Photoionization, cross sections efficiency

The lines of primary interest ia an xps spectmm ate those reflecting photoelectrons from cote electron energy levels of the surface atoms. These ate labeled ia Figure 8 for the Ag 3, 3p, and 3t7 electrons. The sensitivity of xps toward certain elements, and hence the surface sensitivity attainable for these elements, is dependent upon intrinsic properties of the photoelectron lines observed. The parameter governing the relative iatensities of these cote level peaks is the photoionization cross-section, (. This parameter describes the relative efficiency of the photoionization process for each cote electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same cote level of all elements. This difference results ia variable surface sensitivity for elements even though the same cote level electrons may be monitored. 

The distinction between photoabsorption and photoionization is important, particularly near threshold, where the probability that ionization will not occur upon photoabsorption is significant. Thus, the ionization efficiency is defined by TJ. = a. /photoionization cross section and photoabsorption cross section, is related to the absorption coefficient a by a = n(7, n being the absorber density. 

The photoionization efficiency is defined as the number of ions, produced by one incident photon on unit path length at unit gas pressure. The curves in Figures 3-6 represent spectral efficiency curves. On them the ratio of the measured photocurrent to the number of incident monochromatic photons is plotted as function of the photon energy (in e.v.). At the low pressure used in the mass spectrometer (< 10 3 torr) the efficiency curves are proportional to the photoionization cross-section curves.3... 

Total ionization oscillator strengths (see Section V) may be put on an absolute scale by normalizing to the total absorption at energies where the ionization efficiency (see Section II) is unity. For most species this is the case above 20 eV.25 102 Alternatively, absolute photoionization cross sections 122 may be used for normalization. 

The intensity of an XPS peak (Ia) is a strong function of (i) the incoming photon flux, (ii) the concentration of the given element, (hi) its photoionization cross-section (which is excitation-energy dependent), (iv) the mean free path of the emitted photoelectron, and (v) further instrumental parameters (such as photoelectron collection and detection efficiency). By defining atomic sensitivity factors (S, as an overall factor summing up the effects of iii-v), the atom fraction of any element in a sample can be calculated as ... 

DV-Xa molecular orbital calculation is demonstrated to be very efficient for theoretical analysis of the photoelectron and x-ray spectroscopies. For photoelectron spectroscopy, Slater s transition state calculation is very effective to give an accurate peak energy, taking account of the orbital relaxation effect. The more careful analysis including the spin-polarized and the relativistic effects substantially improves the theoretical results for the core level spectrum. By consideration of the photoionization cross section, better theoretical spectrum can be obtained for the valence band structure than the ordinary DOS spectrum. The realistic model cluster reproduce very well the valence state spectrum in details. 

Above the ionization threshold (see Chapter 8), the absolute photodissociation cross section can be obtained as the difference between the absolute total absorption cross section and the absolute photoionization cross section (see for example, for N2, Fig. 6 of Shaw, et ai, 1992). Another experimental quantity is the ionization efficiency defined as the total photoionization cross section divided by the total absorption cross section. 

Fig 10. Photoionization cross section of NH3. Also shown is the photoionization efficiency curve for NH4 produced by reaction of with NH3. 

The amount of the adsorbed inhibitor was characterized by the sum of the P 2p and the P KLL signals, while the Ca/HEDP molar ratio at the sample surface was determined from the calcium/phosphorus intensity ratio. Differences in the respective photoionization cross sections as well as in the spectrometer efficiency for these elements were taken into account by experimentally determining the ratio of the sensitivity factors from a sample of known stoichiometric composition (CaHP04). 

Ions produced by photon impact analysis of photoionization efficiency curves and comparison with ground-state cross section. 

These results indicate that long lived autoionization states with excitation cross-sections comparable to those for excitation of bound high-lying states exist in heavy atoms with complex spectra. Transitions to these autoionization states can radically increase the efficiency of photoionization of atoms, a factor very important in atomic vapor laser isotope separation. 

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