Chemi-ionization process

The clean flames show low levels of ionization close to the reaction zone because of chemi-ionization processes such as ... 

In our laboratory a microwave discharge was generated in helium by a 100 watt Raytheon microtherm generator and sampled through a pinhole leak at the apex of a conical probe into a quadrupole mass filter. Typical variations in the intensities of He+ and He2+ with pressure are given in Figure 1. If the He2+ is formed by the three-body process (Equation 11) the rate constant would have to be about 10"28 cc.2/mole-cule2/sec. which far exceeds the measured value (84). We conclude then that the diatomic ion is formed principally by the chemi-ionization process (Equation 10). 

In 1951, Hornbeck and Molnar successfully determined that the formation of the diatomic rare-gas ions occurred by a chemi-ionization process in which an electronically excited atom reacts with an atom in the ground state to form an ion i.e.. 

As a result of the strong binding between U and O, and the low ionization energies of UO and UO2, these reactions are substantially exothermic, by greater than 200 kJ mol and proceed spraitaneously under low-energy conditions. Associative reactions between U and O2 proceed at approximately the collisional rate, but the chemi-ionization processes account for only 1% of the products, with neutral UO-pO as the dominant products. In the case of the reaction of U and O, where no neutral channel is accessible, the chemi-ionization process proceeds with a rate close to the collisional limit. Dyke et al. (1988) studied the chemi-ionization reaction of U with O2 (Eq. 36) and that of U with N2O, Eq. (37), by chemielectron spectroscopy ... 

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). 

The mechanism of chemi-ionization and of CH( 4 2A) emission has been studied in considerable detail269,331 but it was not possible to decide on a unique mechanism for the emission process. Observed rotational distributions in the CH (A 2 A) state have been interpreted in terms of rotational relaxation270 but the uncertainty in the excitation mechanism renders such an enterprise somewhat ambiguous. 

One may alternatively remove an electron from one of the n bonds of 3,3 -bicyclopropenyl, although in fact this process produces the same radical cation as before because of extensive through-bond interactions. Solution phase, photoexcited quinone-induced chemi-ionization of 3,3 -dimethyl-3,3 -bicyclopropenyl (22) results in the... 

Penning and Associative Ionization of Atoms.—Several workers have described chemi-ionization reactions of He with an atom, M, in terms of autoionizing transitions between adiabatic potential curves, V and V, correlating with He + M and He + M respectively. Figure 2 shows schematic curves and transitions, representing the processes ... 

They found that the intensity of the diatomic ion increased as the second power of pressure and its appearance potential was less by 0.7 to 1.5 eV than the ionization potential of the rare gas in question and, hence, the nature of the reaction was established. Similar chemi-ionization reactions have been observed for a number of systems other than the rare gases. Although chemi-ionization is not an ion-molecule reaction, it has subsequently been studied by a number of mass spectrometrists who were interested in collision reactions of ions, and reports in the literature often contain results on both processes. Consequently, it seems reasonable to include a discussion of the chemi-ionization in this volume. 

Originally, studies of ionization in flames were motivated by the observation that in hydrocarbon flames, the ion concentration far exceeds the value expected if ionization were due to thermal processes alone (see Table I). The objectives of these studies were to explain the source of nonequilibrium ionization and to explore links between flame ionization and the mechanism of flame propagation. An explanation of the source of flame ions was found in the process of chemi-ionization. This led to further studies of the details of ionic reactions which occur in flames and of flame reactions which can be induced by the addition of foreign elements. 

Chemi-ionization may be defined as the process by which ionization is produced by the energy release on the formation of a new chemical bond in general term,. + (4)... 

Lee and co-workers studied the dynamics of the Ba(6p P) -F CI2 system, which yields both BaCl -F Cl reaction products and Ba+ -F CI2 charge-transfer products, the two processes being characterized by their chemi-ionization emission (see Davies et al (1990)). At 3 eV total energy, laser excitation to Ba(6p) enhanced the Ba+ intensity measured in the backward direction. As shown in Figure 22.6, this Ba" " intensity exhibits a maximum when the p-orbital of barium is aligned along... 

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