Delivering high-purity vacuum ultraviolet photons at the Brazilian toroidal grating monochromator (TGM) beamline

Vacuum ultraviolet beamlines based on grating monochromators share the problem of high-order harmonics contamination. In the low-photon energy range up to 25 eV, this problem may be overcome by letting the light beam pass through a high-pressure gas region. Here the photons with energies higher than the gas ionization threshold are absorbed and dumped. We have recently proposed and demonstrated a simple and inexpensive type of mixed gas metallic film filter. We present here the design and results of a complete gas-phase harmonics filter based on a very efficient and affordable differential pumping system which currently works at a bending magnet beamline at LNLS. Besides the efficient depletion of high-order harmonics, a noble gas harmonic filter has additional advantages. It can also provide a precise photon energy calibration, as well as a means for determining the resolving power. The use of such a gas filter is very important for photochemistry and molecular dynamics where even a very small portion of high-energy photons can mask the fragments generated by the first order photons.