Photoelectrons main lines

In order to interpret X-ray photoelectron spectra with main lines and shake-up satellites, one has to study the dynamics of the screening process following the sudden creation of a core hole. This can be done in at least three different ways with many variations ... 

Shakeup Satellites. An unusual feature in many spectra is the shakeup satellite, a line or lines usually several eV lower in kinetic energy than the parent photoelectron line. It arises vdien the photoelectric transition has a significant probability of generating a final ion in an excited state. The extra energy in the excited state is reflected in the energy separation of the satellite from the main line. A common example in organic systems... 

Figure 19. Intensity of photoelectrons from the argon X-shell. The intensity of the photo-electron satellite line for an excited final Ar" state relative to the main line for a final Ar state is plotted. Although the data scatter, the intensity of the satellite line apparently increases with increasing kinetic energy of the photoelectrons, i.e., with increasing energy of the primary photons. (From Ref. 70.)...

Ne Is photoelectron ties are in percent relative to the Is main line, a) Theory Only HF ininitial state, FISCI between Is2s 2p6 and Is2s22p5np (n=3,4,5,6). b) Theory With ISCI between ls 2s22p and singly and doubly excited states in ns and np (n=3,4,5,6), FISCI as in a), c) Experiment (from Martin and Shirley 2) ... 

The radiation source also gives rise to weak additional photoelectron signals, as, for example, both the AlKa, and the MgKa lines have satellites some 10 eV below the main line with around 10% of its intensity. In addition to these satellites, an oxidized or damaged anode (CuKa,) as well as cross-talk from the complementary side of a badly aligned twin anode can lead to the appearance of unwanted radiation. 

Like the resonance lines, the X-ray lines are always accompanied by satellites with intensities of up to 10% of that of the main line. This must be considered in the evaluation of spectra. For example, the small sUiictures seen at the low binding energy side of the photoelectron lines in Figs, la, 10c, and 14 are due to the satellites of the exciting radiation. In addition to the satellites, there is always a bremsstrahlung continuum underlying the characteristic X-ray lines. This continuum can be reduced by inserting a thin metal foil (the nature of which depends on the anode material) between the X-ray source and the collision chamber. The foil must be sufficiently thin and cannot be used to maintain a reasonable pressure difference between the source and the chamber. 

The photoelectron line of main interest is Cls. Different bonding in the environments of the carbon atoms leads to very small chemical shifts of this line. High-resolution XPS is, therefore, required and monochromatic radiation should be used to prevent overlap with satellite lines. 

The photoelectron spectrum of nitrogen (N2) has several peaks, a pattern indicating that electrons can be found in several energy levels in the molecule. Each main group of lines corresponds to the energy of a molecular orbital. The additional "fine structure" on some of the groups of lines is due to the excitation of molecular vibration when an electron is expelled. 

Another important property of PMTs is the pulse height distribution. The amplification of individual photoelectrons by the PMT is a stochastic process that causes variations in the gain of individual photoelectrons. As a result significant jitter in the amplitude of the output pulses is observed, see Fig. 3.6. These pulse height variations can be more than a factor of 10. The lowest pulse heights mainly consist of (thermal) noise, indicated by the dashed line in Fig. 3.6. The pulse height distribution exhibits a peak corresponding to detected photons. The threshold level of the... 

Fig. 4. Photoelectron spectrum of O(ls) for water vapor irradiated by the A1X line. To left of the main peak are the satellites. Solid line, experimental data vertical lines, results of calculations.53 The energy is counted from the main peak corresponding to ejection of I s electrons.

Figure 2.14 Compilation of data for the / parameter of 2p photoionization in neon as a function of the kinetic energy of the photoelectron. The solid line surrounded by the hatched area represents the experimental values including an error of +0.03. The other curves come from theoretical calculations (see main text and [Sch86]). From [Sch86].

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