Auto-ionization process

The other application explained here is the ionic product of water, pKw [11], which is essentially the equilibrium constant of the auto-ionization process of water (Kw =... 

The vibrational distribution of H2"(X2X+ v) ions, created by various ionizing (or auto-ionizing) processes of H2(ArA w) discussed in previous two sub-sections, is subject to further redistribution due to collision processes... 

Since water is amphiprotic, that is, it can act both as an acid and as a base, every aqueous solution is characterized by the auto-ionization process, in which one H2O molecule transfers a proton to another H2O molecule. The water auto-ionization equilibrium must always be satisfied, whether other acids or bases are present in the solution or not. 

To illustrate the above items, let us consider phosphorus oxychloride, POCl3 [22] as an example of ionizing solvents. The auto-ionization process of POCI3 may be described by the following equation ... 

Moreover, it should be emphasized that the classic solvosystem concept considers the auto-ionization process to be primary, whereas substances dissolved are referred to as acids and bases by the formation of their adducts with cations and anions of the solvent. 

What is the proposed mechanism of initiation of polymerization, by the Lewis arid like aluminum chloride or bromide through an auto ionization process ... 

The two color ionization experiment povides a valuable technique to investigate a large number of vibrational levels of an ion, that are not accessible by excitations from the electronic groundstate, and to determine the behaviour of the ionization cross section in the vicinity of the ionization energy. Contributions from auto-ionizing processes can be... 

The second, more direct, piece of experimental evidence is provided by the study of the auto-ionization process... 

S Kinetic Emission Model The Kinetic Emission model proposed by Joyes (.loyes 1963, 1969a, 1969b) was developed to explain the yield trends exhibited by the positive multiply charged atomic ions from elements lighter than Phosphorus. This mechanism describes the formation of multiply charged ions as being formed as a result of the de-excitation of core holes present in the sputtered atomic/ionic populations, i.e. an auto-ionization process. 

This auto-ionization process competes with the radiative decay of the Rydberg level, often also with possible predissociation. However, the spontaneous transition probability is approximately proportional to n 3. For large quantum numbers n the radiative decay therefore becomes very slow and even small auto-ionization probabilities may already result in complete auto-ionization of a Rydberg level if predissociation can be neglected. This means that the excitation of these molecular Rydberg levels above the ionization limit can be very effectively monitored through the resulting ion. 

For most of the molecules, the electron flow is from D to A, which is supported by the anti-Aviram-Ratner mechanism of Fig. 11a. However, there may be yet another possibility, shown in Fig. 21 implicit in the analyses of Figs. 9-11 has been the Aviram and Ratner assumption [79] that auto-ionization is a less efficient competing process. If the electric field induces intramolecular ionization first by sufficiently altering the orbital energies, then the direction of electron flow may occur in the anti-AR direction (Fig. 21) ... 

Fig. 21 (a) Rectification in the AR sense electron transfers occur first between electrode MD and the HOMO of the D-o-A molecule (1 ), and between the LUMO and MA (1), thus establishing the zwitterionic excited state D+-a-A , which then (2) relaxes back into the neutral state, (b) Rectification in the anti-AR sense if auto-ionization occurs first (1), forming the excited state D+-a-A by an interaction of the intense electric field and the molecule, followed by transfers to and from the electrodes (2) and (2 ), resulting in the electron passing from MD to MA. The molecular energy levels, which in reality must perforce shift dramatically during the electron transfer process, are drawn here for simplicity as if the molecule were still isolated in the gas phase ... 

There are also a number of non-radiative processes by which the excited (NA, v) states can decay. Auto-ionization and pre-dissociation are the most prominent ones,... 

The pre-dissociation results from a non-adiabatic coupling between electronic states (NA v) and (N A ec) at internuclear distances (usually smaller than the equilibrium distance) that correspond to the vibrational continuum of N A state, or when the N A state is a purely dissociative one. Autoionization and pre-dissociation probabilities for Npa(v) and Npir(v)(N > 4) states have been calculated in [68], while for the states with N < 3 they have been evaluated from experimental measurements [69,70]. Auto-ionization and pre-dissociation rates are very high ( 108s 1 for N = 3,4), and rapidly increase with increasing v. More experimental or theoretical information is needed for these processes. 

