Ionization autoionization

In the preceding sections, we have assumed that an absorption line has a Lorentzian shape. If this is not true, then the linewidth cannot be defined as the full width at half maximum intensity. Transitions from the ground state of a neutral molecule to an ionization continuum often have appreciable oscillator strength, in marked contrast to the situation for ground state to dissociative continuum transitions. The absorption cross-section near the peak of an auto-ionized line can be significantly affected by interference between two processes (1) direct ionization or dissociation, and (2) indirect ionization (autoionization) or indirect dissociation (predissociation). The line profile must be described by the Beutler-Fano formula (Fano, 1961) ... 

Self-ionization (autoionization) a reaction in which two like molecules react to give ions. (16.6)... 

Conventional associative ionization (AI) occurring at ambient temperature proceeds in two steps excitation of isolated atoms followed by molecular autoionization as the two atoms approach on excited molecular potentials. In sodium for example [44]... 

The Bell equation gives the correct behavior for the ionization cross section at both high- and low-impact energies. In cases where autoionization is important it is not always possible to reproduce the cross section from the single equation above, but if it is used in two separate fits, one from the ionization threshold to the autoionization threshold, and the second above the autoionization threshold, a good fit to the cross section may be obtained over the entire range. 

Identify each of the following terms (a) hydronium ion, (h) Bronsted theory, (c) proton (Bronsted sense), (d) acid (Bronsted sense), (e) base (Bronstcd sense), (/) conjugate, (g) strong, (h) acid dissociation constant, (/) ionization constant, (/) base dissociation constant, (k) autoionization, (/) pH, and f/w) K .. 

Kw ionization constant for the autoionization of water. kelvln the unit of the Kelvin temperature scale. 

Following Platzman (1967), Magee and Mozumder (1973) estimate the total ionization yield in water vapor as 3.48. The yield of superexcited states that do not autoionize in the gas phase is 0.92. Assuming that all of these did autoion-ize in the liquid, we would get 4.4 as the total ionization yield. This figure is within the experimental limits of eh yield at 100 ps, but it is less than the total experimental ionization yield by about 1. The assumption of lower ionization potential in the liquid does not remove this difficulty, as the total yield of excited states in the gas phase below the ionization limit is only 0.54. 

The conductivity of liquid ammonia is sufficiently high to indicate a very slight degree of autoionization. In order for ions to be produced, something must be transferred from one molecule to another, and in solvents such as water or ammonia it is proton transfer that occurs. Accordingly, the ionization of liquid ammonia can be shown as... 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

This reaction represents a neutralization reaction in liquid sulfur dioxide. It makes no difference that the solvent does not ionize or that SOCl2 is a covalent molecule. The utility of the solvent concept is not that it correctly predicts that solvents undergo some autoionization. The value of the solvent concept is that it allows us to correctly predict how reactions would take place if the solvent ionized. Note that in this case SOCl2 does not ionize, but if it did it would produce S02+ (the acidic species characteristic of the solvent) and Cl-. 

The coordination model provides a way to explain many reactions that occur in nonaqueous solvents without having to assume that autoionization takes place. As shown in Eq. (10.17), the fact that FeCl4 is produced can be explained by substitution rather than autoionization. However, as has been shown earlier in this chapter, it is sometimes useful to assume that the solvent concept is valid, and many reactions take place just as if the solvent has ionized to a slight degree into an acidic and a basic species. 

This experiment may be regarded as the forerunner of mass spectro-metric appearance-potential determination in that both are threshold techniques, that is they depend on slow variation in the energy supplied by the impacting electron until a change in the electron-molecule interaction is observed. Thus, just as the Hertz experiment did not distinguish between excitation and ionization potentials, mass spectrometric appearance potential measurements are subject to similar ambiguities in interpretation as between ionization and autoionization. 

Field ionization essentially is a process of the autoionization type, i.e., an internally supra-excited atom or molecular moiety loses an electron spontaneously without further interaction with an energy source. [32] Different from electron ionization, there is no excess energy transferred onto the incipient ion, and thus, dissociation of the ions is reduced to minimum. 

Water undergoes autoionization to a small extent the ionization constant at 25°C is 1.008x10-14 ... 

For collisions involving fast electrons, most of the relevant reactions given by Eqs. (9.1)-(9.13) occur with the primary and secondary electrons leaving the target molecule promptly, in about 10 sec. One the other hand, autoionization and dissociative ionization channels can result in a secondary electron being delayed relative to the primary, and in the case of resonant electron attachment, there may be a measurable delay in the exit of the primary electron. These processes are described in considerable detail by Mark et al. [19]. 

According to the theoretical investigations by Nakamura [126-128] and Miller [129], the collisional energy dependence of these processes can be calculated if the interaction potential V(R) for the system He(2 S)-M and the autoionization rate r K)jh from the intermediate quasi-molecule [He(2 S)M] to the resulting quasi-molecular ion [HeM] are known. For example, by the classical formula of Miller [129], we have theoretical cross sections for Penning ionization as ... 

A elassieal expression for the eross seetion for eollisional de-excitation of He(2 P) is also derived from the formula by Eq. (16). However, the autoionization widths r R) for Penning ionization by resonant atoms are not identical to the empirical form of Eq. (18) for electron exchange. Instead, a direct transition due to a dipole-dipole interaction is proposed to govern this Penning ionization [126,139,140,143], that is,... 

In the present discussion, the total autoionization width for Penning ionization of He M or for decay of the transient molecular autoionizing state of [HeM] is assumed to be divided into two components a dipole-dipole part and an electron exchange part. The maximum width possible is assumed to be the sum of the two components [142], that is. 

The third line describes oscillation of a wave packet in an anharmonic potential (phase term omitted). Eq. (1) would be valid also in the presence of an intermediate resonance. In Eq. (2), the Apl dependence is only in the Plon terms. The fundamental arises from the sin terms (hence from dPion/dx) and the overtone from the sin2 terms. A phase jump of the fundamental is expected at a Tpr where dPlnn/dx and hence dPloir/dIE change sign. From these derivatives (proportional to the fundamental amplitude in Fig. 3) we can infer that Ploa has a maximum at 680 nm and a minimum near 405 nm. The maximum could reflect either an intermediate resonance or a two-photon resonance with an autoionizing state. The minimum is likely to announce a further rise of PIon at shorter Apr due to the lower order of ionization. 

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