Page 427 - 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

Photoelectron Spectroscopy 77

Vapors from liquids usually can he measured in the under UHV conditions. Among the techniques most
same way as gases. If the vapor pressure is lower than 10 −3 widely used for the preparation of suitable surfaces are
torr, the liquid must be heated. To prevent condensation
in the inlet system this should be heated, too. 1. Bombardment of the surface with rare gas ions with a
Todetermineagas-phasespectrumfromasamplethatis kinetic energy of a few hundred to a few thousand
solidundernormalconditions,thesamplemustbevolatile. electron volts
If the volatility is fairly high, the same inlet can be used as 2. Heating of the sample, sometimes in the presence of
for liquids. For samples with low vapor pressure, a direct hydrogen gas (to reduce oxides)
inlet system can be used. A small capillary is filled with 3. Mechanical preparation of fresh surfaces within the
the sample and placed in a heatable sample holder, and UHV.
the opening of the capillary is then brought close to the
PIR. Molecules evaporating from the capillary reach the Which preparation technique is most suitable always
PIR directly, and difficulties with deposition at narrow or depends on the nature of the sample. For example, ion
cold parts of the inlet system do not arise. In addition, bombardment can change the surface constitution and
the molecules do not come into contact with heated metal heating can lead to phase transitions. To prevent contam-
parts, which often leads to catalytic decomposition. The ination of the collision chamber, the cleaning and prepa-
amount of substance needed for the measurement of a ration procedure is usually carried out in a separately
gas-phase PE spectrum is about 20 mg, and it cannot be pumped chamber. The sample is then transferred from the
recovered. preparation chamber to the collision chamber.
To study photoemission from a solid, the sample must
be brought directly to the ionization region. When the
UHV requirements are not too high (up to 10 −10 torr), D. Electron Energy Analyzer
the sample can be introduced through a lock. As extreme
The electron energy analyzer is the heart of a PE spectrom-
UHV conditions are required for most surface investiga-
eter. It is here that the electrons are discriminated with re-
tions, a fully metal sealed vacuum system must be used.
spect to their kinetic energy. The most important features
The sample is usually mounted on a manipulator that can
of an electron kinetic energy analyzer (EKEA) are sensi-
be operated from the outside and is inserted through a
tivity and resolution. High resolution and high sensitivity
system of two or more separate vacuum chambers. This
contradict each other, and a given analyzer will always
prevents the pressure in the analysis chamber from rising
be a compromise between them. As chemical shifts are
upon introduction of a sample and thus guarantees a high
often small (<0.5 eV) the spectral resolution, E,ofan
operational capacity.
analyzer used in XPS should not exceed a few tenths of
If the sample is a conductor, it is brought in direct elec-
an eV. E is mainly determined by three contributions:
trical contact with the sample holder. Metallic indium is
the analyzer resolution E A , the linewidth of the exciting
often used to provide the necessary electrical and thermal
radiation E X , and the intrinsic width of the photoelec-
contact between sample and sample holder. This is espe-
tron line E P . If we assume Gaussian distributions for all
cially convenient because samples that are small particles
three, E can be expressed as
can be pressed directly into the indium. If the sample is
an insulator, even pressing it into indium does not always E = E + E + E 2 0.5 (19)
2
2

X
A
P
prevent charging. In this case, charge compensation can be
achieved by using a floodgun to bombard the sample with As discussed in Section III.B, E X is at best 0.3 eV in
low energy electrons. Powder samples can be mounted on XPS, when using a monochromated X-ray source, and 10
double-stick tape or pressed into a tablet, similar to the meV in UPS. To minimize the influence of the analyzer,
preparation used in infrared spectroscopy. Soluble sam- E A should by no means exceed these values. In both
ples can be dissolved and a drop of the solution brought cases, this corresponds to a relative resolution of at least
to the surface of a metallic sample holder and evaporated. 1:1000.
The latter method can yield a thin film, which is less sen- Modern EKEAs usually consist of a complex combina-
sitive to charging. tion of electron transfer lenses and a hemispherical elec-
When a sample is brought into the vacuum, its surface is trostatic deflection analyzer. Figure 16 shows a sketch of
always contaminated, usually with O 2 ,H 2 O, CO 2 , and hy- the analyzer and input lense system presently used on a
®
drocarbons. In many cases, the sample surface itself has a VG ESCALAB ® 220iXL produced by VG Scientific .
different chemical constitution than the bulk. The surface The input lens captures the maximum possible number of
of most metals, for example, consists of oxides. In nearly emitted phototelectrons and transfers them to the deflec-
all cases, the sample surface must therefore be cleaned tion analyzer.