Photoemission experiment

The short-range order in a material is important in determining optoelectronic properties. For instance, x-ray and electron diffraction experiments performed on amorphous siHcon (i -Si) and germanium (a-Ge) have revealed that the nearest neighbor environments are approximately the same as those found in their crystalline counterparts (6) photoemission experiments performed on i -Si show that the DOS in valence and conduction bands are virtually identical to the corresponding crystal with the exception that the singularities (associated with periodicity) present in the latter are smeared out in the former. 

Shpaisman FI, Salomon E, Nesher G, Vilan A, Cohen H, Kahn A, Cahen D (2009) Electrical transport and photoemission experiments of alkylphosphonate monolayers on GaAs. J Phys ChemC 113 3313-3321... 

In hindsight, it was Hertz who, unknowingly, reported the first photoemission experiments in 1887, when he noticed that electrical sparks induced the formation of a second spark in a variety of samples [1], Hertz correctly recognized that the effect was due to UV light generated by the first spark, but he did not understand the nature of the induced spark. In fact he could not have done so, because the electron had not yet been discovered. About ten years later Thomson identified the radiation in... 

In photoemission experiments, the Fermi level shows up as a sharp edge therefore it is often chosen as a convenient zero of the energy scale. Note, however, that electrons at the Fermi level are bound to the metal. The energy necessary to disconnect them from the metal is equal to the work function (the molecular analog would be the ionization potential). This property is discussed later on. 

In general one can say that those parts of the band that correspond to bonds which have been broken in order to create the surface are narrower. A similar effect can be expected for small particles the average coordination number of the atom decreases and the bands are narrower. This effect can be observed in photoemission experiments an example is shown in Fig. 3.18. 

The surface states observed by field-emission spectroscopy have a direct relation to the process in STM. As we have discussed in the Introduction, field emission is a tunneling phenomenon. The Bardeen theory of tunneling (1960) is also applicable (Penn and Plummer, 1974). Because the outgoing wave is a structureless plane wave, as a direct consequence of the Bardeen theory, the tunneling current is proportional to the density of states near the emitter surface. The observed enhancement factor on W(IOO), W(110), and Mo(IOO) over the free-electron Fermi-gas behavior implies that at those surfaces, near the Fermi level, the LDOS at the surface is dominated by surface states. In other words, most of the surface densities of states are from the surface states rather than from the bulk wavefunctions. This point is further verified by photoemission experiments and first-principles calculations of the electronic structure of these surfaces. 

Fig. 4.9. Schematic of photoemission experiments, A beam of incident photons with energy ftto induces electrons to emit from the sample. The photoelectrons are collected by the velocity analyzer and the electron detector at angles 9 and

Spurious effects due to incompletely removed oxides layers are very likely to be recorded and misinterpreted in photoemission experiments from the very oxidizable U-metal surface. However, considering only high resolution XPS and UPS data for clean surfaces as well as the measurements using synchrotron radiation , it can be... 

Photoemission experiments with flat surfaces revealed that atoms of lower coordination may have a different population of d-orbitals and a different local density of states (138-140). These effects have been also predicted and analyzed theoretically (94-97, 136, 137), and should be always considered. The only question is whether they manifest themselves in the chemisorption and catalytic behavior. In any case, the impression is that by making metal particles small in size, one can cause the electronic structure of a certain fraction of the metal atoms to vary more than by making a bulk solution alloy. 

In photoemission experiments one measures either the photocurrent or the photopotential when an electrode is placed in a solution and is illuminated by light that the solution does not absorb. The photocurrent of emitted electrons depends on the every ho) and the potential

[Pg.335]

It is practically impossible to reconstruct the form of F(x) in detail by measuring the photocurrent. However, its first two moments can be measured in photoemission experiments. The first moment of the function F(x) is the projection of the average thermalization path length of electrons on the normal to the surface of the cathode ... 

Under some simplifications associated with the symmetry of fullerenes, it has been possible to perform calculations of type Hartree-Fock in which the interelec-tronic correlation has been included up to second order Mpller-Plesset (Moller et al. 1934 Purcell 1979 Cioslowski 1995), and calculations based on the density functional (Pople et al. 1976). However, given the difficulties faced by ab initio computations when all the electrons of these large molecules are taken into account, other semiempirical methods of the Hiickel type or tight-binding (Haddon 1992) models have been developed to determine the electronic structure of C60 (Cioslowski 1995 Lin and Nori 1996) and associated properties like polarizabilities (Bonin and Kresin 1997 Rubio et al. 1993) hyperpolarizabilities (Fanti et al. 1995) plasmon excitations (Bertsch et al. 1991) etc. These semiempirical models reproduce the order of monoelectronic levels close to the Fermi level. Other more sophisticated semiempirical models, like the PPP (Pariser-Parr-Pople) (Pariser and Parr 1953 Pople 1953) obtain better quantitative results when compared with photoemission experiments (Savage 1975). 

The key to understand the anomalous behaviors in A vC,o is the electron-electron interaction, U, and the electron-phonon interaction, S. In A vCgo, the important interaction as U is the Coulomb repulsion between the flu electrons. The importance of U has been pointed out based on the results of the photoemission experiments [15]. Also, in AxC o, the important interaction as S is the coupling of the tlu electrons to the intramolecular phonons of the CRaman experiments [16], Nevertheless, a complete understanding of the anomalous behaviors in AxCgo has not been established yet. The reason is that the system with the orbital degree of freedom in which both U and S are important has not been known so far [17-20],... 

The electron dynamics in metals have been investigated with femtosecond time resolution in two photon photoemission experiments [3,16,17], where the idea that the signal can be described by a double convolution of an exponential decay with the laser pulse shape is commonly... 

Valence and core-level photoemission experiments were carried out in the IFW Dresden under the following conditions. Unless otherwise stated, the valence band photoemission data were recorded at room temperature using He Ia radiation (21.22 eV) with a total energy resolution of 100 meV, and the core-level photoemission data, using monochromatised Al Ka radiation (1486.6 eV) with a total energy resolution of 0.4 eV. All such data are angle-integrated in nature ( 4° or more). 

Fig. 6 Main picture valence band photoemission spectra of Tm C82 recorded at photon energies across the Tm 4d-4f threshold. The inset shows the Tm-N4 5 x-ray absorption spectrum, indicating the choice of photon energies for the resonant photoemission experiment. The photon energies are (1) 169.4 eV, (2) 173.7 eV, (3) 177.8 eV and (4) 183.9 eV...

Fig. 4.14. Gracing incidence (

Fig. 4.23. Valence band offsets for CdS/ZnO, CdS/(Zn,Mg)0 and CdS/ZnO Al interfaces as determined by photoemission experiments. Solid symbols are for sputter deposition of the oxides onto CdS, open symbols are for deposition of CdS onto the oxides. The value from Ruckh et al. [102] is included (diamonds). The circled numbers serve to classify the different values as described in the text...

---