What Happens When Individual Atoms and Molecules Collide

What Happens When Individual Atoms and Molecules Collide  

Dudley R. Herschbach (1932- ) began a series of molecular beam experiments at Berkeley starting in 1960. He employed two small ovens with slits producing crossed beams of molecules or atoms under very low pressure that collide in a vacuum reaction chamber. The early apparatus was built upon a turntable directing the stream of products toward a surface ionization detector sensitive to alkah metals and their salts. His first successful experiment crossed beams of potassium atoms and CH3I molecules  

An atom-molecule reaction of this type has a very minute activation harrier. His analysis indicated that the lifetime of the activated complex formed (K. .. I... CH3) was 10 seconds, shorter than the lifetime of rotation or vibration of the complex. In 1963, Herschhach moved to Harvard and huilt a more sophisticated apparatus. He calculated that a reaction that breaks an ionic bond, rather than a covalent bond, to form an ionic bond should have a longer-lived (more persistent ) activated complex (A, B = alkali metals X = halogen)  

This reaction did provide, as predicted, a longer-lived activated complex ( 10 seconds) that rotates and vibrates several times before falling apart. The number of collisions that provided products was smaller than expected. Herschhach and his coworkers realized that this was due to attack by A at the wrong end of X B to form some (A. .. B. .. X) complexes, not so well suited to falling apart to A X and B. 

In 1967, Yuan T. Lee (1936- ) joined Herschhach as a researcher after completing his Ph.D. at Berkeley. Lee designed and built an apparatus with supersonic beam nozzles and an electron bombardment ionizer that functioned as a universal detector. Supersonic beams propel species in the same direction at nearly the same speed and allow very few collisions. For this reason, chlorine atoms, much more reactive than potassium atoms, could be employed in the new and even more sensitive apparatus  

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