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Angle integrated Photoelectron Spectrometer
M.S. Hegde, G.S. Ramesh,
Indian Institute of Science, Bangalore, India.
S.M. Chaudhari, D.M. Phase, A.D. Wadikar.
Inter University Consortium for DAE Facilities, University campus, Khandwa road,
Indore.452 017, India.
The experimental station of
photoelectron spectroscopy beamline is an angle integrated photoelectron
spectrometer, which was designed and fabricated indigenously. This consists of
(1) the energy analyser, (2) the experimental chamber with in-situ heating and
cooling arrangement of the sample mounted on XYZ sample manipulator, (3) sample
preparation chamber equipped with quick load-lock magnetic sample transfer
system, ion gun for controlled etching of the sample and diamond file type
scrapper; and (4) the associated electronics as well as the data acquisition
system. A brief description of the spectrometer is given below.
The electron energy analyser is the most important
part of the spectrometer. The complete analyser system consists of the
following parts: the electrostatic lens, the hemispherical elements and the
detector. The lens is a three-piece cylindrical system. The lens is used to
transport the electrons from the emission area to the hemispherical analyser
through the entrance slit of the analyser plate. The most common configuration
of the three-piece lens is an einzel lens, in which the outer electrodes
are held at the ground potential and beam focusing is achieved by varying the
potential on the centre electrode. This type of lens is commonly used in
electron spectrometers. Each cylinder is machined out of stainless steel and
mirror polished and coated with gold for excellent transmission of the beam. All
the pieces are then mounted inside a stainless steel shield, which in turn is
mounted on the analyser plate.
The inner and outer hemispheres of the analyser are
machined out of aluminium in a numerically controlled, universal milling machine
to accuracy better than +0.001mm. The surfaces are then polished and
coated with gold. This ensures uniform potential energy surfaces and prevents
surface charging. The hemispheres are mounted on a fringe plate (H-plate), also
machined out of aluminium, which has entrance and exit slits. A slit width can
be varied from 1mm to 3mm in discrete steps of 1 mm. The entire analyser
assembly is mounted such that the inner hemisphere, outer hemisphere and the
H-plate are insulated from each other, by using Teflon washers and bushes.
Electrons are focused to the entrance slit of the analyser through the entrance
aperture by the lens system. Energy dispersion takes place as the electrons
travel through the electrostatic field between the inner and outer hemispheres.
There are six concentric rings made out of stainless steel, mounted on the
H-plate to correct the fringe field, which improves the resolution of the
analyser. These rings are positioned within the annular space (gap between the
two hemispheres) equidistantly. The inner and the outer hemispheres have a
radius of 65 mm (r1) and 125 mm (r2) respectively. The mean radius of the
analyser is 95 mm and the annular space is 60 mm.
The detection of electrons is
carried out by applying a high voltage to the channel electron multiplier
(X719BL, Philips make) mounted at the exit slit of the analyser. A single turn
of enamelled copper wire is carefully mounted surrounding the analyser. This can
be used to fine-tune the focussing of the beam into the analyser entrance slit.
A Mu-metal shield surrounds the analyser and lens for shielding it from earth’s
magnetic field. The spectrometer chamber is also shielded by the mu-metal.
The electronics system consists of a spectrometer
control unit to provide various voltages to the energy analyzer, a pulse
amplifier to amplify the detected signal, a rate meter to count the number of
electrons per second. The total electronics system is interfaced to a personal
computer. A windows based software program scans the spectrometer and acquires
the data and stores it in a file for further analysis.
The function of the analyzer
is as follows: When the sample is kept at ground potential, electrons ejected
from a state with binding energy Eb are emitted with a true kinetic
energy Ek given by: Ek = hn-
Eb -f,
where f
is the work function of the sample. The ejected electrons pass though the lens
and are then retarded by an amount R, determined by the lens voltages before
entering the analyzer. The retardation of kinetic energy to pass energy is
necessary to achieve the required resolution. Therefore, the electrons, which
have been transmitted by the analyzer with a retardation R and pass energy HV
would have a kinetic energy given by the equation:
E = R + HV +
f
----(1)
Here, the H, which is 1.403
for our analyzer, is the analyzer constant given by the following relation:
H = EQ \F(r2,r1) - EQ \F(r1,r2) ---(2)
The inner hemisphere is
applied a positive potential with respect to the outer. The analyzer is scanned
by varying the retard voltage applied to the analyzer plate, while holding the
analyzer pass energy constant. This ensures a constant resolution for the whole
range of kinetic energies. The absolute resolution
DE
is usually measured as the full width at half-maximum (FWHM) height of a chosen
observed peak and is given by the relation:
EQ \F(DE,HV) = EQ \F(0.63d,Ro) -----(3)
Where, d = slit width, Ro =
mean radius of hemisphere and HV = pass energy. For a HV= 20eV, the resolution
of our analyser varies from 0.13eV (d=1mm) to 0.39eV (d=3mm).
A schematic diagram of the
photoelectron spectrometer is shown in Fig. 1 a while Fig. 1 b is a photograph
of the completely assembled spectrometer.
Fig.1(a) Schematic of Angle Integrated Photoelectron spectrometer
Fig. 1(b) Photograph of the completely assembled Photoelectron spectrometer
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