Electron ionization processes

Fig. 5.3. Schematic time chart of possible electron ionization processes. Reprinted from Ref. [4] with permission. John Wiley Sons, 1986.

These observations suggest that the 7P] and TP atomic scales are merely reflecting the all valence electron ionization process rather than a single-electron releasing, as is commonly reported relating the electronegativity concept, see Table 4.3. 

To explain the difference of correlation when the wavelength is modified, two effects may be considered. The first effect is the formation of the monomer ions. It is assumed to result from processes similar to electron ionization between neutral species and electrons expelled from the surface by the laser ablation. However, the fact that the correlation is not always linear leads us to consider that electron ionization processes may not be involved alone in the formation of monomer ions. The energy per photon delivered at 193 nm (6.42 eV) or at 266 nm (4.66 eV) may be sufficient to ionize styrene and butadiene compounds whose ionization energies are 8.4 and 9.1 eV, respectively, if it is assumed that molecules are in excited states in our conditions. In this case, butadiene and styrene ionization may be different according to the population of their energetic and vibrational levels. Consequently, the loss of linearity at... 

Applicability of the modern TDDFT approaches for the treatment of multiple electron ionization processes is another problem related to the quality of time-dependent xc-energy functionals. Most of approximate xc functionals lack the important property of the exact functional, the discontinuity of its derivative with respect to the number of particles N, when N passes through integer values [78]. Several attempts to apply TDDFT with such approximate functionals for calculations of nonsequential double ionization were unsuccessful [79,80]. Recently it was shown [81] that the derivative discontinuity is crucial for correct description of double ionization. 

The incident electrons must have an energy greater than the ionization energy of the target gas molecule M, defined as the energy required to remove the electron held most weakly within the molecule. The electron ionization process can be written for the gas-phase sample molecule M ... 

The extra sources of electrons that become important are known as secondary ionization processes and are caused by ... 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

The above direct process does not produce a high yield of ions, but it does form many molecules in the vapor phase. The yield of ions can be greatly increased by applying a second ionization method (e.g., electarn ionization) to the vaporized molecules. Therefore, laser desorption is often used in conjunction with a second ionization step, such as electron ionization, chemical ionization, or even a second laser ionization pulse. 

Electron ionization. Ionization of any species by electrons. The process can be written for atoms or molecules as ... 

Ionization cross-section. A measure of the probability that a given ionization process will occur when an atom or molecule interacts with an electron or a photon. 

The total electron density contributed by all the electrons in any molecule is a property that can be visualized and it is possible to imagine an experiment in which it could be observed. It is when we try to break down this electron density into a contribution from each electron that problems arise. The methods employing hybrid orbitals or equivalent orbitals are useful in certain circumsfances such as in rationalizing properties of a localized part of fhe molecule. Flowever, fhe promotion of an electron from one orbifal fo anofher, in an electronic transition, or the complete removal of it, in an ionization process, both obey symmetry selection mles. For this reason the orbitals used to describe the difference befween eifher fwo electronic states of the molecule or an electronic state of the molecule and an electronic state of the positive ion must be MOs which belong to symmetry species of the point group to which the molecule belongs. Such orbitals are called symmetry orbitals and are the only type we shall consider here. 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

SNMS sensitivity depends on the efficiency of the ionization process. SNs are post-ionized (to SN" ) either hy electron impact (El) with electrons from a hroad electron (e-)heam or a high-frequency (HF-) plasma (i.e. an e-gas), or, most efficiently, hy photons from a laser. In particular, the photoionization process enables adjustment of the fragmentation rate of sputtered molecules by varying the laser intensity, pulse width, and/or wavelength. 

Typical ion sources employ a noble gas (usually Ar). The ionization process works either by electron impact or within a plasma created by a discharge the ions are then extracted from the region in which they are created. The ions are then accelerated and focused with two or more electrostatic lenses. These ion guns are normally operated to produce ions of 0.5-10 keV energy at currents between 1 and 10 pA (or, for a duoplasmatron, up to 20 pA). The chosen spot size varies between 100 pm and 5 mm in diameter. 

---