Photoelectron kinetics

Fig. 6a-c. One monolayer of cobalt on copper (111), grazing incidence, T = 77 K and T = 300 K relevant steps of the EXAFS analysis, a Experimental absorption spectra b fourier transform of the EXAFS oscillations c inverse Fourier transform of the first neighbour peak as a function of photoelectron kinetic energy E... 

In the case of the most common X-ray somces used for XPS (Mg and Al), and of the most widely observed core levels of the actinides (4 f), the photoelectron kinetic energy is of several hundreds of eV. This ensures an analysis depth large enough to reveal the bulk properties. 

Fig. 12. Spherical condenser for measurements of the photoelectron kinetic energy distribution from solids. 1—sample 2—LiF window 3—shutter 4—container of the monochromator exit slit 5—exit slit 6—electron collector 7—fluorescent layer 8—electrostatic screen photomultiplier for intensity measurements of the u.v. light on the left.

As in Section IV for naphthalene vapor, the kinetic spectra of solids are presented by plotting on the abscissa axis the quantity Ep = (hv — incident photon,

work function of the solid (energy of electron abstraction). Ekin is the photoelectron kinetic energy, which is lower than the theoretical upper energy limit of kinetic energy because of internal energy... 

Figure 5.29 Response function of a photoelectron spectrometer equipped with a large-scale detector to ensure the recording of the whole xenon 4d5/2 photoline with constant efficiency. The accepted range of approximately 5% corresponds to +1.35 eV at a photoelectron kinetic energy of 27 eV, and this value is large compared to the photon bandpass (0.4 eV at 94.5 eV), the contribution from the electron spectrometer (0.22 eV), and the natural linewidth (T = 0.12 eV). From [KKS93].

So far, we have fairly extensively discussed the general aspects of static and dynamic relaxation of core holes. We have also discussed in detail methods for calculating the selfenergy (E). Knowing the self-energy, we know the spectral density of states function A (E) (Eq. (10)) which describes the X-ray photoelectron spectrum (XPS) in the sudden limit of very high photoelectron kinetic energy (Eq. (6)). We will now present numerical results for i(E) and Aj(E) and compare these with experimental XPS spectra and we will find many situations where atomic core holes behave in very unconventional ways. 

While the distinction of the electron trajectories as being either direct or indirect and the observation of quantum mechanical interferences among the trajectories can be understood in terms of the DC electric field strength and the photoelectron kinetic energy with respect to the saddlepoint in the Coulomb + DC electric field potential, this is not the only quantity that characterizes the photoelectrons that are emitted. Above the saddlepoint in the Coulomb + DC potential there exists a continuation of the Stark manifold. This Stark manifold manifests itself in the excitation spectrum of the atom, which shows pronounced peaks in the photoionization efficiency as a func-... 

Two variables of a PES experiment are readily altered the input photon energy (hv) and the output photoelectron kinetic energy (KE). In a classical energy distribution curve (EDC) operation mode, one scans KE only and obtains information on the energy level manifold. While this is the only mode possible with fixed-energy VUV photon sources, SR permits two further combinations a constant final-state (CFS) mode where one scans hv and a constant initial-state (CIS) mode with both hv and KE scanned in such a way that their difference remains constant. CIS and CFS modes permit separate studies of the initial (ground electronic states, ionization probabilities) and final (photoelectron perturbed by the molecular ion) stages of photoionization events. 

If the sample volume, from which photoelectrons are to be emitted, is put in electrical contact with the spectrometer, charge flows to establish the contact potential, which is the work function difference Sample — Aspect, and the Fermi levels, ep, of the sample and spectrometer are equalized. This is shown schematically in Fig. 2. The work function in the experiment is Aspect and observation of the photoelectron kinetic energies from an atomic species in two different samples thus provides a measure of the shift of the level in question, level i, with respect to eF if one wishes to consider the level shift with respect to some zero of the potential common to all the samples, then any shifts in eF from sample to sample with respect to that same zero must be taken into account. This point... 

Fig. 2. Energy level scheme of the Fermi level, Ep, and a bound level for a metallic sample. At left the sample is free and uncharged at center it is in metallic contact with a photoelectron spectrometer and the contact potential has been established. The photoelectron kinetic energy associated with photoexcitation of the bound level is indicated...

It is recommended to plot photoelectron spectra with increasing ionization energy to the left, i.e. with increasing photoelectron kinetic energy to the right [14]. 

Photoelectron spectroscopy of valence and core electrons in solids has been useful in the study of the surface properties of transition metals and other solid-phase materials. When photoelectron spectroscopy is performed on a solid sample, an additional step that must be considered is the escape of the resultant photoelectron from the bulk. The analysis can only be performed as deep as the electrons can escape from the bulk and then be detected. The escape depth is dependent upon the inelastic mean free path of the electrons, determined by electron-electron and electron-phonon collisions, which varies with photoelectron kinetic energy. The depth that can be probed is on the order of about 5-50 A, which makes this spectroscopy actually a surface-sensitive technique rather than a probe of the bulk properties of a material. Because photoelectron spectroscopy only probes such a thin layer, analysis of bulk materials, absorbed molecules, or thin films must be performed in ultrahigh vacuum (<10 torr) to prevent interference from contaminants that may adhere to the surface. 

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