Cross section photoionization

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. 

Fig. 9. Scofield s x-ray photoionization cross-section relative to that for C electrons as a function of atomic number (19).

Fig. 10. Comparison of Scofield s calculated x-ray photoionization cross-sections relative to that for F 1 electrons and experimental values (22) as a...

The spectra of Figure 3 illustrate two further points. All the C Is peaks in Figure 3a are of equal intensity because there are an equal number of each type of C atom present. So, when comparing relative intensities of the same atomic core level to get composition data, we do not need to consider the photoionization cross section. Therefore, Figure 3c immediately reveals that there is four times as much elemental Si present as Si02 in the Si 2p spectrum. The second point is that the chemical shift range is poor compared to the widths of the peaks, especially for the solids in Figures 3b and 3c. Thus, not all chemically inequivalent atoms can be distin-... 

J. H. Scofield. J. Electron Spect. 8,129, 1976. This is the standard quoted reference for photoionization cross sections at 1487 eV. It is actually one of the most heavily cited references in physical science. The calculations are published in tabular form for all electron level of all elements. 

Si 2p line, at about 100 eV BE, is also easily accessible at most synchrotron sources but cannot, of course, be observed using He I and He II radiation. On the other hand, the Zn 3d and Hg 4f lines can be observed quite readily by He I radiation (see Table 1) and the elements identified in this way. Quantitative analysis using relative peak intensities is performed exactly as in XPS, but the photoionization cross sections a are very different at UPS photon energies, compared to A1 Ka energies, and tabulated or calculated values are not so readily available. Quantitation, therefore, usually has to be done using local standards. 

Relative photoionization cross sections for molecules do not vary gready between each other in this wavelength region, and therefore the peak intensities in the raw data approximately correspond to the relative abundances of the molecular species. Improvement in quantification for both photoionizadon methods is straightforward with calibration. Sampling the majority neutral channel means much less stringent requirements for calibrants than that for direct ion production from surfaces by energetic particles this is especially important for the analysis of surfaces, interfaces, and unknown bulk materials. 

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

Many kinds of transition probabilities depend on DOs. Photoionization cross sections, are proportional to the absolute squares of matrix elements between DOs and continuum orbitals, or... 

An excellent agreement with the X-ray photoionization spectra of ethylene, butadiene and hexatriene (7) is obtained (12) (Figure 3) when including in our calculations the Gelius (36) photoionization cross sections for an Alka photon beam, by means of Eqs. (4) and (5). Such a direct comparison is impossible for octatetraene, a compound for which there is no available XPS data. 

Fig.l Ground state photoionization cross section. Full line LG results, broken line VG results. 

The adiabatic ionization potential (1A) of a molecule, as shown in Figure 4.1, equals the energy difference between the lowest vibrational level of the ground electronic state of the positive ion and that of the molecule. In practice, few cases would correspond to adiabatic ionization except those determined spectroscopically or obtained in a threshold process. Near threshold, there is a real difference between the photoabsorption and photoionization cross sections, meaning that much of the photoabsorption does not lead to ionization, but instead results in dissociation into neutral fragments. 

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. /photoabsorption cross section, is related to the absorption coefficient a by a = n(7, n being the absorber density. 

In Eq. (4.9), V is the frequency of radiation and a>. and (Ox are the statistical weights of the initial and final states. It should be remembered that Eq. (4.9) refers to the photoionization cross section, not the total photoabsorption cross section (see Sect. 4.2). 

When corrections are applied using two different sets of photoionization cross-sections available in the literature on two separate samples of Hf(Sio.sAso.5)As, electron populations can be extracted, as listed in Table 1. The results suggest a... 

Fig. 9 a Variation in photoionization cross-sections [42]. b PES valence band spectra for Hf(Sio.5Aso.5)As at three different excitation energies, normalized to the As2 4p peak. Reprinted with permission from [35]. Copyright Elsevier... 

Integration of these component peaks, with appropriate corrections applied for different photoionization cross-sections and inelastic mean free paths, gives the electron populations listed in Table 4. The atomic charges obtained are consistent... 

The study of the photoionization cross section as a function of photon energy for the different orbitals of Me4Sn, which can be a powerful tool for the assignment of the spectra and the analysis of the contribution of the various atomic orbitals to the molecular orbitals, has been carried out by the authors of References 11 and 12 by using He I and He II as ionizing source, and of Reference 13 by using synchrotron radiation. Bertoncello... 

Scofield s calculated X-ray photoionization cross sections, 24 86-87 Scoping activities, EIA, 20 234 Scoping methods, EIA, 20 239t Scopolamine, 2 79, 80 Scopolanine hydrobromide, 4 360t Scorch behavior, in rubber compounding, 22 794... 

The 5950A ESCA spectrometer is interfaced to a desktop computer for data collection and analysis. Six hundred watt monochromatic A1 Ka X-rays are used to excite the photoelectrons and an electron gun set at 2 eV and 0.3 mAmp is used to reduce sample charging. Peak areas are numerically integrated and then divided by the theoretical photoionization cross-sections (11) to obtain relative atomic compositions. For the supported catalyst samples, all binding energies (BE) are referenced to the A1 2p peak at 75.0 eV, the Si 2p peak at 103.0 eV, or the Ti 2p3/2 peak at 458.5 eV. 

What role does the dopant play in APPI (Ionization of molecules having a low photoionization cross section (probability) has been shown to be enhanced by the use of a dopant that is introduced into the vaporized plume of analyte molecules the dopant is selected on the basis of its high UV absorptivity and serves as a charge transfer reagent). 

B. Recent Advances in Technique 1. Photoionization cross-section measurement... 

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