Flowing-afterglow

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.

Flow tube studies of ion-moleeule reaetions date baek to the early 1960s, when the flowing afterglow was adapted to study ion kineties [85]. This represented a major advanee sinee the flowing afterglow is a thennal deviee under most situations and previous instruments were not. Smee that time, many iterations of the ion-moleeule flow tube have been developed and it is an extremely flexible method for studying ion-moleeule reaetions [86, 87, 88, 89, 90, 91 and 92]. 

Hamilton C E, Bierbaum V M and Leone S R 1985 Product vibrational state distributions of thermal energy charge transfer reactions determined by laser-induced fluorescence in a flowing afterglow Ar" + CC -> CC (v= 0-6) + Ar J. Chem. Rhys. 83 2284-92... 

Ferguson E E 1992 A personal history of the early development of the flowing afterglow technique for ion molecule reactions studies J. Am. Soc. Mass Spectrom. 3 479-86... 

Adams N G and Smith D 1988 Flowing afterglow and SIFT Techniques for the Study of Ion-Molecule Reactions ed J M Farrar and W FI Saunders Jr (New York Wiley)... 

Flierl P M ef a/1996 Flowing afterglow apparatus for the study of ion-molecule reactions at high temperatures Rev. Scl. Instrum. 67 2142-8... 

Squires R R 1997 Atmospheric chemistry and the flowing afterglow technique J. Mass Spectrum. 32 1271-72... 

One resolution of the problem that gained acceptance for a while was proposed by Adams et al.18 In their flowing afterglow measurements, the apparent recombination coefficient of fell off to a small value in the late afterglow . This finding... 

A careful repeat of the flowing-afterglow measurement of Adams et al. 18 was subsequently carried out by Smith and Spanel.29 The results confirmed the Adams et al. observation that the recombination coefficient appears to fall off to a small value in the late afterglow. The authors concluded that the small recombination coefficient observed in the late afterglow is the proper value for v = 0 ions and that the initial rapid plasma decay in the early afterglow should be ascribed to vibrationally excited ions. 

Since the flowing-afterglow method is quite well established, a brief review of the basic features of this technique should be sufficient. The experiments of Gougousi et al.46 on H3 recombination and those of Adams et al.18 and of Smith and Spanel24 used nearly the same experimental method, but there are significant differences in the data analysis and in the interpretation of the results. 

Figure 7. Schematic diagram of a flowing-afterglow electron-ion experiment. The diameter of flow tubes is typically 5 to 10 cm and the length is 1 to 2 meters. The carrier gas (helium) enters through the discharge and flows with a velocity of 50 to 100 m/s towards the downstream end of the tube where it exits into a fast pump. Recombination occurs mainly in the region 10 to 20 cm downstream from the movable reagent inlet, at which the ions under study are produced by ion-molecule reactions. The Langmuir probe measures the variation of the electron density in that region. A differentially pumped mass spectrometer is used to determine which ion species are present in the plasma.

Alternatively, the translational energy threshold for endothermic proton transfer from MH+ to R can be measured using a flowing afterglow triple quadrupole instrument.127 These data define the proton affinity of M, relative to that of R. Thus, the PA of cyclopropenylidene was found to exceed that of ammonia by 23.3 1.8 kcal/mol (Table 6).128 In order to obtain absolute proton affinities, the enthalpies of formation of both the base and the conjugate acid must be known from other measurements (Eq. 9). Numerous reference compounds with known absolute PA are available.124... 

The sequential removal of H and H+ from isobutene-type structural units (so-called H2+ abstraction ) was also used to generate the radical anion of non-Kekule benzene , i.e. l,3-dimethylenecyclobutane-l,3-diyl (39) (Scheme 11). As shown by Hill and Squires161, this highly unusual, distonic C(,II(, isomer can be produced in pure form by reaction of O with 1,3-dimethylenecyclobutane (38). Working in a flowing afterglow mass spectrometer, subsequent reactions were again used to characterize this radical anion and differentiate it from other ( VdL, isomers. 

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