= Molecular beam
{wiki}
A directed stream of <molecules> used to prepare samples for <spectroscopy>.
A <molecular beam> also includes a stream of <chemical atoms>, then called an <atomic beam>. An oven supplies particles; aligned slits restrict their directions, and a <vacuum> limits subsequent <collisions>. Narrowing the directions does not by itself make every particle's <speed> equal. This gives a useful separation of tasks: prepare a path, select or measure <speed>, apply an interaction, and read the resulting <particle> counts or deflection.
The advantage is unusually tangible. A <magnetic field> can separate <spin> components in the <Stern-Gerlach experiment>; even <gravity> can sort a beam by flight time. The <effusive beam speed distribution> explains which particles leave the source, while <gravity deflection of an atomic beam> explains how that distribution becomes a pattern on a detector.
\Image[/molecular-beam-flight-and-speed-selection.png]
{title=Two ways of measuring <molecular beam> speeds: rotating openings and gravitational flight}
{description=Original functional schematics, not historical apparatus drawings. The lower panel exaggerates the fall for visibility; all numerical settings used in the text are chosen model inputs.}
Back to article page
