Internal ion-molecule reactions

The advent of ultrafast pump-probe laser techniques62 and their marriage with the TOF method also enables study of internal ion-molecule reactions in clus-ters.21,63-69 The apparatus used in our experiments is a reflectron TOF mass spectrometer coupled with a femtosecond laser system. An overview of the laser system is shown in Figure 4. Femtosecond laser pulses are generated by a colliding pulse mode-locked (CPM) ring dye laser. The cavity consists of a gain jet, a... 

An example of the usefulness of the reflectron technique discussed earlier in this chapter is evident for the case of ammonia clusters in Figures 3b and c. Upon ionization, ammonia undergoes an internal ion-molecule reaction leading to protonated cluster ions, and concomitant evaporative unimolecular dissociation. This can be viewed in the context of equations 7-9 and the following ... 

Upon multiphoton ionization of the neutral ammonia clusters, protonated ions are formed via the following internal ion-molecule reactions (Echt et al. 1985 Morgan and Castleman 1989 Wei et al. 1990a,b) ... 

Since the observation made in study of the formation HeH+ indicated that this product was not formed by reaction of He + with H2, it had been assumed that the exothermic heat of reaction of He+ ions with H2 is probably deposited in the product HeH + as internal energy, decomposing the product into H+ and He. This idea was cited by Light (16) in his phase space theory of ion-molecule reactions to account for the failure to observe HeH+ from reactions with He+ ions. The experimental difficulty in the mass spectrometric investigation of this process is that H + formed by electron impact tends to obscure the ion-molecule-produced H+ so that a sensitive quantitative cross-section measurement is difficult. 

In 1960 Tal roze and Frankevich (39) first described a pulsed mode of operation of an internal ionization source which permits the study of ion-molecule reactions at energies approaching thermal energies. In this technique a short pulse of electrons is admitted to a field-free ion source to produce the reactant ions by electron impact. A known and variable time later, a second voltage pulse is applied to withdraw the ions from the ion source for mass analysis. In the interval between the two pulses the ions react under essentially thermal conditions, and from variation of the relevant ion currents with the reaction time the thermal rate constants can be estimated. 

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). 

In evaluating the viability of a new experimental technique for studying ion-molecule reactions, a number of factors must be considered. Ultimately our aims are to measure relative cross sections for reactions as a function of both the internal energy of the ion and the collision energy. It is important that the collision energy can be varied down to = 10 meV where the rotational energy may be comparable with the translational energy. 

With more conventional photoelectron spectroscopy (PES), the photon energy is fixed and the distribution of kinetic energies of the electrons produced by photoionization determined. The latter distribution reflects the distribution of ion internal states produced. Photoelectron spectroscopy has been applied to determine vibrational-state distributions at the same photon wavelengths at which an ion-molecule reaction was studied using photoionization methods (Fig. 4).86d... 

Most of the studies of ions formed by charge transfer have been concentrated on the unimolecular reactions of M+ ions formed in well-defined internal-energy states (e.g., fragmentation patterns6) and more recently have been concerned with rate-coefficient measurements.118 Some work has also been reported on consecutive ion-molecule reactions of M+ ions produced in well-defined internal states (mostly... 

G. G. Meisels, F. Botz, R. K. Mitchum, and E. F. Heckel, Angular Momentum and the Temperature Dependence of Ion-Molecule Reactions," paper presented at Seventh International Mass Spectrometry Conference, Florence, 1976. 

The theoretical treatment here is even more fraught with difficulties than that for ion-molecule reactions. Here, both the reactants and the activated complex are assumed to be dipolar. The theory predicts that there will be a spread in the values of A S leading to p factors greater and less than unity, but these p factors are predicted to be close to unity. The observed p factors vary over a large range, Table 7.3, and are comparable to those for ion-ion-like charge reactions. The conclusion is that the electrostatic treatment may be totally inadequate, or that the effects of the internal structure are important. 

P. P. Caspar, D. Read, Ion-molecule reactions of R-Si", a new class of reactive intermediates with four valence electrons, XIII International Symposium on Organosilicon Chemistry, Guanajuato, Mexico, Aug. 25 - 30, 2002. 

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