Valence band features

The only UPS/XPS photoemission study of Am shows a lanthanide like valence band feature as displayed in Fig. 16. The 5f emission is nearly completely withdrawn from Ep except possibly for some very weak 5 f contribution seen only in high resolution He-I-spectra (AE 0.12 eV) as a very sharp peak just at Ep. The 5 f intensity is concentrated in a structured peak around 2.8 eV binding energy (for MgR excitation, upper curve, the structures are not resolved) as deduced from the excitation energy dependence of the spectra. If one compares with Sm metal, the peaks at 1.8, 2.6 and 3.2 eV are attributed to the H, F, and P states, respectively, of the 5f final state multiplet originating from the initial 5f ground state of trivalent Am. 

Figure 4.31 shows ultraviolet photoelectron spectra recorded during the same interface experiment shown in Fig. 4.26. A clear transition from the Cu(In,Ga)Se2 valence band structure with a valence band maximum at 0.8eV binding energy to the ZnO valence band structure with a valence band maximum at 3eV is observed with increasing ZnO deposition. The well-resolved valence band features are enabled by the in situ sample preparation. Also very sharp secondary electron cutoffs are obtained, which allow for an accurate determination of work functions. The work functions of Cu(In,Ga)Se2 and ZnO are determined as 5.4 and 4.25 eV, respectively. These result in ionization potentials of 6.15 and 7.15 eV for Cu(In,Ga)Se2 and ZnO.

UV excited valence-band spectra (UPS) are not often used in the characterization of real catalyst systems. In the present study they were measured for two purposes. First, in a mixture of iron metal and iron oxide, UPS allows accurate definition of the zero of the energy scale and the detection of charging phenomena. In addition, the characteristic dependence of spectral features on excitation energy permits the qualitative separation of the valence band features due to oxygen-derived states from those of the iron 3d valence states. It is therefore possible to follow the reduction process of the surface. In addition, these spectra can indicate that there are no small molecules such as molecular oxygen, water, or carbon monoxide on the activated surface, by the absence of characteristic fingerprint patterns. 

Figure 2.36. Dependence of the valence band features of a partly reduced catalyst on the excitation energy. This dependence is used to discriminate oxygen states from the iron 3d states (analyzer mode FAT 10 eV for all spectra).

The two-dimensional carrier confinement in the wells formed by the conduction and valence band discontinuities changes many basic semiconductor parameters. The parameter important in the laser is the density of states in the conduction and valence bands. The density of states is gready reduced in quantum well lasers (11,12). This makes it easier to achieve population inversion and thus results in a corresponding reduction in the threshold carrier density. Indeed, quantum well lasers are characterized by threshold current densities as low as 100-150 A/cm, dramatically lower than for conventional lasers. In the quantum well lasers, carriers are confined to the wells which occupy only a small fraction of the active layer volume. The internal loss owing to absorption induced by the high carrier density is very low, as Httie as 2 cm . The output efficiency of such lasers shows almost no dependence on the cavity length, a feature usehil in the preparation of high power lasers. 

Valence band spectra provide information about the electronic and chemical structure of the system, since many of the valence electrons participate directly in chemical bonding. One way to evaluate experimental UPS spectra is by using a fingerprint method, i.e., a comparison with known standards. Another important approach is to utilize comparison with the results of appropriate model quantum-chemical calculations 4. The combination with quantum-chcmica) calculations allow for an assignment of the different features in the electronic structure in terms of atomic or molecular orbitals or in terms of band structure. The experimental valence band spectra in some of the examples included in this chapter arc inteqneted with the help of quantum-chemical calculations. A brief outline and some basic considerations on theoretical approaches are outlined in the next section. 

The size-dependent features of the electronic structure have been explored, with special emphasis on the evolution of the valence band DOS of transition and noble metals as the particle size increases. 

The trans-azo bridge acts as a spacer assisting in the electron exchange between ferrocene moieties, similar to the vinylene bridge, as given for AE° of azoferrocene (25) in Table IV and its IT band feature (92,153). The mixed-valence cation of 25 formed in benzonitrile by le ... 

As a result of the atomic nature of the core orbitals, the structure and width of the features in an X-ray emission spectrum reflect the density of states in the valence band from which the transition originates. Also as a result of the atomic nature of the core orbitals, the selection rules governing the X-ray emission are those appropriate to atomic spectroscopy, more especially the orbital angular momentum selection rule A1 = + 1. Thus, transitions to the Is band are only allowed from bands corresponding to the p orbitals. 

An attractive feature of applying XPS to study these skutterudites is that the valence states of all atoms can be accessed during the same experiment. As in the study of the MnP-type compounds, these types of investigations also provide insight into bonding character and its relation to electronegativity differences. This information is obtained by analysing both core-line and valence band XPS spectra. 

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