The CRESU technique

CRESU - Expansion of a gas initially at rest in a reservoir via a Laval nozzle results in a drop in temperature. The carefully calculated profile of the nozzle results in a uniform supersonic flow at the exit, which can be maintained for some considerable distance downstream, provided the chamber pressure can be maintained at low pressure by the necessary pumping system.

The CRESU technique (a French acronym standing for Reaction Kinetics in Uniform Supersonic Flow) was invented by Bertrand Rowe and Jean-Baptiste Marquette [1] in the 1980’s. Initially used by Rowe and his collaborators for studying ion-molecule reactions at the Aerodynamics Laboratory at Meudon in Paris, the technique entered a new era when, in collaboration with the group of Professor Ian W.M. Smith of the University of Birmingham, a new CRESU was constructed here in Rennes, dedicated to the study of neutral-neutral reactions [2].

The CRESU technique uses the supersonic flow created by the expansion of a gas through a Laval nozzle to create a cold, wall-less reactor, avoiding condensation problems encountered in more conventional cryogenically-cooled cells.

The resulting uniform supersonic flow has a relatively high density (1016 to 1018 cm–3), resulting in frequent collisions, which ensures that thermal equilibrium is maintained in the gas during the expansion and along the flow. This means that we can define a true, thermodynamic temperature for this environment. The rapidity of the expansion and associated cooling avoids condensation, and low vapour pressure species can be maintained in the gas phase under highly supersaturated conditions. The lowest temperature we have obtained with this technique so far is 6 K [3].
 

  • [1] G. Dupeyrat, J. B. Marquette, B. R. Rowe, Phys. Fluids 28, 1273-1279 (1985).
  • [2] I. R. Sims, J. L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, P. Bocherel, B. R. Rowe, I. W. M. Smith, J. Chem. Phys. 100, 4229-4241 (1994).
  • [3] C. Berteloite, M. Lara, A. Bergeat, S. D. Le Picard, F. Dayou, K. M. Hickson, A. Canosa, C. Naulin, J. M. Launay, I. R. Sims, M. Costes, Phys. Rev. Lett. 105, 4 (2010).