Neutrals interaction with excited ions

The complications just described can be minimized if there is greater selectivity in the ionization process, as is sometime possible when photoionization is used as the excitation mechanism. Because the ionization energy can be more precisely controlled, it is possible in selected cases to produce only the desired reactant-ion species, or at least to minimize production of other ions. As already noted in the earlier section on formation of excited ions, it is also possible to populate specific internal-energy states of some reactant ions by using a photoionization source. One of the earliest photoionization mass spectrometers used to study interaction of internally excited ions with neutrals was that constructed by Chupka et al.91 Such apparatuses typically incorporate a photon source (either a line or a continuum source) and an optical monochromator, which are coupled to the reaction chamber. Various types of mass analyzer, including sector type, time-of-flight (TOF), and quadrupole mass filters, have been used with these apparatuses. Chupka has described the basic instrumental configuration in some detail.854 Photoionization mass spectrometers employed to study interactions of excited ions with neutral species have also been constructed in several other laboratories.80,1144,142,143 The apparatus recently developed by LeBreton et al.80 is illustrated schematically in Fig. 7 and is typical of such instrumentation. 

The mode of creation of polymerizable or chemically reactive species, i.e., chemically reactive species, is the interaction with excited neutrals of the carrier gas (no bombardment of electrons and ions). 

Note The process M H- X -> MX" + e , i.e., ionization of internally excited molecules upon interaction with other neutrals is known as chemi-ionization. Chemi-ionization is different from Cl in that there is no ion-molecule reaction involved [8,15] (cf. Penning ionization. Chap. 2.2.1). 

Several books and review chapters devoted to the field of ion-neutral reactions in the gas phase have appeared in recent years, la 8, j,k some of which are concerned at least in part with the special topic of interest for the present review chapter—namely, the role of excited states in such interactions. The present review attempts to present a comprehensive survey of the latter subject, and the processes to be discussed include those in which an excited ion interacts with a ground-state neutral, interaction of an excited neutral with a ground-state ion, and on-neutral interactions that produce excited ionic products or excited neutral products. Reactions in which ions are produced by reaction of an excited neutral species with another neutral, for example, Penning ionization, are not included in the present chapter. For a recent review of this topic, the reader is referred to the article by Rundel and Stebbings.1 Electron-molecule interactions and photon-molecule interactions are discussed here only as they relate to the production of ions in excited states, which can then be reacted with neutral species. 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

A considerable array of instrumentation has been developed in the past 10-15 years for the study of ion-neutral collisions. Such instruments can be classified into several categories, with each type having features that make it suitable for investigating particular aspects of the ion-neutral interaction. The present discussion is limited to those experimental techniques that have been specifically applied for examining the role of excited states in these processes. 

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