Physics and Chemistry with Cold Molecules

The previous century saw the emergence of techniques that made it possible to extract microscopic information—and understanding—from the chaos of macroscopic gaseous and other systems. Among the first was the molecular beam technique that demonstrated the principal virtues of controlled atomic and molecular motion for elucidating the burning questions of the day, including those concerning the validity of quantum mechanics. As emphasized by Otto Stern in 1946, “the most distinctive characteristic property of the molecular ray method is its simplicity and directness. It enables us to make measurements on isolated neutral atoms or molecules with macroscopic tools. For this reason it is especially valuable for testing and demonstrating directly fundamental assumptions of the theory.” In his 1988 article “Molecular Beams: our legacy from Otto Stern,” Norman Ramsey compiled a list of what he called “major contributions to physics from the field of molecular beams.” There were thirty two items on that list, ranging from space quantization to molecular scattering to tests of time reversal symmetry. In the 1960s, the molecular beam technique made inroads into chemistry as well, by fulfilling the pipe dream of disentangling from gaseous chaos elementary chemical reactions as single binary collisions of chemically well-defined reagents. Chemical reaction dynamics that came about as a result has remained one of the chief preoccupations of chemical/molecular physics to date.