Page 426 - 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

76 Photoelectron Spectroscopy

where n is the diffraction order, λ the X-ray wavelength,
d the crystal spacing, and ϕ the Bragg angle. Anode, crys-
tal, and sample surface are all arranged on the circum-
ference of a so-called Rowland circle, usually 0.5 m in
diameter. The size of the monochromator influences the
size of the X-ray footprint, which is about 0.05% of the
diameter of the Rowland circle. Commercial monochro-
mators, which are now found in many XPS instruments,
all rely on quartz as the 110 plane fulfills the required
conditions for Al K α radiation. With slight adjustments of
anode and crystal position such a monochromator can also
be used for Ag L α (n = 2) or Ti K α (n = 3). Linewidths
down to 0.3 eV have been reached for Al K α with this
technique. The use of a monochromator has the addi-
tional advantage that the K α satellites and the brems-
strahlung continuum are removed. The spectrum shown
in Fig. 9 was obtained with a monochromator equipped
instrument.
For the investigation of deeper lying core levels, which
are not accessible with Al K α radiation (1483.6 eV), like
the 3d 5/2 core level of Yb in YbP at a binding energy
of 1527.8 eV, monochromatized Si K α radiation is very
useful. Monochromatization is again achieved with quartz
(110) crystals. The Bragg angle for the Si K α energy of
1740 eV is 56.85 ; the diameter of the Rowland circle is photons
◦
FIGURE 15 Brilliance [in ] of the syn-
650 mm. sec(mm · mr ad ) 2 0.1% BW
chrotron radiation obtained from BESSY I (now closed) and
Synchrotron radiation has become increasingly impor-
BESSY II in Berlin. The curves for wigglers (W) and undula-
tant as exciting radiation. When charged particles travel tors (U, V) give an impression of the increase in brilliance that
along a bent path with a velocity near the speed of light, is gained with these devices. [From BESSY II (1986). “Eine opti-
as occurs in an electron synchrotron or a storage ring, mierte Undulator/Wiggler-Speicherring Lichtquelle f¨ur den VUV-
an intense beam of light is emitted tangential to the path. und XUV- Spektralbereich,” p. 20, BESSY, Berlin.]
This light has a smooth continuous energy distribution ex-
tending far into the vacuum UV and is strongly polarized
in the plane of the ring. Figure 15 shows the brilliance opened new dimensions for a great variety of experiments.
(number of photons emitted per square millimeter, sec- One such example is shown in Section IV.B.
ond and steradiant) of the radiation provided by the new
Berlin Synchrotron Radiation Facility BESSY II. Below
C. Collision Chamber and Sample Inlet System
the cut-off energy, which depends on the design of the syn-
chrotron, the intensity is by two or three orders of magni- For the measurement of gaseous samples the target gas
tude higher than that of conventional line sources. Special is introduced through a capillary about 1 mm in diame-
devices, called wigglers and undulators, lead to a further ter. The amount of gas entering the collision chamber is
intensity increase of two to four orders of magnitude (see regulated by a leak valve. The beam of molecules com-
Fig. 15). Such devices are now available at all synchrotron ing from the capillary and the radiation coming from the
radiation facilities. source, which is usually a beam with a diameter of about
To use synchrotron radiation as an excitation source in 1 mm, can be either parallel or perpendicular to each other.
PES it is necessary to select a small energy range from the The former arrangement leads to a rodlike photoionization
continuous synchrotron radiation. This is done with dif- region, the latter to a more pointlike one. The gas pres-
ferent types of monochromators, which are provided by sure in the PIR is about 10 −3 torr. The photoelectrons
the synchrotron radiation facility. Great progress has been created are usually observed perpendicular to the two in-
achieved over the last two or three decades in the construc- coming beams. This avoids having the beam of target gas
tion of the necessary far UV and soft X-ray monochroma- point directly to the entrance of the kinetic energy analyzer
tors. The tunability of the excitation energy and the polar- and helps minimize contamination of the surfaces of the
ization of the synchrotron radiation add advantages that analyzer.