Dissociative Recombination in Ion-Electron Collisions: New Directions

Abstract

The dissociative recombination of molecular ions, which can be represented as

$$\text{e}\; + \mathop {AB}\nolimits^ + \; \to \;\mathop {AB}\nolimits^{**} \; \to \;\mathop A\nolimits^* \; + \;B,$$

(9.1)

where A and B can be atoms or molecules, is a difficult process to study experimentally for it is only significant at low collision energies. Two main approaches to the problem have been used: plasma decay methods and intersecting beam experiments. In the former, a plasma containing the ion to be studied is created and the decay of the electron concentration is monitored as a function of time. If the particular ion under study dominates the plasma and competing electron removal mechanisms such as diffusion to the walls of the apparatus are minimised, then the rate coefficient for the recombination can be determined from the rate of the electron density decay. In the latter approach, a mass selected ion beam is made to intersect an electron beam and the resulting recombination products are detected directly. In this way, the recombination cross section can be determined. A variety of individual variants of these approches have been developed, including the microwave afterglow technique [9.1], the flowing afterglow Langmuir probe method (FALP) [9.2], the merged [9.3], inclined [9.4], and crossed [9.5] beam techniques. Hybrid techniques such as the quadrupole ion trap [9.6] and the electron beam trap techniques [9.7] have also been applied to this subject.