  We propose a method for spectrally filtering radiation in the VUV wavelength
range by means of a monochromator constituted by a cell filled with a
resonantly absorbing rare gas. Around particular wavelengths, the gas exhibits
narrow-bandwidth absorbing resonances following the Fano profile. In
particular, within the photon energy range 60 eV - 65 eV, the correlation index
of the Fano profiles for the photo-ionization spectra in Helium is equal to
unity, meaning that the minimum of the cross-section is exactly zero. For
sufficiently large column density in the gas cell, the spectrum of the incoming
radiation will be attenuated by the background cross-section of many orders of
magnitude, except for those wavelengths close to the point where the
cross-section is zero. Remarkable advantages of a gas monochromator based on
this principle are simplicity, efficiency and narrow-bandwidth. A gas
monochromator installed in the experimental hall of a VUV SASE FEL facility
would enable the delivery of a single-mode VUV laser beam. The design is
identical to that of already existing gas attenuator systems for VUV or X-ray
FELs. We present feasibility study and exemplifications for the FLASH facility
in the VUV regime.
