Photoemission analyzer

Finally, we have tested the photoemission stability after exposition to temperature. Samples were analyzed after 1, 2, 3, 10, 30 and 60 min of heating. The hybrid... 

In order to appreciate the meaning of a binding energy, it is necessary to consider final state effects. In practice we use XPS data as if they were characteristic of the atoms as they are before the photoemission event takes place. We must realize that this is not correct photoemission data represent a state from which an electron has just left. Let us analyze the event in more detail. 

One way of experimentally exploring the electronic structure of solids is by means of photoemission spectroscopies such as UPS and X-ray photoelectron spectroscopy (XPS), where photoexcited electrons are analyzed dispersively as a function of their kinetic energy. The electronic structure of the reference material TTF-TCNQ will be extensively discussed in Section 6.1. Figure 1.31 shows the XPS spectra of the S2p core line for (TMTTF)2PF6 (black dots) and BEDT-TTF (grey dots). 

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

Using inverse photoemission, the unoccupied electronic states of solid surfaces are being studied. Here, instead of injecting an UV light onto the surface and analyzing the emitted electrons, an electron beam is injected onto the surface and the spectrum of the emitted photons is analyzed. Fig. 4.11 shows a summary of the results of photoemission and inverse photoemission of one of the most exhaustively studied surfaces, W(OOl) [Drube et al. (1986)]. As shown, strong surface states immediately below and above the Fermi level are observed. Both are of a character. 

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. 

The core-level x-ray photoemission spectrum of the Mn 2p core level for (Ga,Mn)As with x = 0.074 was measured and was analyzed by a configuration interaction (Cl) cluster-model assuming a Mn2+ and Mn3+ ground state (Okabayashi et al. 1998). For the d5 configuration, the p-d exchange energy (which is conventionally referred to as Nop for DMS) should be negative and NoP —1.2 eV is obtained for /t-(d5) centers with an optimized parameter set. 

An important technique is UV photoemission spectroscopy (UPS) which is based on the outer photoelectric effect (in contrast to XPS, where we use the inner photoelectric effect). Photons with energies of 10-100 eV are used to ionize atoms and molecules at the surface. The energy of emitted electrons is detected. To study adsorption of molecules to surfaces, often difference spectra are analyzed which are measured before and after the adsorption. These difference spectra are compared to the spectrum of the molecules in the gaseous phase. 

In the field of not only traditional metallurgy but also recently developed nano-technology, it is very interesting and important what change is introduced when it is surrounded by other atoms. Such a change in electronic states has been investigated as chemical shift detected by X-ray (XPS) and UV (UPS) photoemission spectroscopy [1] as well as X-ray emission and absorption spectroscopy [2,3]. Also, such a chemical shift has been simulated by theoretical calculation [4]. However, many problems have been unsolved. In the case of XPS and UPS, since the most outer layers of substances are analyzed, the spectra are easily affected by absorbed gaseous molecules. Also, with the X-ray emission and absorption spectroscopy it is difficult to analyze the complicated X-ray transition states for substances composed of heavy metal elements. Therefore, a complementary method has been demanded for the spectroscopy such as XPS, UPS and X-ray emission and absorption spectroscopy. The coefficient y of the electronic contribution to heat capacity, Cp, near absolute zero Kelvin reflects the density of states (DOS) in the vicinity of Fermi level (EF) [5]. Therefore, the measurement of y is expected to be one of the useful methods to clarify the electronic states of substances composed of heavy metal elements. 

Fig. 24. Ultraviolet photoemission (hv = 7.7 eV) and analyzed field emission spectra of tungsten as a function of exposure to H2. Log R of the field emission results is related to the local density of states at the surface. Ref. (61)...

Almost all of the existing quantitative data on surface structure was obtained through experimental techniques that involve the propagation and scattering of electrons in solids. This class of surface probes includes the whole range of fine-structure techniques, in addition to LEED and angle-resolved photoemission experiments. This chapter will provide a theoretical description of these techniques and the methods of analyzing the experimental data to determine surface structure. 

For time-resolved 2PPE spectroscopy, a combined set-up of an ultrafast laser system and an ultrahigh-vacuum photoemission spectroscopic system is indispensable. Typical electron energy analyzers have been used as the spectrometer, such as a cylindrical mirror analyzer, a hemispherical analyzer and a time-of-flight (TOF) analyzer. The TOF analyzer is mainly used for low repetition rate (<1 kFlz) laser sources, and the others are used for the lasers with multi-kldz or MHz repetition rates [11-14]. 

In our x-ray photoemission studies, a monochromatized photon beam (A1 Ka, hv = 1486.6 eV) was focused onto the sample surface, and the emitted electrons were energy analyzed with a Surface Sciences Instruments hemispherical analyzer. The take-off angle of the photoelectrons relative to the surface normal was 60° unless otherwise specified. A position-sensitive detector with 128 channels was used with a dedicated HP9836C computer to facilitate data acquisition. (22.) ... 

Several NaCl-type binary compounds, including UN, have been analyzed (14) using both theoretical band structure (itinerant) and crystal field (localized) approaches (although the applicability of one of these approaches generally means that the other is inappropriate). UN may be an intermediate case where neither approach will yield very satisfactory results. Photoemission spectroscopy can yield valuable insights into this problem. 

Tarr. For the invnse photoemission (IRES) measuiements, a monochromatic low energy electron beam was directed onto a sample at normal incidence and the emitted photons were monochromatized and then energy analyzed with a position sensitive detector. The size of the electron beam was -1x5 mm and the over l resolution was 0.3-0.6 eV for photon energies 12-44 eV (g). For interface studies, overlayers were deposited from well outgassed thermal evaporation sources at pressures below 5 x IQ-io -pQjy evaporation rates of -1 A/min. 

The main characteristics of a photoemission experiment are presented in Fig. 2. The simplest instrument consists of a UH V chamber with a light source, the sample and the analyzer with its detection unit. 

Fig. 2. The main constituents of a photoemission experiment. The relationships between the instrument and the resulting spectrum are indicated. FAT Fixed analyzer transmission FRR fixed retardation ratio...

Quantitative spectra with optimized resolution are recorded in the FAT mode with optimized pass energies. Figure 6 exemplifies the drastic effects of the analyzer convolution function on a photoemission spectriun for a valence band UPS experiment on graphite. The spectra characterize the very same surface and are both recorded with Hel radiation. The characteristic differences of the two operation modes of the hemispherical analyzer (LH EA 200) can be clearly seen as well as the different suitability of the relevant spectra for qualitative and quantitative analysis. In XPS data, the same differences occur for the two modes of operation. The intrinsically more symmetric line profile of core level lines tends, however, to obscure the effects of the analyzer in narrow scan spectra. 

Photoemission spectroscopy (PES) is by far the most widely used and powerful spectroscopic technique for interface research. XPS and UPS are complementary techniques that utilize different light sources, e.g., x-ray and ultraviolet, to excite electrons in solids via photoelectric effect and then collect the escaped photoelectrons with an energy analyzer. In general, photoemission experiments for interface formation studies are performed in the following way. The study begins with the photoemission analysis of a clean surface of the material that will eventually form one side of the... 

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