Photoionization cross-sections, partial

The H2 molecule is a system for which quite recently it has been possible to measure in unprecedented detail state-selected vibrationally and rotation-ally resolved photoionization cross sections in the presence of autoionization [27-29]. The technique employed has been resonantly enhanced multiphoton ionization. The theoretical approach sketched above has been used to calculate these experiments from first principles [30], and it has thus been possible to give a purely theoretical account of a process involving a chemical transformation in a situation where a considerable number of bound levels is embedded in an ensemble of continua that are also coupled to one another. The agreement between experiment and theory is quite good, with regard to both the relative magnitudes of the partial cross sections and the spectral profiles, which are quite different depending on the final vibrational rotational state of the ion. 

Figure 14. Partial photoionization cross section for 3s electron of argon.130...

Some partial photoionization cross sections, derived in this way for neon, are shown in Fig. 2.11 as a function of photon energy. The uppermost curve is the total absorption cross section. At the onset of the ionization thresholds for the ejection of Is, 2s and 2p electrons this quantity shows the corresponding absorption edges (see the discussion related to equ. (2.11)). The partition of the total cross section into partial contributions cr(i) clearly demonstrates that the dominant features are due to main photoionization processes described by the partial cross sections satellite transitions from multiple photoionization processes are also present. If these are related to a K-shell ionization process, they are called in Fig. 2.11 multiple KL where the symbol KLX indicates that one electron from the K-shell and X electrons from the L-shell have been released by the photon interaction. Similarly, multiple I/ stands for processes where X electrons from the L-shell are ejected. Furthermore, these two groups of multiple processes are classified with respect to ionization accompanied by excitation, (e, n), or double ionization, ( ,e). If one compares in Fig. 2.11 the magnitude of the partial cross sections for 2p, 2s and Is photoionization at 1253.6 eV photon energy (Mg Ka radiation) and takes into account the different... 

The partial differential photoionization cross-section for producing photoelectrons with a kinetic energy e at time t ejected in the LF direction is then... 

Figure 14.11 Site-specific experimental valence X-ray photoelectron spectrum of the rutile Ti02 single-crystal sample, obtained using the XRSW technique, in comparison with the calculated partial density of states (corrected for individual angular-momentum-dependent photoionization cross sections) [32]. (a) Ti, (b) O. The spectra are normalized to equal peak height.

Figure 15. Partial-channel photoionization cross sections for 2bj, 5aj and 2b2 orbitals of H2S obtained in static-exchange approxi tlon.

Figure 4. Calculated partial photoionization cross sections for the K-shell of N2 over a broad energy range. The dashed line represents twice the K-shell photoionization cross section for atomic nitrogen, calculated using a Hartree-Slater potential.

For molecules, the problem is more complex than for atoms because the continuum must usually be expressed as a sum of many partial waves. It is highly unlikely that the Cooper minima for all of the important /-partial wave components will occur near the same value of e. An example of a Cooper minimum in a molecular photoionization cross section is observed for HI, corresponding to ionization from the 5p7t orbital (Carlson et al., 1984). Other examples of Cooper... 

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