Merged beam

As with most methods for studying ion-molecule kinetics and dynamics, numerous variations exist. For low-energy processes, the collision cell can be replaced with a molecular beam perpendicular to the ion beam [106]. This greatly reduces the thennal energy spread of the reactant neutral. Another approach for low energies is to use a merged beam [103]. In this system the supersonic expansion is aimed at the tluoat of the octopole, and the ions are passed tluough... 

Since the experimental work on H3 recombination provided much of the impetus for this work, we will compare two recent experiments, a merged-beam and a plasma afterglow experiment. We conclude that the results can be reconciled by postulating the formation of long-lived Rydberg states and that these are quite sensitive to the environment in which the recombination occurs. In beam experiments (Section TV. B ) the observed cross sections are shown to be sensitive to the presence of electric fields. In the afterglow experiments, the concentration of third... 

We will be rather brief in describing the principles of the experimental methods that are most commonly used in studies of dissociative recombination, afterglows and colliding beams (merged beams and ion storage rings). It should suffice to recount some of their generic properties. 

Merged-beam measurements 23-26 have consistently shown that the measured recombination cross section depends on conditions in the ion source. The authors have ascribed the effect to differing vibrational distributions. In one of the later measurements,16 the Hj vibrational state was inferred from the threshold energy for electron-ion dissociative excitation,... 

Figure 2. Dissociative recombination cross section for H (all v) measured using the inclined-beam method (solid triangles, from ref. 30) and by the merged-beam method (curve, from ref. 31).

Figure 3. Cross sections for the dissociative recombination of H3 leading to H2 + H-formation. Inclined-beam results open circles (from ref. 36). Merged-beam results closed circles (from ref. 37).

Figure 4. Comparison of measured dissociative recombination cross sections for H. Inclined-beam results for cold ions (triangles with dots, from ref. 38). Afterglow results for cold ions (open squares with dots, from ref. 15). Merged-beam results (from ref. 23) for excited ions (solid circles), for cold ions (open squares) (from ref. 23), and for ions with intermediate vibrational excitation (solid squares and open circles).

Figure 6. Cross sections for the dissociative recombination of H3 (v = 0). Storage-ring results (ref. 28) dashed line. Merged beam results (ref. 42) for a deflector field of 3kV/cm (closed circles) and a deflector field of 200 V/cm (open circles).

The anomalous plasma decay suggests that the deionization coefficient is larger at higher electron densities and higher H2 densities. The second interpretation proposes that electrons and H2 molecules can act as stabilizing third bodies and that this process involves the same long-lived intermediate complexes that are observed in merged beam experiments (see Section IV.B). 

In merging beam experiments, in which the metastable beam is formed by charge exchange with alkali atoms and the target beam by resonant... 

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. 

Since the dissociation energy of 02" , 6.16 eV, is higher than that of O2, this reaction is equally or less probable than reaction (19). To date, it has only been observed by Rol and Entemann in the reactions of O2+ -f Na. The former merged beam study, however, is questionable given the failure to observe NaO+ in a number of sodium studies based on reactions (19) and (20) ° as well as in high temperature mass spectrometry measurements. Lo et have also reported CaO+ formation in ther-... 

Only three methods proved to yield beams of an intensity high enough to perform excitation experiments beam production by ion neutralization, sputtered beams and seeded supersonic beams. The merging beam technique26 does not seem to be suited for the measurement of optical excitation cross sections because of the very long beam interaction path. 

MERGED-BEAMS STUDIES OE ELECTRON-MOLECULAR ION INTERACTIONS IN ION STORAGE RINGS... 

The present article will be restricted to discussions of dissociative recombination and resonant ion-pair formation (process 3). Dissociative excitation, dissociative ionization, and electron impact detachment of negative ions have been reviewed recently.The field of merged-beams experiments in atomic and molecular physics has recently been reviewed, and the reader is referred to this excellent article for a detailed discussion of the merged-beams technique. 

The merged-beams geometry in the electron cooler section gives the possibility to study electron-molecular ion interactions at meV collision energies. The spread of relative energies of electrons and ions can be made very small and depends essentially only on the quality of the electron beam. 

Most of the studies in this decade were carried out with conventional single source mass spectrometers, which limited the kind and accuracy of the information. During the next decade, however, various sophisticated techniques for the study of ion—molecule reactions, such as tandem mass spectrometers, photoionization sources, pulsed sources, flowing afterglow and drift tube methods, crossed and merging beams and ion cyclotron resonance, have been developed. Much detailed information on various aspects of ion—molecule reactions has accumulated, and this has consequently stimulated the theoretical studies as well. This decade was, so to speak, the second epoch in the history of ion—molecule studies. 

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