Angle-resolved photoemission system

The observation of the CT gap should be contrasted with the predictions of band theory. Local-density-approximation (LDA) calculations performed for a number of undoped materials predict these systems to be metals since die Cu 3d and O 2p orbitals form a conduction band which is half-filled (for a review see Pickett 1989). The spin-polarized version of this band theory is not sufficiently accurate to yield an antiferromagnetic state of the insulating compound (Pickett et al. 1992). Thus LDA calculations fail to account for the two principal features of undoped materials the insulating gap and antiferromagnetic ordering. However, despite these serious problems these calculations do yield accurate values for the Fermi surface crossings as observed by angle-resolved photoemission (for a review see Pickett et al. 1992 see also ch. 201 of this Handbook). 

The electronic structure of the TMC(IOO) surface has been studied most extensively among the low-index TMC surfaces. It is well known that the use of angle-resolved photoemission spectroscopy (ARPES) can give direct information about the valence band structure around the surface (18,19), and extensive ARPES studies have been performed on the valence band structure of TMC(IOO) surfaces such as TiC(lOO) (20,21), ZrC(lOO) (22,23), VC(IOO) (24-27), NbC(lOO) (28-31), and TaC(lOO) (32,33). These studies have shown that most of the features in ARPE spectra can be understood as emissions from the bulk bands, and thus the electronic structures of TMC(IOO) are well regarded as the cross sections of the bulk electronic states. However, in some systems, surface induced electronic states have been identified as described in the following. 

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. 

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