Molecular beam

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. Molecular beams are useful for fabricating thin films in molecular beam epitaxy and artificial structures such as quantum wells, quantum wires, and quantum dots. Molecular beams have also been applied as crossed molecular beams. The molecules in the molecular beam can be manipulated by electrical fields and magnetic fields. Molecules can be decelerated in a Stark decelerator or in a Zeeman slower. The first to study atomic beam experiments was Louis Dunoyer de Segonzac 1911, but were simple experiments to confirm that atoms travelled in straight lines when not acted on by external forces. Since 1921, the teams of Otto Stern in University of Hamburg and University of Frankfurt am Main, improved and pioneered the experiments with atomic beams, and later with molecular beams. 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 However, the first paper on molecular beams was by Hartmut Kallmann and Fritz Reiche, who before Stern who started with atoms, were interested in dipole moments and the deflection of beams of polar molecules (not just atoms) in an inhomogeneous electric field. In 1922, with Walther Gerlach, Stern rushed to publish their results on the Stern–Gerlach experiment demonstrating the existence of the spin, after reading on the experiments of Kallman and Reiche. However the paper of Kallman and Reiche would go largely unnoticed until its rediscovery 2009.