[1][2][3] The technique involves the expansion of a gas or mixture of gases through a de Laval nozzle from a high-pressure reservoir into a vacuum chamber.
There are relatively few CRESU[4] apparatuses in existence for the simple reason that the gas throughput and pumping requirements are huge, which makes them expensive to run.
Most species have a negligible vapour pressure at such low temperatures, and this means that they quickly condense on the sides of the apparatus.
Essentially, the CRESU technique provides a "wall-less flow tube", which allows the kinetics of gas-phase reactions to be investigated at much lower temperatures than otherwise possible.
Chemical kinetics experiments can then be carried out in a pump–probe fashion, using a laser to initiate the reaction (for example, by preparing one of the reagents by photolysis of a precursor), followed by observation of that same species (for example, by laser-induced fluorescence) after a known time delay.