The solid-gas interface 139
versus energy spectrum is produced. This kinetic energy, £e|, is given
in terms of the irradiation frequency, v, and the ionisation potential,
/, of the electron by Einstein's equationEel = hv-J (5.14)If it is assumed that photoionisation occurs without any adjustment of
the remaining electrons (Koopman's theorem), the ionisation poten-
tial and the orbital energy, c, of the ejected electron are numerically
equal (/ = — c). A spectrum of electron orbital energies is thus
obtained.
The ejected electrons will interact strongly with matter because of
their charge, therefore, when studying solids, the detection of
electrons as described above will be limited to those ejected from
atoms at or close to the surface.
In ultraviolet photoelectron spectroscopy (UPS or U-PES), the
irradiation (usually a Hc(I) (21.2 eV) or He(II) (40.8 eV) source)
causes the displacement of a valence electron. Although an important
method of studying the electronic nature of molecules in the gas
phase, it is less useful for studying the surfaces of metals, since the
valence electrons are in a continuous (conducting) energy band with a
spread of about 10 eV. Adsorbed layers can, however, usefully be
investigated in terms of the difference between the spectrum
following adsorption and that for the clean metal surface.
In X-ray photoelectron spectroscopy (XPS or X-PES), the irradiation
(usually a Mg K a (1253.6 eV) or Al K a (1486.6 eV) source) causes a
core electron to be ejected. This is a more useful technique than UPS
for surface studies, since the binding energies of core electrons are
characteristic of the elements in question and surface elements can
thus be identified by the traditional 'spectroscopic fingerprinting'
procedure. In this respect, XPS is sometimes referred to by its
alternative name, electron spectroscopy for chemical analysis (ESCA).
In order to emphasis the contribution from surface atoms, the X-ray
beam is usually set at a grazing angle to the surface. Most of the signal
originates from within a nanometre of the surface.
The ionisation potential of a core electron depends, to a small
extent, on the chemical environment of the atom in question, and
chemical shifts of up to about 10 eV can be observed. For example,
the C(ls) XPS signal for molecular!y adsorbed carbon monoxide on
polycrystalline iron at 290 K shows a peak at 285.5 eV, which is