Photoelectron Spectroscopy PES

The photoelectric effect, first outlined by Einstein in the early 1900s, refers to the ejection of electrons from a surface due to photon impingement. However, it was not until the 1960s that this phenomenon was exploited for surface analysis - a technique referred to as X-ray photoelectron spectroscopy (XPS), or electron spectroscopy for chemical analysis (ESCA). This technique consists of the irradiation of a sample with monochromatic X-rays (e.g., A1 (1.487 keV), Mg 

Since each carbon atom is sp hybridized, a p orbital perpendicular to the plane of the ring remains on each carbon atom. These six p orbitals can be used to form tt molecular orbitals [Fig. 9.47(a)]. The electrons in the resulting ir molecular orbitals are delocalized above and below the plane of the ring [Fig. 9.47(b)]. This gives six equivalent C—C bonds, as required by the known structure of the benzene molecule. The benzene structure is often written as 

Very similar treatments can be applied to other planar molecules for which resonance is required by the localized electron model. For example, the NO3 ion can be described using the tt molecular orbital system shown in Fig. 9.48. In this molecule each atom is assumed to be sp hybridized, which leaves one p orbital on each atom perpendicular to the plane of the ion. These p orbitals can combine to form the tt molecular orbital system. 

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Energy of electron = energy of photons used kinetic energy of the electron 

Helium gas usually is used as the source of high-energy photons in PES because, when excited, helium atoms anit photons with a wavelength of 58.4 ran (21.2 eV). These very-high-energy photons are energetic enough to eject electrons from many conunon atoms and molecules. 

During an interface experiment, an overlayer is stepwise deposited onto a substrate. By monitoring the substrate and overlayer core-level binding energies during deposition, the evolution of the valence band maxima of the substrate and of the overlayer can be followed during interface formation [33-36], The procedure is outlined in Fig. 4.3. As will be shown in Sects. 4.2.3.3 and 4.3.3, care has to be taken when applying this standard procedure to study surfaces and interfaces of sputter-deposited ZnO films, as BEvb(CL) depends on the deposition parameters for this material. 

Photoelectron spectroscopy is a highly surface sensitive technique because of the inelastic mean free path of the photoelectrons Ae, which depends on the electron kinetic energy Ekin and has typical values of 0.2-3nm [31,37,38]. Determination of Schottky barrier heights Z b, or valence band discontinuities AEyB, can be performed by following the evolution of the position of the valence band maxima with respect to the Fermi level of substrate and overlayer with increasing thickness of the overlayer. For layer-by-layer growth the attenuation of the substrate intensities is given by the inelastic 

Experimental determinations of barrier heights on oxide semiconductor interfaces using photoelectron spectroscopy are rarely found in literature and no systematic data on interface chemistry and barrier formation on any oxide are available. So far, most of the semiconductor interface studies by photoelectron spectroscopy deal with interfaces with well-defined substrate surfaces and film structures. Mostly single crystal substrates and, in the case of semiconductor heterojunctions, lattice matched interfaces are investigated. Furthermore, highly controllable deposition techniques (typically molecular beam epitaxy) are applied, which lead to films and interfaces with well-known structure and composition. The results described in the following therefore, for the first time, provide information about interfaces with oxide semiconductors and about interfaces with sputter-deposited materials. Despite the rather complex situation, photoelectron spectroscopy studies of sputter-deposited 

ZnO films can provide substantial information on chemical and electronic properties of ZnO surfaces and interfaces, which occur in real thin film solar cell structures. In addition, general information on the interface formation of oxide materials can be extracted. In the following we describe  

It was mentioned earlier that EPR spectra may show fine structure due to interaction between the unpaired electron(s) and nuclei with non-zero spin. These nuclei may be those of the paramagnetic ion (for example, Mn, with spin and 100% natural abundance) or those of a ligand. EPR spectra of copper(II) complexes with nitrogen ligands show evidence for interactions of this sort and have been extensively studied—an example is shown in Fig. 

where the nitrogen fine structure is additional to the quartet pattern mentioned above and is indicated by N. The best known example, however, is that of the EPR spectrum of the [IrCl ] ion, involving a strong-field complex of iridium(IV), with a d configuration, incorporated as an impurity in a crystal of Na2[PtCl6] -6H20. The fine structure observed, shown in Fig. 

Careful analysis of such vibrational frequencies can even indicate the symmetry of the orbital from which the electron was ionized. 

In Fig. 12.23 are shown the He photoelectron spectra of Cr(CO)g (a) and W(CO)e (b). Their general similarity is obvious. The interpretation of these spectra is aided by the data given in Chapter 10 in Table 10.4. At 

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Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

Ultraviolet photoelectron spectroscopy (UPS) is a variety of photoelectron spectroscopy that is aimed at measuring the valence band, as described in sectionBl.25.2.3. Valence band spectroscopy is best perfonned with photon energies in the range of 20-50 eV. A He discharge lamp, which can produce 21.2 or 40.8 eV photons, is commonly used as the excitation source m the laboratory, or UPS can be perfonned with synchrotron radiation. Note that UPS is sometimes just referred to as photoelectron spectroscopy (PES), or simply valence band photoemission. 

One of the most direct methods is photoelectron spectroscopy (PES), an adaptation of the photoelectric effect (Section 1.2). A photoelectron spectrometer (see illustration below) contains a source of high-frequency, short-wavelength radiation. Ultraviolet radiation is used most often for molecules, but x-rays are used to explore orbitals buried deeply inside solids. Photons in both frequency ranges have so much energy that they can eject electrons from the molecular orbitals they occupy. 

