Page 430 - 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

80 Photoelectron Spectroscopy

analyzing weak features in the spectrum. Most modern G. Instrumentation for Angle-Resolved
instruments use a sweep technique in which the investi- Photoelectron Spectroscopy
gated energy range is measured within a few seconds, and
Instruments designed for angle-resolved measurements
the measurement is repeated at intervals. The data from
must meet two specific requirements: (1) The analyzer
each sweep are accumulated and stored in a data handling
must be movable around the ionization region and (2) the
system. Well-designed sweep programs also take care of
acceptance angle at the first input lens of the analyzer
sensitivity differences between individual channeltrons in
system must be small enough (usually a few degrees) to
thearray.Thedevelopmentofthespectrumcanbewatched
achieve angular resolution. The requirements for an angle-
on a screen, and the measurement can be terminated
resolving spectrometer differ depending on whether the
when the signal-to-noise ratio has reached an acceptable
sample is a gas or a solid.
limit.
The angular distribution for atoms or molecules in the
For spin-resolved PES (Section I.H), the detector must
gas phase is fully described by the asymmetry parameter
be sensitive to the direction of the electron spin. Such a
β (Section I.F). To measure β, the angle θ between the
detector was constructed in 1943 by Shull et al. based on
incoming radiation and the outgoing electrons must be
the theoretical prediction by Mott, that the scattering of
varied[Eq.(6)].Itissufficientforthistypeofmeasurement
an electron from a nucleus with a high atomic number Z
to move the analyzer in one plane.
depends on the direction of the incident electron’s spin, if
Forsolidsamplestherearethreedegreesoffreedom:the
the electron has an energy greater than 50 keV and is scat-
angles
, θ, and φ defined in Section I.G. If the incident
tered through an angle greater than 90 . This Mott detector
◦
radiation is polarized, an additional degree of freedom is
has been widely used to measure the spin polarization of
obtained because of the orientation of the polarization vec-
electrons. In the 1980s, Mini Mott detectors were real-
tor. Angle-resolved measurements with unpolarized radi-
ized by the use of cylindrical or sperical retarding optics.
ation can be carried out with an instrument in which the
Among them, the SPLEED and the low-energy diffusive
analyzer can be moved in one plane, provided that the sam-
scattering detector are of practical importance. The lat-
ple is mounted on a manipulator that allows adjustment of
ter is preferable because the SPLEED demands frequent
the direction of the surface normal. With this setting
,
cleaning of the target crystal under UHV. The low-energy
θ, and φ can be varied over a fairly wide range. However,
diffuse scattering detector is based on the spin–orbit inter-
a better range for all three angles is achieved if the ana-
action between an incident electron and a polycrystalline
lyzer can be moved in two planes that are perpendicular
gold film that is used as scattering target, because gold
to each other. Because of the additional degree of freedom
is not chemically reactive and exhibits strong spin-orbit
obtained with polarized radiation, it is necessary to use an
interaction due to its high atomic number Z. According to
instrument that allows movement of the analyzer in two
the Ls term of the spin–orbit interaction, the spin analyzer
dimensions in order to obtain all the available information.
is sensitive to the transverse component of the spin polar-
Some limited information on the angular distribution
ization which is normal to the scattering plane, while the
of the electrons can be derived even with an instrument in
longitudinal component along the incident electron direc-
which the angle between excitation source and analyzer is
tion cannot be observed.
fixed: if the sample is mounted on a suitable manipulator,
it is possible to vary the orientation of the analyzer to
F. Magnetic Shielding the surface normal while keeping the angle of incident
radiation constant.
Low-energy electrons are very sensitive to the stray mag-
netic fields that exist in any laboratory, such as the mag-
neticfieldoftheearth.Thesestrayfieldsstronglyinfluence IV. SOME EXAMPLES
the trajectories of the traveling electrons. As a result, all
parts of a PE spectrometer between ionization region and In this section we will discuss a few examples of the appli-
detector must be carefully shielded. This can be done with cation of PES in different fields of research. The number
Helmholtz coils, which compensate the external fields, or of examples is too limited to allow more than a glimpse
with magnetically shielding materials like mu-metal. The onto the broad and highly active field of PES. Further ex-
lattertypeofshieldingisusedinmostmoderninstruments, amples and other types of application can be found in the
because shielding with Helmholtz coils is difficult for the references cited in the bibliography. Current work in PES
dynamic stray fields produced by many types of labora- is published in a variety of scientific journals. A journal
tory equipment. Improper shielding leads to reduced sen- specifically devoted to this field is the Journal of Elec-
sitivity and resolution, as well as to asymmetric skewed tron Spectroscopy and Related Phenomena, published by
lines. Elsevier, Amsterdam.