Molecular beam

A directed stream of molecules traveling through a low-pressure environment, where collisions with background gas are rare. Its controlled motion enables precise studies of molecular properties and reactions.

Effusive beam: A molecular beam formed when gas escapes through a small aperture from a reservoir into vacuum. This common source creates a beam by allowing faster molecules to escape more readily.
Molecular beam epitaxy: A thin-film growth method that deposits atomic or molecular beams onto a heated substrate in ultrahigh vacuum. It uses directed beams to build crystalline films one layer at a time.
Kinetic theory of gases: A model that explains gas properties through the motion and collisions of microscopic particles. It connects molecular speeds and collision rates to the conditions needed for a beam.
Effusion: The escape of gas through an opening smaller than the gas’s mean free path. Effusion describes the source process; an effusive beam is the directed stream it can produce.
Supersonic expansion: A gas expansion through a nozzle that accelerates and cools molecules into a narrow velocity distribution. It produces intense, cold molecular beams with reduced internal motion.
Crossed molecular beam experiment: A reaction experiment in which two molecular beams intersect and products are analyzed by scattering angle and speed. Crossing beams reveals the dynamics of individual gas-phase chemical collisions.
Mean free path: The average distance a particle travels between collisions with other particles. A long mean free path allows beam molecules to travel without being scattered.
Molecular flow: Gas transport in a regime where molecules collide with walls more often than with one another. This flow regime commonly occurs in beam apparatus, but does not itself imply a directed beam.
Collimation: The restriction of particles to a narrow range of travel directions. Apertures and skimmers collimate beams to define their direction and reduce divergence.
Molecular spectroscopy: The study of how molecules absorb, emit, or scatter electromagnetic radiation. A beam’s controlled motion and low collision rate support precise spectroscopic measurements.

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