In the direction of beam propagation, the random directional motion of gas molecules is collimated by a nickel predator, and the acute angle of the sampling cone prevents the backscattered gas from entering th e orifice, thus only the sample is emitted directly from the reactor and is extracted. The sample is finally ionized by an orthogonal tuna ble VUV light beam in the ionization chamber. With the 200 lines mm -1 grating, the photon flux of the beamline is higher than 1×10 13 photons s -1 in the energy range of 5 to 25 eV with the full width at half-maximum (FWHM) of 1.87 meV at 7.3 eV (where energy resolution is 3900 at about 7.3 eV). The ion signal Si of species i under photon energy E can be written as