Selection rules photoemission

In the investigations of molecular adsorption reported here our philosophy has been to first determine the orientation of the adsorbed molecule or molecular fragment using NEXAFS and/or photoelectron diffraction. Using photoemission selection rules we then assign the observed spectral features in the photoelectron spectrum. On the basis of Koopmans theorem a comparison with a quantum chemical cluster calculation is then possible, should this be available. All three types of measurement can be performed with the same angle-resolving photoelectron spectrometer, but on different monochromators. In the next Section we briefly discuss the techniques. The third Section is devoted to three examples of the combined application of NEXAFS and photoemission, whereby the first - C0/Ni(100) - is chosen mainly for didactic reasons. The results for the systems CN/Pd(111) and HCOO/Cu(110) show, however, the power of this approach in situations where no a priori predictions of structure are possible. 

The various forms of photoelectron spectroscopy presently available permit a straightforward determination of occupied and unoccupied surface states. The most comprehensive and authoritative collection of reviews is in the book edited by Feuerbacher et al. [44], while Ertl and Kiippers [15] also provide useful information. Here, we will only attempt to summarize how the principal versions of the technique can be used in the determination of surface electronic structure. In this context the crucial factor is that photoemission spectra represent a direct manifestation of the initial and final density of states of the emitting system. Because selection rules (matrix element effects) can be involved in the transition, the state densities may not always correspond to those derived from the band structure, but in practice there is frequently a rather close correspondence. 

Surface sensitivity is an intrinsic property of photoemission measurements. The incident light penetrates far into the solid, but the escape depth of excited electrons is very short (Fig. 5), although there are local variations related to direction-dependent band structure effects. Surface sensitivity can be further enhanced by appropriate choice of experimental parameters such as photon energy, angles of incidence and emission, etc., which take advantage of selection rules favouring surface processes. 

As already mentioned, the phenomenon of magnetic circular dichroism in photoemission originates from spin-orbit and exchange interactions in combination with the dipole selection rules. In the atomic model picture, the splitting of the 3p level (into sublevels with orbital momentum m) is caused by the electrostatic interaction of the core level with the magnetically polarized valence electrons [57]. The observed intensity differences and the respective asymmetry values in photoemission from the Fe 3p levels are small (typically 3%) compared to the large MCDAD and MLDAD asymmetries (up to about 12%) observed in valence band photoemission [27]. 

Spicer and his coworkers (Berglund and Spicer, 1964a,b Blodgett and Spicer, 1966) proposed a very different model for the excitation probability. They argued that the momentum conservation is not relevant in photoemission (nondirect transition), and the only selection rule is the conservation of energy. Therefore D E, ) is proportional to the product of the initial and final densities of states. 

Eor reasons discussed in Section II, the selection rules are quite different for both types of spectroscopy. The number of transitions allowed in photoemission is usually much smaller than the number of allowed optical transitions, especially at high excitation energies. Therefore, PE spectra are considerably easier to interpret than optical spectra, as will be seen in the following sections. 

These terms represent a bulk effect, a classical surface potential effect, and a surface field effect, respectively. Selection rules are stated through which these various contributions to the net photocurrent can be resolved. A complete theoretical description of photoemission, therefore, will enable resolution of each of these components as functions of the angles and j8), the available light intensity with depth, and the substrate epitaxy. This theory is discussed in detail in Ref. 51 note also Chapters 4 and 6 in this book. 

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