Pulsed CRESU technique

If pressures of about 1 mbar are acceptable in the supersonic flow for the study of a given process, then a significant reduction in pumping capacity can be accepted as is the case for the mini-CRESU that has been constructed at the Universite de Bordeaux 1 by M. Costes and co-workers. This apparatus however cannot reproduce temperatures lower than 50 K in the supersonic flow because of its limited pumping capacities. Another way to reduce the size of a CRESU apparatus is to develop pulsed supersonic flows. A special section will be dedicated to this evolution of the CRESU technique in Section 2.3.4. 

The limit of conventional, cryogenically cooled pulsed laser photolysis experiments is 80 K, and the technique suffers from the problem noted for flow tube experiments on ion + neutral reactions, viz. freezing out of reactants or precursors on the cold walls of the reaction cell or the pipes leading into the cell. The CRESU technique has been applied to neutral + neutral reactions by Smith and co-workers to overcome this problem. A diagram of the apparatus is shown in Fig. 3.3. Temperatures as low as 13 K have been obtained. An alternative approach is to introduce the gas mixture into the nozzle via a pulsed valve. This is less demanding on the pumping capacity, but produces less stable flows. It is employed in a number of laboratories. Mullen and Smith [55], for example, have studied NH - - NO at temperatures down to 53 K. 

The reaction has been studied experimentally at temperatures up to 3500 K because of its importance in combustion. Sims et al. have measured the overall rate coefficient using conventional pulsed laser photolysis [61] and the CRESU technique [62, 63], both with LIF detectiOTi of CN, over the temperature range 13-761 K, obtaining k = 2.5 x 10 (77298 The channel branching ratio... 

Rowe and co-workers are developing a so-called diffusion technique to extend the temperature and pressure range. The technique will use the conversion of the initial kinetic energy (per unit volume) of the jet into a pressure increase downstream of the mass spectrometer, when the flow is brought from a supersonic to a subsonic regime through suitably shaped tubing. Also, it has been shown that the use of pulsed Laval nozzles reduces the appreciable amounts of gas that are consumed in the continuous flow CRESU apparatus [55]. 

Finally, a low temperature chemical reactor can be obtained using collimated uniform supersonic flows generated by the isentropic expansion of a buffer gas through an axisymmetric convergent-divergent Laval nozzle. Uniform supersonic flows were initially developed in a continuous flow version for the study of ion-molecule reactions. Later, the methodology was adapted to the study of reactions between neutral species in a continuous flow version and also in a pulsed flow version.This technique known as CRESU will be described in detail in Section 2.3. 

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