A novel interferometer has been developed with which the complex index of refraction of thin self-supporting foils can be measured in the vacuum ultraviolet and soft x-ray region. It consists of two collinear undulators and a grating spectrometer. Taking advantage of the low emittance 855 MeV electron beam from the Mainz Microtron, distinct intensity oscillations have been observed as a function of the distance between the undulators. A foil placed between the undulators causes a phase shift and an attenuation of the oscillation amplitude. The complex index of refraction has been measured at the $K$-absorption edge of carbon.

• Received 13 January 1998

DOI:https://doi.org/10.1103/PhysRevLett.80.5473