Photoelectron spectroscopy (PES) has been applied to determine the structure of 1-aza- and 1,4,7-triazatricy-clo[5.2.1.04,10]decane 37 and 40 <1997JMT(392)21>. The PES spectrum of ATQ shows four composite bands in the region 7-17 eV. A first band peaked at 7.80 eV is attributed to the NLPO (nitrogen lone-pair orbital). A second prominent broad band system, extending from 10.5 to 13.0 eV is associated with photoionizations from the cr-orbital manifold. The third composite band is produced by two photoemissions. The second band may be attributed to emissions arising from a sequence of seven near-lying MOs. 

Most of what we know about the structure of atoms and molecules has been obtained by studying the interaction of electromagnetic radiation with matter. Line spectra reveal the existence of shells of different energy where electrons are held in atoms. From the study of molecules by means of infrared spectroscopy we obtain information about vibrational and rotational states of molecules. The types of bonds present, the geometry of the molecule, and even bond lengths may be determined in specific cases. The spectroscopic technique known as photoelectron spectroscopy (PES) has been of enormous importance in determining how electrons are bound in molecules. This technique provides direct information on the energies of molecular orbitals in molecules. 

Photoelectron spectroscopy (PES) of organometallic compounds with C-M (M = Ge, Sn, Pb) bonds... 

The electronic properties of potential homoaromatics are effectively probed by means of photoelectron spectroscopy (PES). In this technique electrons are ejected from the valence level of the molecules under... 

Molecular photoelectron spectroscopy (PES) is widely used to study the electronic structure of molecules, and compounds can be characterized by their PE spectra. In this chapter the results of ultraviolet PE spectroscopic (UPS) studies of molecules which incorporate amino, nitroso or nitro groups will be summarized. 

Photoelectron spectroscopy (PES, a non-mass spectral technique) [87] has proven to be very useful in providing information not only about ionization potentials, but also about the electronic and vibrational structure of atoms and molecules. Energy resolutions reported from PES are in the order of 10-15 meV. The resolution of PES still prevents the observation of rotational transitions, [79] and to overcome these limitations, PES has been further improved. In brief, the principle of zero kinetic energy photoelectron spectroscopy (ZEKE-PES or just ZEKE, also a nonmass spectral technique) [89-91] is based on distinguishing excited ions from ground state ions. 

The vertical IPs of CO deserve special attention because carbon monoxide is a reference compound for the application of photoelectron spectroscopy (PES) to the study of adsorption of gases on metallic surfaces. Hence, the IP of free CO is well-known and has been very accurately measured [62]. A number of very efficient theoretical methods specially devoted to the calculation of ionization energies can be found in the literature. Most of these are related to the so-called random phase approximation (RPA) [63]. The most common formulations result in the equation-of-motion coupled-cluster (EOM-CC) equations [59] and the one-particle Green s function equations [64,65] or similar formalisms [65,66]. These are powerful ways of dealing with IP calculations because the ionization energies are directly obtained as roots of the equations, and the repolarization or relaxation of the MOs upon ionization is implicitly taken into account [59]. In the present work we remain close to the Cl procedures so that a separate calculation is required for each state of the cation and of the ground state of the neutral to obtain the IP values. 

Gas-phase photoelectron spectroscopy (PES) has been used in conjunction with theoretical calculations to investigate the hole-vibrational and electron-vibrational couplings in fused benzodithiophenes. The first ionization energies of benzojl,2- 5,4- ]dithiophene 21 and benzo[l,2-A4,5- ]dithiophene 22 were found to be to be 7.585 and 7.573eV, respectively <2006CEJ2073>. 

A very useful thermodynamic cycle links three important physical properties homolytic bond dissociation energies (BDE), electron affinities (EA), and acidities. It has been used in the gas phase and solution to determine, sometimes with high accuracy, carbon acidities (Scheme 3.6). " For example, the BDE of methane has been established as 104.9 0.1 kcahmol " " and the EA of the methyl radical, 1.8 0.7 kcal/mol, has been determined with high accuracy by photoelectron spectroscopy (PES) on the methyl anion (i.e., electron binding energy measurements). Of course, the ionization potential of the hydrogen atom is well established, 313.6 kcal/ mol, and as a result, a gas-phase acidity (A//acid) of 416.7 0.7 kcal/mol has been... 

A non-Kekule molecule conceptually formed by fusion of two TMM units and also predicted" " to have a triplet ground state is 2,4-dimethylenecyclo-butane diyl (20), which ultimately was prepared by two independent syntheses. Matrix ESR spectroscopy and gas-phase photodetachment photoelectron spectroscopy (PES) (see Section 4.1.4) eventually agreed that the ground state is triplet. 

Photoelectron spectroscopy (PES) is also carried out in the gas phase photons of known energy (E/,v), for example, the He(I) line (21.21 eV), ionize a substrate the kinetic energy (Eyn) of the emitted electrons is measured and the vertical ionization potentials (/y) derived (Eq. 13). The PES provides information on the energies of occupied molecular orbitals (MOs) " the highest occupied molecular orbital (HOMO) of the parent reveals the bond(s) likely to be weakened or broken upon ionization. The PES data reflect the geometries of the parent molecule and need not have any bearing on the equilibrium structure of the radical cation. 

The singlet-triplet splitting of NH was determined experimentally by spectroscopy of neutral NH and by negative ion photoelectron spectroscopy (PES) of the NH anion. In the latter experiment, the anion NH is prepared in the gas phase and exposed to monochromatic ultraviolet (UV)-laser hght. This photolysis leads to ejection of photoelectrons whose kinetic energies ( k) are analyzed. As the energy... 

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