Cross-sections ionization

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. 

A particular strength of Equation (7) is that the intensity ratio is formed between mea-surements of the same X-ray energy in both the unknown and standard. This procedure has significant advant es First, there is no need to know the spectrometer s efficiency, a value that is very difficult to calibrate absolutely, since it appears as a multiplicative factor in both terms and therefore cancels. Second, an exact knowledge of the inner shell ionization cross section or fluorescence yields is not needed, since they also cancel in the ratio. 

A tabulation of the ECPSSR cross sections for proton and helium-ion ionization of Kand L levels in atoms can be used for calculations related to PIXE measurements. Some representative X-ray production cross sections, which are the product of the ionization cross sections and the fluorescence yields, are displayed in Figure 1. Although these A shell cross sections have been found to agree with available experimental values within 10%, which is adequate for standardless PKE, the accuracy of the i-shell cross sections is limited mainly by the uncertainties in the various Zrshell fluorescence yields. Knowledge of these yields is necessary to conven X-ray ionization cross sections to production cross sections. Of course, these same uncertainties apply to the EMPA, EDS, and XRF techniques. The Af-shell situation is even more complicated. 

Figura 1 Calculated K X-ray production cross sections for protons using the tabulated ECPSSR Ionization cross sections of Cohen end Harrigan, and the fluorescence yields calculated es In Johansson et al. (1 barn h IIT cm l.

Fig. 2.18. Electron-impact ionization cross-section for the Ni K shell, as a function of reduced electron energy U [2.128] U = Ep/Ek, where Ep is the primary electron energy and E

Fig. 3.31. Distributions (i)/(Ee) dEe of electron energy (E ) for a low-pressure HF-plasma (suffix pi, Maxwellian with temperature = 80000 K) and an electron beam (suffix eb, simplified to Gaussian shape with 40 eV half-width) (ii) rTx (Ej) ofthe Ej dependent electron impact ionization cross-section for X=Ti...

As illustrated in Fig. 3.41, several laser schemes can be used to ionize elements and molecules. Scheme (a) in this figure stands for non-resonant ionization. Because the ionization cross-section is very low, a very high laser intensity is required to saturate the ionization process. Scheme (b) shows a simple single-resonance scheme. This is the simplest but not necessarily the most desirable scheme for resonant post-ionization. Cross-... 

The major source of error In calculating the free energies of Pu0(g) and Pu02(g) from Battles et al. probably results from the derived equations for the partial pressures of 0(g) and Pu(g) as a consequence of uncertainties In Ionization cross sections. The thermodynamic assessments of Ackermann et al. Involve extrapolations of oxygen potentials reported by Markin and Rand (4) In temperature of the order of 500 K. However, a second and third... 

The mass spectrum of 1-torr ethylene in 20-torr He is also shown in in Figure 14. Remembering that the (electron impact) ionization cross-section for ethylene is 20 times higher than that for He, we expect almost... 

Of course, it is possible to contemplate experiments that examine photoionization of oriented chiral molecules. An expression has been given for the angle integrated (total) ionization cross-section in such circumstances [48] and CDAD-type measurements have been reported on adsorbed chiral molecules [49, 50], but the interplay of natural and geometric chirality in angle-resolved dichroism measurements remains very much a topic for future investigation. 

In RF discharges of silane, SiH usually is the most abundant ion, but others (SiH, and Si H+ with 1 < n < 9) also are present [319]. The relative abundance of SiH ions increases with increasing pressure, while that of SiH decreases [319]. The ionization cross section of SiHt is higher than that of SiHj [320], but SiH is lost via the reaction SiH -1- SiH4 —> SiHj -F SiH3 [305], At pressures lower than 0.1 Torr SiHt becomes the dominant ion. 

POSITIVE ION-ELECTRON IMPACT IONIZATION CROSS SECTIONS ... 

Quantum mechanical and selected semiclassical and semiempirical methods for the calculation of electron impact ionization cross sections are described and their successes and limitations noted. Experimental methods for the measurement of absolute and relative ionization cross sections are also described in some detail. Four theoretical methods, one quantum mechanical and three semiclassical, have been used to calculate cross sections for the total ionization of the inert gases and small molecules and the results compared with experimental measurements reported in the literature. Two of the theoretical methods, one quantum mechanical and one semiclassical, have been applied to the calculation of orientation-dependent electron impact ionization cross sections and the results compared with recent experiments. 

In this chapter we focus attention on the efficiency of ionization, the ionization cross section, and consider some recent experimental measurements and theoretical studies of the ionization process. A sketch of electron impact ionization curves, the variation of the ionization cross section as a function of the electron energy, using CO as an example, are shown in Figure 1. The mass spectrum, collected at the electron energy corresponding to the maximum in the ionization cross section, is also shown, although there will be no further discussion of fragmentation in this... 

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