipticity of linearly polarized synchrotron radiation are measured at the Co L3 edge upon transmission through an amorphous Co film. The observed x-ray Voigt rotation is about 7.5 degrees /microm. On the basis of ab initio calculations it is shown that the x-ray Voigt effect follows sensitively the amount of spin polarization of the 2p core states. Therefore it provides a unique measure of the spin splitting of the core states.