Contribution Invited Talk
CRESU experiments in astrophysics
The CRESU $^1$ technique is based on the isentropic expansion of a gas through a Laval Nozzle which acts as a guide and makes the gas velocity streamlines parallel at its exit. This generates a cold supersonic flow which is uniform in temperature and density along several tens of centimetres downstream of the Laval Nozzle. As the cold gas is never in contact with the walls of the experiment, reactants can be introduced in supersaturation conditions without risking condensation. In that sense, it can be viewed as an ideal chemical reactor for gas phase reaction kinetics studies at ultra-low temperatures.
The CRESU technique was developed in the early eighties in order to study reaction kinetics of ion-molecule reactions at interstellar relevant temperatures. A decade later the technique was adapted for the study of radical-neutral reactions and inelastic collisions as well. Demonstration that such reactions could be very fast at temperatures as low as 20 K aroused a great deal of interest in the astrochemical community. Since then the technique spread worldwide essentially in a pulsed version for cost reasons although some continuous versions were also constructed [1,4]
For a long time the technique was limited to the measurement of the total loss rate of a radical without offering any information with respect to the potential products of reactions. Recently new technological developments make the technique even more attractive. Association of the CRESU technique to synchrotrons  or to the chirped-pulse  method open it to the detection of products and determination of branching ratios. Pulsing techniques have been improved as well especially the aerodynamic chopper device which delivers pulsed supersonic flows of the same quality than in the continuous mode henceforth . Finally, a new class of reactions has been recently explored by several groups indicating that slow reactions at room temperature can dramatically change their temperature dependence and become quite efficient at ultra-cold temperatures due to the enhancement of tunnel effect or/and stabilization of a pre-reactive complex weakly hydrogen bonded.
The present talk will intend to present an updated picture of the technique and will briefly describe some striking results recently obtained by different groups worldwide.
 A. Canosa, F. Goulay, I.R. Sims, and B.R. Rowe, in Low Temperatures and Cold Molecules, Ed. I.W.M.Smith, 55-120 Imperial College Press, (2008).
 S. Soorkia, C.L. Liu, J.D. Savee, S.J. Ferrell, S.R. Leone, & K.R. Wilson. Rev. Sci. Instrum., 82 (2011).
 C. Abeysekera, B. Joalland, N. Ariyasingha, L.N. Zack, I.R. Sims, R.W. Field, & A.G. Suits. J. Phys. Chem. Lett., 6, 1599-1604 (2015).
 E. Jiménez, B. Ballesteros, A. Canosa, T.M. Townsend, F.J. Maigler, V. Napal, B.R. Rowe, & J. Albaladejo, Rev. Sci. Inst., 86, 045108 (2015).
$^1$ Cinétique de Réaction en Ecoulement Supersonique Uniforme / Reaction Kinetics in a Uniform Supersonic Flow