Page 436 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN012C-568 July 26, 2001 15:32

86 Photoelectron Spectroscopy

negative relaxation shifts. As suggested in Section II.B,
the final state relaxation is largest for the molecules that
contain easily polarizable substituents. In Fig. 27, the ε
values obtained from the experimental binding energy
shifts and the experimentally estimated relaxation contri-
bution are compared to the charge at the phosphorus atom
and the off-atom potential, both calculated by a semiem-
pirical quantum chemical method. The excellent correla-
tion obtained for this example tells us that the theoretical
method used is suitable for predicting reasonable atomic
charges.
The energies shown in Table VII were obtained from
gaseous samples. For solids, an additional problem arises
that is connected with the reference problem (Section I.E).
For solids, the binding energies are usually referred to the
Fermi level and not to the vacuum level. The binding en-
ergy shifts which appear in Eq. (12), however, refer to the
vacuum level. Binding energy shifts referred to the Fermi
level and those referred to the vacuum level are equal only
when the work function is the same for both samples. If
this is not the case, the work function difference, which
in principle is also an experimentally obtainable quantity,
must be taken into consideration.

D. Investigation of Adsorbates
When a molecule is adsorbed at the surface of a solid it
FIGURE 28 Comparison of C 1s and O 1s spectra for free
can be either physisorbed or chemisorbed. We speak of CO, W(CO) 6 , and CO adsorbed on W(110) and Cu(100). [From
“physisorption” when the bonding is mainly caused by Freund, H. J., and Plummer, E. W. (1981). Phys. Rev. B 23, 4859.]
van der Waals interactions and of “chemisorption” when
we have some type of a chemical bond between adsorbate
and substrate. However, there is a more or less continuous contribute strongly to the understanding of the chemisorp-
transition between both types of bonding. The nature of tion bond. Figure 28 shows an example. The C 1s and O
the bonding between adsorbate and substrate is very im- 1s spectra of gaseous CO are shown at the bottom. Weak
portant for the understanding of reactions that take place satellite structures at the high binding energy side of the
in heterogeneous catalysis. Catalytic processes of this type main lines are blown up in the figure in order to reveal
have great technical importance but often they are not fully their structure. The indicated satellites are connected with
understood on a molecular level. Because of its extreme final ion states in which, in addition to the removal of a
surface sensitivity, PES is a powerful tool in achieving core electron, a second electron is excited from the π to
∗
such an understanding. The applications of PES to the the π orbital. When CO is adsorbed on the 110 surface
study of adsorbates are too widespread to be reviewed of tungsten, a new satellite is observed closer to the main
here. We only show two examples to give an impression line. This new satellite increases strongly in intensity with
of how these studies can be carried out. decreasing strength of the chemisorption bond. For the
Foravarietyofadsorbatesystemssuchastheadsorption system CO on copper (100) (at the top in Fig. 28), where
of small molecules (CO, N 2 ,H 2 O, etc.) on transition metal CO is only weakly bound, this new satellite becomes very
surfaces, a “molecular view” of the bonding often provides strong. What is the nature of this satellite and what does
a useful description. In a molecular view we do not look it tell us about bonding? The fact that the same satellite as
at the substrate as a metal with band structure and all the observed for CO on W(110) is also observed in the metal
typical solid-state properties. Instead, we look primarily complex W(CO) 6 reveals that this satellite cannot result
at a few metal atoms to which the adsorbed molecule is from the special properties of metallic tungsten. It must
bound. Such a view immediately connects the bonding in be related to the local bonding between CO and a tungsten
an adsorbate to the bonding in a metal complex. Thus, atom. The satellite is connected with a final state where, in
studying adsorbates together with related complexes can addition to the core ionization, an electron is transferred