A molecular beam is produced by allowing a gas at higher pressure to expand through a small orifice into a chamber at lower pressure to form a beam of particles (atoms, free radicals, molecules or ions) moving at approximately equal velocities, with very few collisions between the particles.
[2] In 1921, Hartmut Kallmann and Fritz Reiche wrote[3] about the deflection of beams of polar molecules in an inhomogeneous electric field, with an ultimate aim of measuring their dipole moments.
[4]) When the 1922 Stern-Gerlach paper appeared is caused a sensation: they claimed to have experimentally demonstrated "space quantization": clear evidence of quantum effects at a time when classical models were still considered viable.
The advances of Stern and collaborators led to decisive discoveries including: the discovery of space quantization; de Broglie matter waves; anomalous magnetic moments of the proton and neutron; recoil of an atom of emission of a photon; and the limitation of scattering cross-sections for molecular collisions imposed by the uncertainty principle[2] The first to report on the relationship between dipole moments and deflection in a molecular beam (using binary salts such as KCl) was Erwin Wrede in 1927.
The invention of the maser in 1957 by James P. Gordon, Herbert J. Zeiger and Charles H. Townes was made possible by a molecular beam of ammonia and a special electrostatic quadrupole focuser.