The crossed molecular beam machine established at 21A end station consists of three components, the rotating source chamber, main chamber and detector. Two source chambers fixed at 90° to each other are rotatable with respect to the detector. For a bimolecular reaction two molecular beams will cross in the main chamber and for a photodissociation reaction a molecular beam will intersect with a photolysis beam. Reaction products after 10 cm free flight will be ionized with the synchrotron radiation. The desired ions are selected with a quadruple mass filter and then detected by a Daly-type ion counter. The background pressure in the ionization region can be suppressed down to 5×10^-12 Torr which makes H₂ product measurement doable as a liquid nitrogen cooler combined with a He-refrigerator is used.

In this crossed molecular beam machine, reaction products are ionized using VUV photons instead of conventional electron impact method. One of the advantages is to suppress dissociative ionizations as a proper ionization wavelength is used. An insertion device U9-undulator installed in the storage ring is employed to generate a multiple-harmonics light source. Only the first harmonic photons are desired thus a trickily designed gas cell filling with noble gas is used to absorb high order harmonic light. The output wavelength can be tuned by adjusting the gap of undulator. At this beamline a typical resolution E/ΔE 20 and a photon flux 1×10¹⁶ photons/sec can be offered for the photon ionization.