CRESU

Figure A3.5.5. Rate constants for the reaction of Ar with O2 as a fiinction of temperature. CRESU stands for the French translation of reaction kinetics at supersonic conditions, SIFT is selected ion flow tube, FA is flowing afterglow and HTFA is high temperature flowing afterglow.

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. 

CRESU Cinetique de reaction en ecoulement supersonique uniforme... 

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]. 

Due to the inverse temperature dependence of the Arrhenius relationship, these very low temperature studies cover a wide range on an Arrhenius plot. Combining CRESU results with higher temperature studies enables the competition of reaction channels with opposing temperature dependencies to be observed in some reactions. This produces an Arrhenius plot... 

Figure 1 shows second-order rate coefficients measured in a CRESU flow apparatus over a wide temperature range at a variety of pressures and bath gases. 

Figure. 1 Second-order rate coefficients for the reaction NH3 + NIU + M —> N2H7" + M from CRESU experiments in [16] (tiill line = Su-Chesnavich prediction of eqs. (28) and (29), circles M = He, squares M = Ar, diamonds M = N2).

Figure 6. Rate constants for the reaction of with NO as a function of average translational plus rotational energy. The HTFA, CRESU, ° flow drift tube, and static drift tube data are shown as squares, circles, triangles and inverted triangles, respectively.

Because the radicals decay according to pseudo-first-order kinetics, condensation of the photochemical precursor for the radicals is unimportant in CRESU experiments, as in those in cooled cells, as long as sufficient precursor survives to provide a measurable concentration of radicals. 

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... 

The complementarity of cooled cell and CRESU experiments is further demonstrated by studies on the reactions of OH radicals with CO (down to 80 K) [11(b)] and 0(T) atoms (down to 158 K) [16]. 

Reaction (11), which may also proceed via collisional stabilisation to a HOCO radical has been the subject of numerous kinetic and dynamics studies over the past 30 years. At room temperature its rate constant is k29g = 1.5 x 10 cm molecule s but below 500 K the activation energy is almost zero. Recent measurements [11(b)] show that the rate constant only falls slightly as the temperature declines to 80 K. Unfortunately, the reaction is too slow to measure at lower temperatures in the present generation of CRESU experiments. The linear flow is so fast, that the first-order rate constant for decay of the radical concentration must be > 5 x 10 s with the result that any bimolecular reactions for which the rate constant is < 10 cm molecule s" cannot be successfully studied. 

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. 

The overall kinetics of this reaction have been studied experimentally over an exceedingly wide range of temperature. Thus, in some of the earliest CRESU... 

The kinetics of the reactions of several atomic and molecular free radicals with alkenes and alkynes have been studied down to low temperatures in CRESU experiments.22 The results of these experiments have been reviewed and analysed by Smith et Based on semi-empirical arguments, as well as correlations of room temperature rate constants, they suggested which reactions of radicals with unsaturated molecules are likely to be fast at ca. 10 K, that is, the temperatures found in the cold cores of dense interstellar clouds. 

Smith el pointed out that the values of I.E. — E.A.) for reactions of 0( P) with alkenes straddle the critical value of 8.75 eV and would therefore comprise an interesting test case for the proposals in their paper. CRESU experiments were subsequently performed on these reactions by Sabbah et al. and their results are shown in Figure 1.7. They are compared with the results of calculations based on the two transition state model which made use of ab initio calculations of the potential at long and medium range. The ab initio calculations confirmed that all the reactions that were studied, apart from that of 0( P) with ethene, for which (I.E. — E.A.) = 9.05 eV, exhibit submerged barriers at separations shorter than those associated with the van der Waals minima. As the curves in Figure 1.7 show, the agreement of theory with experiment is excellent. 

ABSTRACT. Calculation of the rate constant at several temperatures for the reaction +(2p) HCl X are presented. A quantum mechanical dynamical treatment of ion-dipole reactions which combines a rotationally adiabatic capture and centrifugal sudden approximation is used to obtain rotational state-selective cross sections and rate constants. Ah initio SCF (TZ2P) methods are employed to obtain the long- and short-range electronic potential energy surfaces. This study indicates the necessity to incorporate the multi-surface nature of open-shell systems. The spin-orbit interactions are treated within a semiquantitative model. Results fare better than previous calculations which used only classical electrostatic forces, and are in good agreement with CRESU and SIFT measurements at 27, 68, and 300 K. ... 

Figure 5. Rate coefficients for the He" + HCl reaction. AC (closed circles) and CRESU (open circles) rate coefficients are shown [20].

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