Page 424 - 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

74 Photoelectron Spectroscopy

A. Vacuum Requirements
Since PES measures electrons, the whole path between
the region where photoionization takes place and the de-
tector must be kept at a pressure of about 10 −5 torr or
less to prevent collisions with the rest gas. For an excit-
ing radiation with hν> 11.3 eV, no material exists that is
sufficiently transparent to serve as a window. Therefore,
the path from the radiation source to the photoionization
region (PIR) must also be kept under vacuum.
Photoelectron spectrometers are very sensitive to con-
tamination of their inner surfaces. Deposition of the sam-
FIGURE 13 Scheme of a photoelectron spectrometer.
ple or its decomposition products on these surfaces can
lead to local charges that strongly influence the path of
the basic structure of this instrument, which is the same the electrons. Therefore, the vacuum must not only pro-
whether the instrument is designed for gases or for solids. vide a sufficient mean free path for the electrons but also
The technical details of the instrument, however, differ prevent contamination as much as possible. Since even
considerably depending on its application. heavy pumping does not always prevent contamination,
all vital parts of a PE spectrometer should be bakeable to
remove contamination.
Photoelectron spectrometers designed for the study of
solids require even better vacuum conditions. Because of
the surface sensitivity of PES (see Section I.E and Fig. 4),
any contamination of the sample surface contributes
strongly to the measured PE spectrum. For a gas with
a sticking coefficient of 1 (the sticking coefficient is the
probability that a particle hitting the surface will remain
on the surface), a clean surface is covered by a monolayer
if 1 Langmuir of gas is offered to the surface (1 Langmuir
is an exposure to 10 −6 torr · sec). To maintain a reasonably
1
3
clean surface for / 2 hr (about 10 sec), a vacuum better
than 10 −9 torr is necessary in the collision chamber. If
well-defined adsorbates in the mono- or submonolayer
range are the object of investigation, the vacuum in the col-
lision chamber must be even better (down to 10 −11 torr).
Spectrometers designed for this type of investigation are
therefore built in ultrahigh-vacuum (UHV) technology.

B. Radiation Source
The type of radiation source strongly depends on the kind
of PE spectroscopy that is performed. With UPS the com-
monly used radiation source is a resonance lamp, usually
operated with helium. The radiation emitted from this
2
1
1
source results from the transitions P(1snp) → S(1s )
(compare Table VI). The radiation is produced by a high-
voltage, direct-current discharge in a capillary, a high-
current arc discharge with a heated cathode, or a mi-
crowave discharge. The helium pressure is usually a few
hundred millitorrs. Since this is higher than the pressure
maintained in the collision chamber, differential pump-
FIGURE 14 Au 4f doublet measured with different excitation ing is necessary. The lamps have an intensity of about
sources. 10 12 photons/cm · sec and a half-width of 10–15 meV,