Ionization probabilities

Solutions of alkah metal and ammonium iodides in Hquid iodine are good conductors of electricity, comparable to fused salts and aqueous solutions of strong acids. The Hquid is therefore a polar solvent of considerable ionising power, whereas its own electrical conductivity suggests that it is appreciably ionized, probably into I" and I (triodide). Iodine resembles water in this respect. The metal iodides and polyiodides are bases, whereas the iodine haHdes are acids. 

Local Thermodynamic Equilibrium (LTE). This LTE model is of historical importance only. The idea was that under ion bombardment a near-surface plasma is generated, in which the sputtered atoms are ionized [3.48]. The plasma should be under local equilibrium, so that the Saha-Eggert equation for determination of the ionization probability can be used. The important condition was the plasma temperature, and this could be determined from a knowledge of the concentration of one of the elements present. The theoretical background of the model is not applicable. The reason why it gives semi-quantitative results is that the exponential term of the Saha-Eggert equation also fits quantum-mechanical expressions. 

The element sensitivity is determined by the ionization probability of the sputtered atoms. This probability is influenced by the chemical state of the surface. As mentioned above, Cs" or OJ ions are used for sample bombardment in dynamic SIMS, because they the increase ionization probability. This is the so-called chemical enhancement effect. 

The product netx(Px.o) fx is equal to the post-ionization probability a ... 

Fig. 3.36. Experimental, Fe-related HF- calculated according to [3.74] from plasma SNMS sensitivity factors S(pe)x Ref [3.71] (salts) [3.72] alloys, [3.73] with elements X ordered according to round robins (r.r.). their post-ionization probabilities...

Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

The power dissipated at two different frequencies has been calculated for all reactions and compared with the energy loss to the walls. It is shown that at 65 MHz the fraction of power lost to the boundary decreases by a large amount compared to the situation at 13.56 MHz [224]. In contrast, the power dissipated by electron impact collision increases from nearly 47% to more than 71%, of which vibrational excitation increases by a factor of 2, dissociation increases by 45%, and ionization stays approximately the same, in agreement with the product of the ionization probability per electron, the electron density, and the ion flux, as shown before. The vibrational excitation energy thresholds (0.11 and 0.27 eV) are much smaller than the dissociation (8.3 eV) and ionization (13 eV) ones, and the vibrational excitation cross sections are large too. The reaction rate of processes with a low energy threshold therefore increases more than those with a high threshold. 

All elements can be detected and measured with a sensitivity of the order of 1 ppm. The analysed volume is typically 0.1 cubic micrometers, so a sensitivity of 10 19g is possible. Different isotopes can be distinguished, but the ionization probability is matrix dependent, which makes quantitative analysis difficult. 

The interaction will become weaker as the electron wavelength becomes greater or less than the molecular diameter with a consequential decrease in the cross section. This leads to an expression for the ionization probability as a function of electron energy, giving the shape of the ionization efficiency curve,... 

Recent experiments11 have shown that for symmetric top molecules, such as methyl chloride, electron impact ionization is more probable at the positive end of the dipole. This follows intuitively from simple electrostatics, since it would be expected that an electron should be more strongly attracted to the positive end of the dipole, leading to a greater ionization probability at this end of the molecule. The effect may be quantified in terms of a steric ratio R. 

Experimental considerations Sample preparation and data evaluation are similar to membrane osmometry. Since there is no lower cut-off as in membrane osmometry, the method is very sensitive to low molar mass impurities like residual solvent and monomers. As a consequence, the method is more suitable for oligomers and short polymers with molar masses up to (M)n 50kg/mol. Today, vapour pressure osmometry faces strong competition from mass spectrometry techniques such as matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) [20,21]. Nevertheless, vapour pressure osmometry still has advantages in cases where fragmentation issues or molar mass-dependent desorption and ionization probabilities come into play. 

In the frame of the itinerant model, the surface is represented by a potential barrier of various origins and shapes, in most cases treated as onedimensional problem (e.g., 56-60), without taking into account the potential variation in the plane of the surface3 [with the exception of (61) where this effect is qualitatively discussed in connection with the field ionization probability]. Obviously, the nonlocalized model is suitable and often used for the theoretical interpretation of the changes of the bulk properties of the metals caused by the surface effects (the changes of the electrical resistance, magnetic properties, galvanomagnetic effects, etc.). 

Recently the potential variation in the plane of the surface was taken into account in the calculation of the field ionization probability (86). 

Figure 3. Classical phase portraits (upper panel), residual quantum wavefunctions (middle panel), and ionization probability versus time (in units of the period T) (bottom panel). The parameters are (A) F = 5.0, iv = 0.52 (B) F = 20, iv = 1.04 and (C) F = 10 and u> = 2.0. Note that the peak structure of the final wavefunction reflects both stable and unstable classical fixed points. For case C, the peaks are beginning to coalesce reflecting the approach of the single-well effective potentiai (see text). 

The essential quantities that determine the yield of secondary ions in a SIMS spectrum are thus the sputter yield Y and the ionization probability R . 

The ionization probabilities It vary over some five decades across the elements in the periodic table. In addition, they vary also with the chemical environment of the element. This effect, usually referred to as the matrix effect, makes quantitation of SIMS spectra extremely difficult. As illustrated in Table 4.1, positive secondary ion yields from metal oxides are typically two orders of magnitude higher than those of the corresponding metals. A similar increase in yields from metals is observed after adsorption of gases such as oxygen or carbon monoxide. 

As said before, the ionization probability, which accompanies sputtering, is at best qualitatively understood. There have been several attempts to develop models for secondary ion formation. The interested reader may consult the literature for reviews [2,4]. Here we will briefly describe one model that accounts quantitatively for a number of observations on metals, namely the perturbation model of Nprskov and Lundquist [11]. This assumes that the formation of a secondary ion occurs just above the surface, immediately after emission. Then ... 

These features are recognized in the following expression for the ionization probability ... 

In SNMS, sputtered neutrals are post-ionized before they enter the mass spectrometer. In contrast to SIMS, SNMS does not suffer from the matrix effects associated with the ionization probability of sputtered particles. Here, the sensitivity for a cer-... 

The initial low rate of increase of all negative ion yields with exposure is again indicative of an increasing work function which generally reduces the ionization probability for negative ions—an effect opposite to the initial increase in ionization probability observed for positive ions. 

The probability of ionization is given by the geometrical probability, i.e. by the ratio of that effective area to the total area considered, which is unit. That is why (3b) gives the ionization probability of the i element per incident electron for the excited volume of our thin layer. Although the concentration dependence described in (3a) is identical to that given in (3b), it is expressed simpler in (3b), which describes a linear dependence on its variable (c ) than how it is expressed in (3a) where both the numerator and the denominator depends on the variable (c ). 

To increase the ionization probability, a homogeneous weak magnetic field is used to keep the electrons on a spiral path. At the end of the ionization chamber, electrons are collected in a positively charged trap, where the electron current is measured and kept constant by the emission regulator circuitry. 

Instruments with indirect pressure measurement. In this case, the pressure is determined as a function of a pressure-dependent (or more accurately, density-dependent) property (thermal conductivity, ionization probability, electrical conductivity) of the gas. These properties are dependent on the molar mass as well as on the pressure. The pressure reading of the measuring instrument depends on the type of gas. 

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