Molecular beam
A molecular beam is a stream of gas that expands from a high-pressure region through a tiny opening into a lower-pressure chamber. As it expands, the particles (atoms, molecules, or ions) move together at similar speeds, with very few collisions between them. This creates a clean, well-controlled beam.
Molecular beams are used to grow thin films and real devices in molecular-beam epitaxy, including quantum wells, quantum wires, and quantum dots. They can also be used in crossed-beam experiments where two beams intersect. The molecules in a beam can be steered or slowed by electric or magnetic fields. For example, they can be decelerated with a Stark decelerator (using electric fields) or a Zeeman slower (using magnetic fields).
Historically, atomic beam experiments began in 1911 to show that particles travel in straight lines unless acted on by forces. In 1921, Kallmann and Reiche studied how polar molecules deflect in inhomogeneous electric fields to measure their dipole moments. This work helped spur the famous Stern–Gerlach experiments, which demonstrated quantum effects of angular momentum and spin, and led to many important discoveries in quantum physics and molecular beams.
In 1927, Erwin Wrede reported that dipole moments affect the deflection of molecules in a beam. In 1939, Isidor Rabi introduced molecular-beam magnetic resonance, creating inhomogeneous magnetic fields to measure magnetic moments of lithium isotopes using LiCl, LiF, and dilithium; this work was a precursor to NMR.
The invention of the maser in 1957 relied on a molecular beam of ammonia and a special electric-focusing device. The study of molecular beams also spurred the development of molecular-beam epitaxy in the 1960s, a key technique for making precise crystal layers.
This page was last edited on 2 February 2026, at 17:33 (CET).