Photoelectron process

Consider what happens if, for example, an ensemble of carbon atoms is subjected to X rays of 1486.6 eV energy (the usual X-ray source in commercial XPS instruments). A carbon atom has 6 electrons, two each in the Is, 2s, and 2p orbitals, usually written as C Is 2s 2p. The energy level diagram of Figure la represents this electronic structure. The photoelectron process for removing an electron from the... 

In order to examine more deeply the information contained in the photoelectron processes described above, we begin by representing the photoemission as a process in which the initial state of a system of N electrons, having wavefunction is changed by a photon of energy hv to a final state, having wavefunction Wg ... 

Somorjai, G. A. Abstract, Solar Energy and Photoelectronic Processes Symposium, p. 420, 109th AIME Annual meeting,... 

Fig. 15. Core level splitting of the lr-photoelectron lines in the paramagnetic molecules NO and 02 29. While molecular nitrogen N2 with a closed shell configuration shows no splitting, the NO -ion formed in the photoelectron process can exist in a singlet or a triplet state giving rise to the observed N lr-doublet with an intensity ratio of 1 3, while the oxygen line is only broadened. Molecular oxygen 02 finally displays a splitting of 1.1 eV and a ratio of 1 2. All spectra were recorded in the gas phase...

In addition to the removal of one core electron by the incident X-rays (photoelectron process), the Auger process is taking place approximately 10 seconds after the photoelectron event. In this process an outer electron falls into the inner orbital vacancy from the photoelectron process and a second electron is emitted... 

A major complication occurs when more than one electron is promoted during the photoelectron process, rather as if the two stages of equation 3 above take place simultaneously so that one electron occupies a formerly unoccupied or partially occupied orbital whilst the other breaks free from the molecule altogether. This situation is encountered for example in the He(I) photoelectron spectrum of cyclopropene where the intensity of the bands from the fifth onwards is reduced by admixture with two-electron processes. Thus the peak in the He(I) photoelectron spectrum at 16.68 eV (5ai, Section III) has been assigned to the ionization of an orbital of Saj type in a process mixed with simultaneous ionization and promotion to the n level of electrons from the uppermost pair of occupied orbitals at higher ionization energies the intensities of the bands become scattered over numerous processes and the bands become barely detectable. The 16.68 eV peak is due partly to... 

This flourishing experimental activity has created a need to understand the observed phenomena from a theoretical point of view. The observed fingerprint pattern of the f"->f photoelectron process has been well explained by the fractional parentage method (Cox 1975, Beatham et al. 1979, Gerken 1983). Similarly, the intensity distribution for the inverse process, f"->f is also well accounted for by the same method (Lang et al. 1981, chapter 62 by Baer and Schneider in this volume). In the present chapter we will, however, turn most of the attention to the energy position of the 4f level in rare-earth materials. Firstly, we will discuss the pure elements where both the occupied and unoccupied f states will be treated. Secondly, as one of the simplest examples of a chemical shift we... 

In liquids and solutions a chemical shift model should ideally account for the dynamical disordering of the solvent structures. This calls for models that are based on a decomposition of the intermolecular contributions to the shift and a parameterization of these contributions in terms of solvent stmcture, for example, atom-atom distribution functions. Such models should ideally account for the dependence of shift on temperature and pressure. From the distribution functions the shifts can be derived as well as the full photoelectron spectral function, including shift, width, and asymmetry, upon condensation. A basic assumption is that photoionization is vertical, meaning that both initial and final states can be associated with the same nuclear conformation. This approximation is well grounded considering the time scales between the photoelectron process and the rearrangement of the solvent molecules, which means that the solvent is not in equilibrium with respect to the final state. A common assumption behind such models is also that the internal solute nuclear motion is decoupled from external forces. This means that the spectral function/ can be written as a convolution of internal and external parts,/ and/ / respectively,... 

The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

Figure Bl.25.1. Photoemission and Auger decay an atom absorbs an incident x-ray photon with energy hv and emits a photoelectron with kinetic energy E = hv - Ej. The excited ion decays either by the indicated Auger process or by x-ray fluorescence.

Wlien photons of sufiBciently high frequency v are directed onto a metal surface, electrons are emitted in a process known as photoelectron emission [ ]. The threshold frequency Vq is related to the work fimction by the expression... 

