Solidstate
Physics (India), 585-588, No.44, 2001
Development of
Photoelectron Spectroscopy Beamline at Indus-1 Synchrotron Radiation Source
S N Jha†, N C Das†,
S V N Bhaskara Rao†, B N Rajasekhar†, A K Sinha+,
K Bandyopadhyay+, Jagannath*,
A V Korganokar* and K G Bhushan*
†Spectroscopy
Division, Bhabha Atomic Research Centre, Mumbai-400 085
+Centre
for design and manufacture, BARC, Mumbai - 400 085
*Technical
Physics & Prototype Engineering Division, B.A.R.C., Mumbai-400 85
A beamline for
photoemission spectroscopy using Indus-1 synchrotron source has been recently
commissioned at Centre for Advanced Technology, Indore. This beamline consists
of a pre-focussing toroidal mirror, 1.4 meter toroidal grating monochromator (TGM)
and a post-focussing toroidal mirror housed in ultra high vacuum chambers. At
the end there is an electron spectrometer chamber which houses the analysers,
sample manipulator and holders and other facilities. This facility will be
useful for both the angle integrated and angle resolved photoelectron
spectroscopic studies of solid samples in the spectral region of 40Å to 1000Å.
Details of optical and mechanical designs, intensity characteristics and the
overall status of the beamline will be discussed in this paper.
Introduction
By
measuring the energy of the photoemitted electrons from materials as a function
of the incident tunable synchrotron radiation source, information can be
obtained about the electronic energy levels. In addition, measurement of the
angular distribution of the photoelectrons from oriented crystal provides
information on the wave vector of electron states. Therefore, angle resolved
photoelectron spectroscopy (ARPES) is a unique tool to map the energy dispersion
curves of electrons in solids. Angle resolved photoemission experiments need a
high flux, moderate resolution monochromator covering photon energy range about
40 Å to 1000 Å. A beamline incorporating toroidal grating monochromator (TGM)
has been designed and developed for operation at Indus-1 synchrotron radiation
source (SRS), at CAT, Indore. The experimental station (electron spectrometer)
for carrying out PES studies has been designed and built by TP & PED, BARC.
Beamline Design
The
source used in optical design is bending magnet with horizontal and vertical
divergence angles of 8.9 mrad and 5.5 mrad respectively. The photon flux1
available from this source in the wavelength range of 40Å to 1000Å is of the
order of 1012 photons/sec/100mA/0.1% BW. The maximum flux is 9x1012
at 58eV(213.7Å).
The optical layout of the beamline is shown in Fig.1. The
whole beamline can be divided into three sections: 1) pre-focussing mirror M1,
2) monochromator with entrance and exit slits, gratings, and 3) refocusing
mirror M2. All the focussing optical elements have toroidal shapes
and are coated with platinum on bulk zerodur material. The side view of the
beamline shows that the rays originating from the source point S1 and
lying in the vertical plane are focussed by toroidal mirror M1 at the
entrance slit S2 of the monochromator. Since the dispersion plane is
the plane of the paper, the diffracted rays are perfectly focussed at the exit
slit S3 of the monochromator and diverges further to get focussed at
sample position by a second toroidal mirror M2.
Monochromator:
The monochromator used in this beamline is a toroidal grating monochromator. It is the simplified grazing incidence configuration since the rotation of the single optical element is sufficient to achieve monochromatic light with high throughput behind the exit slit.
The angle subtended by entrance and exit arms (2q) determines the short wavelength limit of the spectral range. We have chosen 2q =1620 , sufficiently large to get photons of energy around 300 eV, but as small as possible to reduce focussing aberrations. Three gratings having groove densities of 200g/mm, 600grooves/mm and 1800g/mm are to be used for covering the wavelength range of 40Å to 1000Å. The monochromator was constructed by M/S Jobin-Yvon, France as per our design.
To study the imaging characteristics of the beamline, ray tracing work was carried by the software
program SHADOW2. While operating SHADOW, the sources for various wavelengths and a large no. of rays confined in the horizontal and vertical acceptance angles of the beamline were used for ray tracing purpose. From a calculation of the image blur at the sample position it was found that most of the energy is concentrated within a spot size of 1mm X 1mm.
Mechanical Layout
The mechanical layout of the beamline was finalised based on
the basis of optical layout. As shown in Fig.2, the entire beamline consists of
two horizontal sections and two inclined sections and the light
enters the beamline from the left end. The first horizontal
section consists of bellows, UHV cross to house laser alignment device, gate
valve and UHV chamber for the pre-focussing toroidal mirror M1 which
are connected by 35CF flanges and beam pipes. It is followed by an inclined
section, which consists of bellows, beam pipe, straight through valve and
entrance arm of the monochromator, which are connected by 35CF flanges. The
last section constitutes the UHV chambers for post focussing mirror and electron
spectrometer chamber, which are connected through beampipes, bellows and gate
valves having 35CF flanges. The mechanical design and fabrication of most of the
components have been carried out at Central Workshops, BARC, CAT, Indore and few
outside private agencies within India. Various subassemblies of the beamline
such as mirror chambers, laser alignment box, TGM, etc. have been first tested
for its UHV compatibility and then integrated by UHV bellows, beam pipes and
gate valves. The entire beamline has been maintained at the UHV condition of 10-9
Torr. This was achieved by a combination of Turbo Molecular Pumping (TMP)
stations and Sputter ion Pumps (SIP). Supporting structures for various sections
are provided with fine adjustment mechanisms. These mechanisms help in aligning
the beamline with respect to source point. The two focussing mirrors are mounted
on UHV compatible mounts with five degrees of freedom for accurate positioning
of the mirrors.
