Beam techniques merged

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 molecular beam technique, which was first applied to a chemical system in 1954 by Bull and Moon [165] and the first crossed beam study of Datz and Schmidt [123] in 1967, has reached maturity with the development by Greene and co-workers [373] in 1974 of a molecular beam apparatus for student teaching experiments. Future developments will probably be in the study of the reactions of beams of metals other than alkali and alkaline earths and atoms and radicals. Hopefully, the theories of such reactions will merge with the theories of ion—molecule reactions. 

In summary, it is perfectly obvious that the complete experimental investigation of the factors which may determine the rate of an ion-molecule reaction lies beyond our present limits, both by techniques already realized and by techniques conceivable but unrealized. (Even the above list is not exhaustive due to the possibility of spin-orientation of paramagnetic reactants.) Only in the past four years has it proved possible, by merged-beam techniques, to control the collision energy below 1 eV, at this time there exist only a few excitation functions at the level of refinement discussed in Section 1.4.1. A smattering of data exists for the conditions specified in Section 1.4.2, mainly in the form of relative cross sections. At the present time, the attainment of this stage, excitation functions for reactants in selected states, seems a sensible goal. For practical applications this is all that is needed. Yet, even if such excitation functions were available their... 

The actual situation is not as bleak as the above formal discussion of the problem might suggest. Accurate excitation functions are few in number, but the development of merged-beam techniques offers some hope of making an effective start on this problem in the immediate future. On the other hand, a vast body of information on rate constants has already been... 

The careful establishment of the merging-beams technique is an excellent example of what is needed. McDaniel s recent formulation of necessary criteria to be met for a reliable drift-tube measurement is another example. In contrast, one notes the lack of the necessary control experiments during the past development of the mass-spectrometer ion-source technique (Section 3.4.1) and during the recent development of the ion cyclotron resonance technique (Section 3.4.7). The necessary techniques may not always be available, but the effort must be directed to that end and not to the premature acquisition of data of unproven reliability. 

The alternative beam-beam technique involves confluent, superimposed, or merged beams. A most useful review of this important new development has recently appeared. Remarks here will be restricted to its use for ion-neutral reactions. The technique is important simply because it alone at the present time is yielding directly reliable excitation functions in the collision energy range of most importance to chemists,... 

TABLE II. Excitation Functions Measured by the Merged-Beam Technique... 

Fig. 6. Excitation function for the reaction H2 (H2,H)H3 obtained by the merged-beam technique. the cross section at collision energy WqV, is expressed relative to Qj, the cross section for W= eV. Each dot represents a single measurement a dot accompanied by a digit indicates that a number of measurements gave the same value a cross indicates the arithmetic average of the measurements for the particular value of W. An absolute determination gave Qx = 12 3 A. ...

It is surely desirable to obtain experimental evidence to demonstrate the relationship (3) between the temperature dependence of thermal rate constants (as measured by the flowing afterglow technique) and the energy dependence of the microscopic cross section (as measured by the merged-beam technique). Again, data from ion-molecule studies should be obtained to demonstrate the principle of microscopic reversibility, whose validity is unquestioned and whose utility is beginning to be explored. ... 

P. K. Rol, Low-Energy Interaction Studies by a Merging Beams Technique, Space Science Laboratory, General Dynamics/Convair, Report AFCRL-69-0324 (GDC-DBE69-006) (1970). 

The merging-beam technique has been used recently in studies of ion-molecule reactions of species such as 02" and The apparatus is... 

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. 

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