Soft X-ray Monochromators for Third-Generation Undulator Sources

Over the past ten years, the resolving power from new designs of bending-magnet-based soft X-ray monochromators has increased by more than an order of magnitude. This has led to a revolution in soft X-ray spectroscopy, but the limited flux at this high resolution has allowed only relatively efficient measurements to be made, such as photo-absorption. Application of this new tool of high-resolution spectroscopy to photoemission, energy-resolved fluorescence spectroscopy and microscopy has now been made possible with the advent of undulator sources of soft X-rays. Here we have reviewed the recent development of undulator-based soft X-ray monochromators, the special features of undulators in general and the resulting benefits and problems, and describe the state of the art undulator beamline, 7.0 at the Advanced Light Source. In addition, we offer some speculation as to the possible routes to the next or ultimate generation of soft X-ray monochromator.