Photoelectron Spectra of Solids

In the case of solids some special aspects, such as the very limited escape depth, must be considered. A photo-electron created inside a solid must escape into the vacuum to be measured. Since the probability of inelastic scattering is very high as long as the electron moves inside the solid, only electrons created close to the surface have a chance to escape without a secondary energy loss. Figure 4 shows the average escape depth as a function of the kinetic energy of the electron together with some of the most useful excitation lines (see Section III). For a kinetic energy of about 100 eV, the escape depth is lowest,

FIGURE 4 Connection between escape depth (attenuation length) and kinetic energy of the photoelectron. [From Cardona, M., and Ley, L., eds. (1978). "Topics in Applied Physics," Vol. 26, "Photoemission in Solids I," p. 193, Springer-Verlag, Berlin.]

with an average value of only a few angstroms. Even for a kinetic energy of 10 or 1000 eV, the escape depth is only on the order of 20 A. PES probes only the few outermost atomic layers of a solid, which can be a disadvantage if one wants to study the bulk material. First, the composition of the surface is often different from the composition of the bulk because of segregation effects or surface contamination (Section III.A). Even if there is no difference in composition, there is usually a strong contribution from the outermost layer, especially for kinetic energies around 100 eV. The outermost layer is chemically always different from the interior, since the atoms in this layer have fewer neighbors. The surface sensitivity of PES is advantageous, however, if we want to study the surface itself. Therefore, PES has become one of the most powerful tools in surface science. It allows us to study not only a surface, but also atoms or molecules sitting at the surface. Nowadays it is possible to detect coverages down to a fraction of a monolayer. Thus, PES is extremely useful for the investigation of adsorbates.

Another special aspect of solids is the "reference problem." For an atom or molecule in the gas phase, ionization leads to the creation of an electron and a positive ion. The electron is either detected or lost at the walls of the instrument. The ion also leaves the ionization region rapidly. By calibration with accurately known binding energies (see Table I), the binding energies of the sample can be referred to the vacuum level that corresponds to an infinite separation of electron and ion.

TABLE I Useful Calibration Lines

Atom

Level

Compound/phase

Energy (eV)"

Ne

1s

Gas

870.37

F

1s

CF4/gas

695.52

O

1s

CO2/gas

541.28

N

1s

N2/gas

409.93

C

1s

CO2/gas

297.69

Ar

2p3/2

Gas

248.62

Kr

3p3/2

Gas

214.55

Kr

3d5/2

Gas

93.80

Ne

2s

Gas

48.47

Ne

2p

Gas

21.59

Ar

3p

Gas

15.81

Cu

2p3/2

Metal

932.8

Ag

3p3/2

Metal

573.0

Ag

3d5/2

Metal

368.2

Cu

3s

Metal

122.9

Au

4f7/2

Metal

83.8

Pt

4f7/2

Metal

71.0

a For the metals the energies refer to the Fermi level instead of the vacuum level.

FIGURE 5 Reference schemes for solid samples: (a) conducting sample; (b) insulating sample with reference material on top. S, Sample; Sp, spectrometer; R, reference.

a For the metals the energies refer to the Fermi level instead of the vacuum level.

FIGURE 5 Reference schemes for solid samples: (a) conducting sample; (b) insulating sample with reference material on top. S, Sample; Sp, spectrometer; R, reference.

In the case of solids it is necessary to distinguish between conductors and insulators. If the sample is a conductor and in electrical contact with the spectrometer, the Fermi levels EF equilibrate (Fig. 5a). The same is true for any metal that is used to calibrate the binding energy scale. The binding energy ESB of an arbitrary conducting sample can therefore be referred to the Fermi level of the spectrometer, which is the reference level used in most investigations. To refer ESB to the vacuum level of the sample, the work functions of the reference material and sample must be known.

The situation is more difficult for insulating samples. Photoionization creates positive charges within the sample that are not equilibrated immediately, and the sample becomes charged. At the same time there is usually a relatively high density of low-energy electrons close to the sample surface, which can neutralize the positive charges. The equilibrium between outgoing and incoming electrons depends on the measuring conditions, specifically on the intensity of the ionizing radiation and the cleanness of the surrounding metal parts. Therefore, the actual charging potential (Fig. 5b) depends on the measuring conditions. The charging may not even be homogeneous over the surface area investigated (differential charging), resulting in a broadening of the observed lines. Sample charging can be reduced by use of very thin samples or a separate source of low-energy electrons (flood gun). Alternatively, sample charging can be taken into account by depositing small amounts of a reference material (usually gold) onto the sample surface or by using the carbon that is found on nearly every surface as a reference material. Assuming that the reference material and the sample are at the same potential in the irradiated area (Fig. 5b), the binding energies for the sample are then referred to the binding energies of the reference material.

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