Dr Ian R Sims

CRESU is a French acronym standing for Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow. It takes advantage of the flow properties of gaseous expansions from convergent-divergent Laval nozzles into low pressure environments, producing a flow of gas which is uniform in temperature, density and velocity, and which endures for several tens of centimetres and some hundreds of microseconds downstream of the nozzle exit. Frequent collisions occur during the controlled expansion within the nozzle and in the subsequent uniform region downstream where the gas density (10¹⁶–10¹⁷ molecule cm^-3) is relatively high. The expansion is slow enough to maintain thermal equilibrium, but rapid enough that condensation is avoided (strongly supersaturated conditions prevail in the subsequent flow). A uniform, 'collimated' flow results at the exit of the nozzle.

This uniform supersonic flow provides an excellent environment in which to perform experiments on collisional processes at extremely low temperatures, not only between ionic and neutral species, but also between exclusively neutral species. We are currently, or have recently been, using this apparatus to study:

rotational energy transfer in collisions of CO with He at temperatures down to 15 K
rate constants at extremely low temperatures for the reaction O + OH ® O₂ + H, an important interstellar reaction, but one which is very difficult to study, as it involves two unstable species
studies of the kinetics of formation of weakly-bound dimers, the first step in homogeneous condensation
reactions of atomic carbon, C(³P), initially using an indirect chemiluminescent probe, and most recently by employing direct VUV detection. Reactions of both fundamental and interstellar interest have already been studied down to 15 K, and we hope to extend both the range of reactants and the temperature range (down to 7 K).