Crossed molecular beam

Crossed molecular beam experiments are chemical experiments where two beams of atoms or molecules are collided together to study the dynamics of the chemical reaction, and can detect individual reactive collisions.

In a crossed molecular beam apparatus, two collimated beams of gas-phase atoms or molecules, each dilute enough to ignore collisions within each beam, intersect in a vacuum chamber. The direction and velocity of the resulting product molecules are then measured, and are frequently coupled with mass spectrometric data. These data yield information about the partitioning of energy among translational, rotational, and vibrational modes of the product molecules.

The crossed molecular beam technique was developed by Dudley Herschbach and Yuan T. Lee, for which they were awarded the 1986 Nobel Prize in Chemistry. While the technique was demonstrated in 1953 by Taylor and Datz of Oak Ridge National Laboratory, Herschbach and Lee refined the apparatus and began probing gas-phase reactions in unprecedented detail.

Early crossed beam experiments investigated alkali metals such as potassium, rubidium, and cesium. When the scattered alkali metal atoms collided with a hot metal filament, they ionized, creating a small electric current. Because this detection method is nearly perfectly efficient, the technique was quite sensitive. Unfortunately, this simple detection system only detects alkali metals. New techniques for detection were needed to analyze main group elements.

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