CRESU apparatus

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Fig. 7. Schematic of the CRESU apparatus devoted to the measurement of ion/molecule reactions at low temperatures [50]...

Fig. 8. Variation of the rate coefficient with temperature as measured with the CRESU apparatus for the reaction of N+ with ammonia [50]. The open circles represent early CRESU (at Meudon) results [52] while the solid circles are newer CRESU (at Rennes) results [50]. The open square is a room-temperature result obtained by Adams et al. [53] with a SIFT apparatus. The solid line is a theoretical prediction by Troe using the statistical adiabatic channel model [54]...

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]. 

Unique as they are in accessing ultra-low temperatures, there are other limitations to the experiments performed in a CRESU apparatus. The fact that a... 

Table 1 shows the processes for which rate constants have been obtained by the PLP-LIF technique applied in the CRESU apparatus and the lowest temperature at which the rate of each such process has been measured. A rich variety of temperature dependences have been observed, although for all the processes identified in Table 1, the general trend is for the rate constant to increase as the temperature is lowered. 

Table 1 Minimum temperatures (in Kelvin) at which reactions of CN, OH, CH(v=0) and CH(v=l) have been studied in the CRESU apparatus. 

A particularly reactive atom in neutral-neutral reactions is atomic carbon, since, not only can it react with radicals and semi-radicals such as O2, it can also react with a variety of non-radicals such as unsaturated hydrocarbons. Indeed, the radicals CN and CCH have the same ability. Such reactions have been studied with the CRESU apparatus down to temperatures as low as Often the rate coefficients show a weak inverse... 

Other techniques are available to study electron attachment. However, most of them do not permit one to vary the temperature of the molecular species from room temperature to low temperature in true LTE. For the latter purpose, the CRESU apparatus may be the most suitable technique. Details are presented in Section 2.3.2. 

Beyond the significant mass flow rates that must be introduced to generate a uniform supersonic flow downstream of a Laval nozzle, it is also important to stress that the inner shape of the divergent part of the Laval nozzle and the temperature of the reservoir completely constrain the flow conditions i.e. nature of the buffer gas, gas flow rate, supersonic temperature and pressure. In other words, for a given Laval nozzle, the temperature in the supersonic flow is not a timeable parameter. Hence, a series of different Laval nozzles are required to match the range of temperature that needs to be explored. The typical temperatures that can be achieved in the present working CRESU apparatuses are usually in the range 15-300 K. This temperature is directly linked to the reservoir temperature by the relation ... 

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. 

Fig. 2.2. Sketch of the CRESU apparatus adapted for the study of neutral-neutral reactions and inelastic collisions.

In order to reduce the high cost for the development and use of a continuous flow CRESU apparatus, another alternative can be to pulse the supersonic flow. Using pulsed injection valves the total gas flow rate, and therefore the pumping capacity, can be considerably reduced. In this case the large flow rate through the Laval nozzle is achieved for only a short time, typically a few milliseconds. 

Although many kinetics studies can be performed in both the continuous and the pulsed versions of the CRESU apparatus, some specific reactions, for which one of the reagents is not available in large quantity, require the use of a pulsed Laval nozzle. This is the case for unstable species that need... 

Table 2.3 summarises the ion association reactions that have been studied in CRESU experiments. Due to the low pressure in the CRESU apparatus, especially in its first Meudon version, many of these results are very close to the low pressure limit. However, as the temperature is lowered, association becomes so efficient that the reaction often enters the fall-off zone. Indeed in this case for some reactions, such as CH3 +H2O, the reaction proceeds nearly at the capture rate i.e., at the high pressure limit. 

Since 1992 and the first success of combining laser techniques with the CRESU apparatus, a good number of neutral-neutral processes have been studied.These include bimolecular reactions, three-body reactions, and inelastic collisions, such as spin orbit relaxation of atoms (Al, Si and C), rotational relaxation of molecules such as NO and CO, and even for some specific cases vibrational relaxation (CH, NO and toluene). In this section we will only concentrate on reactive processes. 

Table 2.4. Bimolecular neutral-neutral reactions studied using the continuous and pulsed versions of the CRESU apparatus. The numbers indicate the minimum temperature at which the experiment has been carried out. 

With respect to ion-molecule reactions, the development of pulsed supersonic flows in Rennes (see Section 2.6.5) will also offer the possibility of achieving very low pressures in the supersonic flow and therefore will open the way to a revival of the ion-molecule CRESU apparatus by the use of a selective injection of ions which will be directly derived from expertise gained previously. Studies at temperatures close to 1K will be a decisive test in comparing present collision theories and are obviously of major importance. 

Canosa A, Sims IR, Travers D, Smith IWM, Rowe BR. (1997) Reactions of the methyUdine radical with CH4, C2H2, C2H4, C2H6 and but-l-ene studied between 23 and 295 K with a CRESU apparatus. Astron. Astrophys. 323 644-651. 

The overall rate coefficient for this radical + molecule reaction has been studied over the temperature range 25-716 K by Sims et al. [63] using the CRESU apparatus and by Sims and Smith [61] using a conventional heated and cryogenically cooled cell. The rate coefficient shows a strong, negative temperature dependence, with k = 2.77... 

Molecular oxygen was included in chemical networks at the beginning of astrochemistry [80], In the gas-phase, O2 is a product of the neutral-neutral reaction O + OH —> O2 + H, which is an exothermic, relatively fast reaction. The rate coefficient of this reaction has been measured at low temperature, using the CRESU apparatus (see Chap. 3) by Carty et al. [81]. They found a value of 3.5 X 10 cm s between 39 and 142 K. The OH radical is formed by the dissociative recombination of protonated water H30, itself formed in the following sequence of reactions ... 

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