Diffractive wavefront correction for Fe L-edge spectroscopy on the meV scale.

We propose a wavelength-dispersive instrument for high-resolution soft X-ray spectroscopy at large-scale facilities like synchrotron radiation sources or free-electron lasers. Demonstrated by simulations at the Fe L-edge in the range from 700 eV to 730 eV, an energy resolution of (12-15) meV is enabled by a wavefront-corrected reflection zone plate (RZP) with an aperture of 40 mm × 190 mm on a spherical Si substrate (radius 69 m). Its tangential slope error and deformation are measured on-axis along the RZP to ±0.1 arcsec (r.m.s.) and ±6.1 nm (r.m.s.), respectively, and simulated at off-axis positions of the substrate with a similar amplitude. The surface waviness is compensated around the design energy of 715 eV by a two-dimensional, adapted grating groove distribution. As a benefit, the diffracted beam is collimated to a one-dimensional focal line whose length of 40 mm nearly equals the sagittal detector size (50 mm): no photons in the +1st diffraction order with an efficiency of 6.4% are lost, and the acceptance solid angle of 16 mrad × 3.3 mrad provides a high transmission of the signal from the source.