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 @[email protected] reaction, beams of the two reactants are caused to impinge on one another, often at an @[email protected] of 90°. In a beam-gas @[email protected] experiment a collimated beam is introduced into a gas, and the @[email protected] patterns are observed.