Stationary afterglow

While electrons in conventional beams have velocities too high to have large cross sections, thermal electrons have large cross sections for state changing collisions with Rydberg atoms, and these collisions have been studied in a systematic fashion. Specifically, metastable He atoms in a stationary afterglow have been excited to specific Rydberg states with a laser.37 38 The populations of... 

A decade ago, while considerable data had been compiled on the kinetic measurement of dissociative recombination (DR) reactions of small polyatomic ions, laboratory information on the product distributions of such reactions was restricted to the results of a few merged-beam and stationary-afterglow studies on DR of C02 and of H( [157,158], and the first explorations of combined flow tube/Langmuir probe/spectroscopic detector techniques, independently pursued by Rowe and co-workers (at Rennes) [159,160] and by Adams and co-work-ers (at Birmingham, and subsequently Atlanta) [161, 162]. Considerable advances have since been made, both in measurement of recombination coefficients (particularly for larger ions) and in the elucidation of product distributions for a still small but growing sample of important IS ions. 

Rather limited and sporadic use has been made of the stationary afterglow technique. Historically, it played an important role as the first method to produce data on reactions of aeronomic interest. Typically, a gaseous sample is subjected to a pulse of radiation or excitationf and the subsequent history of the decaying plasma is followed by sampling from it into a mass spectrometer. The phenomena governing this history, such as ambipolar diffusion of the ions and electron-ion recombination, are complex, and ion-neutral reactions constitute only one part of this. As always, the whole must be understood before reliable quantitative information can be obtained for a part, and the deciphering of the history to yield quantitative information on reaction rates is thus difficult. 

A discussion of these problems and a survey of results are covered in reviews of the stationary afterglow technique which appeared a few years ago (32,128,129) constants for a rather restricted number of reactions had been reported, including three-body processes, such as He" " + 2He Hc2 + He, and bimolecular processes of ionospheric interest. Efforts to study the reactions of negative ions had been unsuccessful. It could fairly be said at that time that the technique had produced few rate data which were not measured more readily by other techniques, but which nevertheless provided useful, independent corroboration. Such a remark... 

These papers represent an important advance in the understanding and technical control of stationary afterglows and hence of their use to produce rate data. The technique would seem to be better suited to three-body processes than fast bimolecular reactions indeed, the possibility of observing the latter for negative ions would seem to be excluded by the need to use long reaction times, during which positive-ion-negative-ion ambipolar diffusion dominates the transport loss processes. In conclusion, it seems unlikely at this time that the method will become a major method for the determination of rate parameters. 

Fig. 18. Compilation of cross-section data for the reaction He (N2,He + N)N as a function of the He" ion energy, made by Maier, to which reference should be made for the sources of the data. The long-dashed line is beam data the short-dash/long-dashed line is obtained from stationary afterglow data the solid line refers to longitudinal tandem data and the short-dashed line is the Langevin cross section.

Stationary Afterglow (B.H. Mahan) 300K >20 qj is determined. High pressures,therefore cluster ions are a potential problem. Extrapolation to zero pressure is required. 

X 10- 300K Stationary Afterglow Leu et al 1973b Impurity ions... 

X lO-" 300K Stationary Afterglow Macdonald et al 1984 Impurity ions present... 

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