Beamline Alignment
The alignment of the beamline starts with the determination
of the optical axis of the SR. Before setting up the beamline, but after
completion of the front end (an interface between the bending magnet port and
the beamline), the visible part of SR light passing through a window flange
(view port) of front-end was used to determine the optical axis. All the
subsections of the beamline were then placed at their respective positions and
were subsequently integrated with suitable gate valves and bellows. The entire
beamline was then kept at less than 10‑9 Torr vacuum. Once beamline
was integrated the final visible spot size at the sample position was observed
by keeping the TGM in zero order position. It was observed that the beam spot
size was confined to an area of less than 2mm2.
Recording of Soft X-rays
A Si photo diode detector
(AXUV-PS1, McPHERSON) was used to detect the soft x-ray component of SR. Its
quantum efficiency/photon is also well calibrated. The detector assembly was
mounted at the end of the beamline (before the sample position) and evacuated to
10-9 Torr vacuum. In Fig.3 we show the intensity distribution at the
sample position obtained using the 1800 grooves/mm.
The maximum near the 90 Å (≈137.7eV) corresponds to
normalised photon flux of ≈ 3x1010 photons/s/100mA. Comparing this
with the number of photons emitted by the Indus-1 ring into the beamline results
in beamline efficiency of ≈ 0.5%.
Resolution Test
Resolution test was performed with laboratory source. The HeI
and HeII lines recorded with TGM showed a FWHM of 2.5Å and 0.8Å at wavelengths
of 584Å and 304Å respectively. This was achieved with slit width settings of 100
mm
for both entrance and exit slits.
Experimental Station
Angle resolved photoelectron spectrometer (Fig.4) consists of
a ultrahigh vacuum chamber in which a spherical sector analyzer mounted on a
goniometer for angle resolved studies, an angle integrated analyzer, a LEED-Auger
apparatus for sample characterization, an ion gun to sputter etch the sample
etc. are housed. This chamber is to be coupled to the beamline through a gate
valve to receive a focused monochromatic synchrotron light beam on the sample.
A long travel sample manipulator is mounted on the top of the chamber to bring
the sample at different locations for transfer, sputter-etching,
characterization and analysis. The chamber is also coupled to a sample
preparation chamber through a gate valve. A sample transfer mechanism is
mounted on the sample preparation chamber through which a sample can be
transferred to the spectrometer chamber and handed over to the manipulator.
ARPES system as shown in Fig.4 is completely assembled and
has been tested for ultimate vacuum. With a mild and short baking a pressure of
1x10-9 Torr is obtained. With a high temperature prolonged baking of
the system, it is possible to achieve an ultimate pressure of 1x10-10
Torr in the spectrometer chamber.
Spherical Sector Analyser:
It is a hemispherical electrostatic analyzer consisting of two concentric
stainless steel (conducting) hemispheres. The radius of the inner hemisphere is
40 mms and the inner radius of the outer hemisphere is 60 mms. They are
mounted on a plate such that its location is precisely fixed and the two
hemispheres are isolated by proper insulating spacers. A 5-element electrostatic
lens is mounted on the plate to focus the photoelectrons from the sample on to
the virtual entrance slit of the analyzer. The acceptance angle of the angle
resolved analyzer lens is 0.5 degrees and the acceleration/retardation factor is
continuously variable. The final resolution of the analyser at pass energy of
50eV is 50 meV. The energy analysed photoelectrons are detected by an electron
multiplier and the counts are measured by a rate meter system. The feeding of
the electrical inputs to the lens and to the analyzer and processing of the
output signal are done through a PC. Angle-integrating analyzer is identical to
the angle resolved analyser except that its acceptance angle is 22.5 degrees and
the resolution is 100 meV at the 50 eV pass energy. A test result of the
electron analyser using electron gun as a source is shown in Fig.5.
Goniometer
The angle resolved analyser for ARPES measurements is mounted on
double axis goniometer. Using this goniometer the analyser can be fixed at any
angle in the range of 0 to 360 degrees in both horizontal and vertical axes in
steps of 0.01degree. It is driven by two stepper motors mounted on the underside
of the base flange. The fixing of the position of the analyser is done by
computer controlled stepper motors, which transfer the rotation of the axes of
the goniometer.
Acknowledgement
We
gratefully acknowledge the kind cooperation rendered by members of SUS section
and Workshops of Centre for Advanced Technology, Indore.
References
1.
Broucher on characteristics of Indus-1 synchrotron radiation source, published
by Centre for Advanced Technology, Indore
2.
http://www.xaylith.wisc.edu/shadow