Figure 8.1 Processes occurring in (a) ultraviolet photoelectron spectroscopy (UPS), (b) X-ray photoelectron spectroscopy (XPS) and (c) Auger electron spectroscopy (AES)...

Figures 8.1(a) and 8.1(b) illustrate the processes involved in UPS and XPS. Both result in the ejection of a photoelectron following interaction of the atom or molecule M which is ionized to produce the singly charged M. ...

When M is an atom the total change in angular momentum for the process M + /zv M+ + e must obey the electric dipole selection mle Af = 1 (see Equation 7.21), but the photoelectron can take away any amount of momentum. If, for example, the electron removed is from a d orbital ( = 2) of M it carries away one or three quanta of angular momentum depending on whether Af = — 1 or +1, respectively. The wave function of a free electron can be described, in general, as a mixture of x, p, d,f,... wave functions but, in this case, the ejected electron has just p and/ character. 

Figure 8.21 shows schematically a set of lx, 2s, 2p and 3s core orbitals of an atom lower down the periodic table. The absorption of an X-ray photon produces a vacancy in, say, the lx orbital to give A and a resulting photoelectron which is of no further interest. The figure then shows that subsequent relaxation of A may be by either of two processes. X-ray fluorescence (XRF) involves an elecfron dropping down from, say, fhe 2p orbifal fo fill fhe lx... 

A common example of an Auger process involves the ejection of a photoelectron, as shown in Figure 8.23, from the K shell (i.e. a lx electron), with energy E, which is not considered further. Following the ejection of a if electron it is common for an electron from the L shell, specifically from fhe (or 2s) orbifal, fo fill fhe vacancy releasing an amounf of energy... 

In Figure 8.26 is shown the AXumLum Auger spectrum of sodium in crystalline NaCl. Once again, the formation of the >2 weakly, the core states can be observed. Also shown are peaks resulting from additional processes in which the initial photoelectron with... 

Figure 9.50(a) illustrates the ionization process in a UPS experiment. In this type of experiment the incident radiation always has much more energy than is necessary to ionize the molecule M into either the zero-point level or a vibrationally excited level of M. The excess energy is then removed as kinetic energy of the photoelectron. 

Figure 9.50 Processes involved in obtaining (a) an ultraviolet photoelectron spectrum, (b) a zero kinetic energy photoelectron (ZEKE-PE) spectrum by a one-photon process and (c) a ZEKE-PE spectrum by a two-photon process in which the first photon is resonant with an excited electronic state of the molecule...

More commonly, the resonant two-photon process in Figure 9.50(c) is employed. This necessitates the use of two lasers, one at a fixed wavenumber Vj and the other at a wavenumber V2 which is tunable. The first photon takes the molecule, which, again, is usually in a supersonic jet, to the zero-point vibrational level of an excited electronic state M. The wavenumber of the second photon is tuned across the M to band system while, in principle, the photoelectrons with zero kinetic energy are detected. In practice, however, this technique cannot easily distinguish between electrons which have zero kinetic energy (zero velocity) and those having almost zero kinetic energy, say about 0.1 meV... 

Xps is based on the photoelectric effect when an incident x-ray causes ejection of an electron from a surface atom. Figure 7 shows a schematic of the process for a hypothetical surface atom. In this process, an incident x-ray photon of energy hv impinges on the surface atom causing ejection of an electron, usually from a core electron energy level. This primary photoelectron is detected in xps. 

The lines of primary interest ia an xps spectmm ate those reflecting photoelectrons from cote electron energy levels of the surface atoms. These ate labeled ia Figure 8 for the Ag 3, 3p, and 3t7 electrons. The sensitivity of xps toward certain elements, and hence the surface sensitivity attainable for these elements, is dependent upon intrinsic properties of the photoelectron lines observed. The parameter governing the relative iatensities of these cote level peaks is the photoionization cross-section, (. This parameter describes the relative efficiency of the photoionization process for each cote electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same cote level of all elements. This difference results ia variable surface sensitivity for elements even though the same cote level electrons may be monitored. 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

The opposite phenomenon, a decrease of sensitivity, is known as desensitization. The main reasons for densensitization ate the results of relative electron level positions as weU as the secondary processes of the photoelectrons, for example (97),... 

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