TOWARD A SOFT-X-RAY FOURIER-TRANSFORM SPECTROMETER

The use of Fourier transform spectroscopy in the soft X-ray region is advocated as a possible route to spectral resolution superior to that attainable with a grating system. A technical plan is described for ap plying Fourier transform spectroscopy to the study of the absorption s pectrum of helium in the region of double ionization around 60-80 eV. The proposed scheme includes a Mach-Zehnder interferometer deformed in to a rhombus shape to provide grazing-incidence reflections. The path difference between the interfering beams is to be tuned by translation of a table carrying four mirrors over a range +/- 1 cm which, in the absence of errors generating relative tilts of the wave fronts, would provide a resolving power equal to the number of waves of path differe nce: half a million at 65 eV, for example. The signal-to-noise ratio o f the spectrum is analyzed and, for operation on an Advanced Light Sou rce bending magnet beam line, should be about 330.