The spectroscopy of very cold gases.

The technique of supersonic free jet spectroscopy can be used to study the structure and dynamics of molecules which have been cooled to far below their boiling points but which remain in the gas phase. Cooling of the internal degress of freedom, the molecular rotations and vibrations, produces a highly resolved and greatly simplified molecular spectrum. The principles of the technique are discussed and its utility is demonstrated by two examples. the spectroscopy of porphyrins in the gas phase and the photochemistry of van der Waals molecules.