In the case where an acid (base) dissolved in the solvent is stronger than the acid (base) formed by the solvent auto-ionization, the phenomenon of levelling of acidic (basic) properties is observed. It consists in complete solvolysis of the dissolved acid or base with the formation of an equivalent quantity of the acid or the base of the solvent. For example, in aqueous medium the following process takes place ... 

In order to generalize the definition of the solvent system for the case of ionic media, we shall analyse Franklin s definition. First, it should be noted that the term auto-ionization in this definition should be substituted by auto-dissociation or intrinsic acid-base equilibrium of the solvent , as a more common case of heterolytic break down of the constituent particles of a liquid. Indeed, for molecular solvents or those which are slightly ionized at room temperature, the terms autoionization and intrinsic acid-base equilibrium of the solvent , relate to the same process, whereas for ionic liquids they differ considerably. For example, although sodium nitrate (NaNOs) is subject to practically... 

Nevertheless, the behaviour of oxide-containing melts as a kind of background for acid-base reactions can be described in a manner similar to the solvosystem concept. However, for this purpose the solvosystem concept should be generalized in some ways. As mentioned above, the main object in the basis of Franklin s solvosystem concept is a molecular solvent prone to auto-ionization, and this process results in the formation of small concentrations of cations and anions of the solvent. Water, glycol, different spirits and some other room-temperature liquids are to be mentioned as typical examples of such solvents. 

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) ... 

If the electronic excitation energy of a metastable atom A exceeds the ionization potential of another atom B, their collision can lead to an act of ionization, the so-called Penning ionization. The Penning ionization usually proceeds through the intermediate formation of an unstable excited quasi molecule in the state of auto-ionization cross sections of the process can be very high. Cross sections for the Peniung ionization of N2, CO2, Xe, and Ar by metastable helium atoms He(2 S) with an excitation energy of 19.8 eV reach gas-kinetic... 

This process proceeds by the two-stage Bloch-Bradbury mechanism (Bloch Bradbury, 1935 Alexandrov, 1981) starting with the formation of a negative ion (rate coefficient att) in an unstable auto-ionization state (r is the time of collisionless detachment) ... 

A water molecule has amphoteric character. This means it can act as both an acid and a base. The auto-ionization equilibrium process in water,... 

This blockage of the process can often be avoided for elements with more than one valence electron, by tuning the ionization laser to an auto-ionizing state, which decays rapidly within its lifetime of far less than 1 ns into an ion and a free electron. This is particularly effective for lanthanide and actinide elements, which have a high density of auto-ionizing states near the... 

The lower trace in Fig. 10A shows the number of ions Nign(2v2) produced with LI off. The peaks correspond to resonant transitions (4), as could be proved by observation of the corresponding fluorescence (lowest trace in Fig. 10B). When the pump laser LI is switched on, the upper trace in Fig. 10A is recorded. The difference between the two curves (note that the upper curve is vertically shifted by an amount indicated by the arrow in order to separate the two recordings) gives the number of ions produced by one photon hvg on a transition from a labelled level (v, J ) in the intermediate state. The auto-ionization lines can be seen superimposed on a continuum which increases with decreasing wavelength Aj. The continuum is attributed to the direct photoionization process (3) and from its onset at... 

Compared to atomic auto-ionizing states, where lifetimes below 10"12 s are common, the molecular levels exhibit a much weaker coupling to the ionization continuum. The reason is that the molecular ionization process takes place through a coupling between nuclear motion and electronic configurations, whereas in atoms a direct electronic coupling of the excited electron to the continuum makes the auto-ionization probability by far larger. 

This process is referred to as auto-ionization (AI). It does not follow that all AI transitions lead to completely uncorrelated carrier pairs. Most of the emitted electrons will thermalize within the Coulomb capture radius of the geminate positive ion, forming a transient charge-transfer state that can either decay to the ground state or dissociate by the absorption of ambient energy. Following Jortner (3), the wavefunction of such an AI state can be written as... 

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