Processes Accompanying Electron Ionization

The electron could also be captured by the neutral to form a negative radical ion. However, electron capture (EC) is rather unlikely to occur with electrons of 70 eV since EC is a resonance process because no electron is produced to carry away the excess energy [16]. Thus, EC only proceeds effectively with electrons of very low energy, preferably with thermal electrons (Chap. 7.4). Nonetheless, 

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9] 

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Two features of electron affinity to remember are (1) The electron affinity of an element is equal to the enthalpy change that accompanies the ionization process of its anion, and (2) a large positive electron affinity means that the negative ion is very stable (that is, the atom has a great tendency to accept an electron), just as a high ionization energy of an atom means that the atom is very stable. 

Our formal and numerical analysis of the EOM-Green s function methods has indicated a number of generalizations of these methods that are necessary for an accurate description of the electronic processes that accompany excitation, ionization, and electron attachment. Although the bulk of the numerical work has centered on the IP-EA variant of the EOM theory, similar behavior should be expected in the excitation energy version of the theory. These generalizations necessarily introduce complications in the implementation of the EOM approach. 

The mode of radioactive decay is dependent upon the particular nuclide involved. We have seen in Ch. 1 that radioactive decay can be characterized by a-, jS-, and y-radiation. Alpha-decay is the emission of helium nuclei. Beta-decay is the creation and emission of either electrons or positrons, or the process of electron capture. Gamma-decay is the emission of electromagnetic radiation where the transition occurs between energy levels of the same nucleus. An additional mode of radioactive decay is that of internal conversion in which a nucleus loses its energy by interaction of the nuclear field with that of the orbital electrons, causing ionization of an electron instead of y-ray emission. A mode of radioactive decay which is observed only in the heaviest nuclei is that of spontaneous fission in which the nucleus dissociates spontaneously into two roughly equal parts. This fission is accompanied by the emission of electromagnetic radiation and of neutrons. In the last decade also some unusual decay modes have been observed for nuclides very far from the stability line, namely neutron emission and proton emission. A few very rare decay modes like C-emission have also been observed. 

It has already been seen that, for an oxidation process to proceed, under conditions where the two reactants are separated by the reaction product, it is necessary to postulate that ionic and electronic transport processes through the oxide are accompanied by ionizing phase-boundary reactions and formation of new oxide at a site whose position depends upon whether cations or anions are transported through the oxide layer. 

Cluster IP s can be derived theoretically in various ways. At the simplest theoretical level (in a wave function based method), one may use Koopmans theorem in HF theory and take the negative of a cluster orbital energy, as a crude measure of the energy required to remove one electron from this orbital. This procedure neglects the electronic relaxation which will accompany the ionization process. Such relaxation effects can be accounted for by computing the energy difference between a neutral cluster and its cation. 

Not much is known about these processes, but they must be included to give a total picture. Emissions of Lyman and Balmer spectra of the H atom upon e-impact on hydrocarbons, H2, and H20, discussed in Sect. 4.3.2, fall in this category. Similarly, many of the excited states observed in dissociated radicals via electron impact on stable molecules (Polak and Slovetsky, 1976) also belong to this category. It is known from the dipole oscillator spectrum of H20 (Platzman, 1967) that most ionizations are accompanied by considerable excitation. Excitation transfer to the neighboring neutral molecule followed by fast dissociation cannot be ruled out. 

In principle, refined and relatively reliable quantum-theoretical methods are available for the calculation of the energy change associated with the process of equation 2. They take into account the changes in geometry, in electron distribution and in electron correlation which accompany the transition M(1 fio) — M+ (2 P/-), and also vibronic interactions between the radical cation states. Such sophisticated treatments yield not only reliable predictions for the different ionization energies 7 , 77 or 7 , but also rather precise Franck-Condon envelopes for the individual bands in the PE spectrum. However, the computational expenditure of these methods still limits their application to smaller molecules. We shall mention them later in connection with examples where such treatments are required. 

We have tacitly assumed that the photoemission event occurs sufficiently slowly to ensure that the escaping electron feels the relaxation of the core-ionized atom. This is what we call the adiabatic limit. All relaxation effects on the energetic ground state of the core-ionized atom are accounted for in the kinetic energy of the photoelectron (but not the decay via Auger or fluorescence processes to a ground state ion, which occurs on a slower time scale). At the other extreme, the sudden limit , the photoelectron is emitted immediately after the absorption of the photon before the core-ionized atom relaxes. This is often accompanied by shake-up, shake-off and plasmon loss processes, which give additional peaks in the spectrum. 

For molecules containing atoms of high electron affinity the photocurrent at the first threshold may be due to a dissociation into ions. This phenomenon was already demonstrated in the early 1930 s by Terenin and Popov28 for TIHal vapors, which split into T1+ + Hal - as a primary photoprocess. Such was the first instance of the application of mass spectrometry to the study of the photoionization of gases. A similar process has been later shown by Morrison et al.8 for Br2 and I2, the first threshold, corresponding to a pre-ionization, accompanied by the dissociation into Hal+ + Hal-. 

Since the nature of chemical combination is electrical it is natural to inquire whether there is any essential connexion between chemical activation processes and ionization. From the early days of the electron theory experiments have been made with the object of establishing such a relationship, but most of the evidence seems to indicate that any ionization accompanying ordinary chemical reactions is very small and probably of a purely secondary character. ... 

Even though the differences are probably smaller in large molecules, they may still be large enough to explain why the calculated values of the ionization potentials are systematically higher than the observed ones. Such a behavior is compatible with a mechanism in which a redistribution of electronic densities accompanies the process of ionization, thus increasing the stability of the ion and lowering the ionization potential. 

Some partial photoionization cross sections, derived in this way for neon, are shown in Fig. 2.11 as a function of photon energy. The uppermost curve is the total absorption cross section. At the onset of the ionization thresholds for the ejection of Is, 2s and 2p electrons this quantity shows the corresponding absorption edges (see the discussion related to equ. (2.11)). The partition of the total cross section into partial contributions cr(i) clearly demonstrates that the dominant features are due to main photoionization processes described by the partial cross sections satellite transitions from multiple photoionization processes are also present. If these are related to a K-shell ionization process, they are called in Fig. 2.11 multiple KL where the symbol KLX indicates that one electron from the K-shell and X electrons from the L-shell have been released by the photon interaction. Similarly, multiple I/ stands for processes where X electrons from the L-shell are ejected. Furthermore, these two groups of multiple processes are classified with respect to ionization accompanied by excitation, (e, n), or double ionization, ( ,e). If one compares in Fig. 2.11 the magnitude of the partial cross sections for 2p, 2s and Is photoionization at 1253.6 eV photon energy (Mg Ka radiation) and takes into account the different... 

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