A molecular beam is produced by allowing a gas at higher pressure to expand through a small orifice into a container at lower pressure. The result is a beam of particles (atoms, free radicals, molecules or ions) moving at approximately equal velocities, with few collisions occurring between them. In a crossed molecular-beam experiment a reaction is studied using collimated beams of reactant molecules. For a @M03989@ reaction, beams of the two reactants are caused to impinge on one another, often at an @A00346@ of 90°. In a beam-gas @S05487@ experiment a collimated beam is introduced into a gas, and the @S05487@ patterns are observed.