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Indus-1 Synchrotron Source
Angle Resolved Photoelectron Spectroscopy Beamline at Indus-1
N C  Das, S N  Jha, S V N  Bhaskara Rao
Spectroscopy Division, BARC

Jagannath, A V  Korgaonkar
Technical physics & Prototype Engineering Division, BARC

1. Introduction

Photoemission spectroscopy is the most powerful technique to probe the state of electrons in atoms, molecules, solids and surfaces. In the photoemission process, an electron is excited from its ground state by the absorption of a photon. Due to the increase of its kinetic energy, it can free itself from the system to which it is bound. Once in vacuum, this freed electron, called photoelectron, can be collected and analysed, and its ground state properties can be retrieved from this analysis. In addition, measurement of the angular distribution of photoelectrons from oriented crystals provides information on the wave vector of electron states. The technique, therefore, named as angle resolved photoelectron spectroscopy (ARPES) is a unique tool to map the energy dispersion curves of electrons in solids. ARPES experiments need a high flux photon source for which Indus-1 synchrotron source is ideally suited.

The angle resolved photoelectron spectroscopy beamline has been commissioned at Indus-1 storage ring, CAT, Indore. This beamline will be used to investigate the ground state electronic states for valence electrons in chemisorbed or physisorbed atoms or molecules. These states correspond to the chemical bond between adsorbed species and substrate and give the most direct picture of the chemisorption bonding process.  This approach will be used very extensively to study adsorption of gas phase species and thin evaporated films.

2. Status:

The heart of the beamline is a 1.4 meter toroidal grating monochromator (Jobin Yvon-TGM-1400) covering the wavelength region of 40Å to 1000Å. The monochromator consists of three interchangeable gratings to cover the entire wavelength region. The optics1 of the beamline consists of a pre-focussing mirror, monochromator with entrance and exit slits, gratings, and a refocusing mirror. All the focussing optical elements have toroidal shapes and are coated with platinum on bulk zerodur material. At the initial stage, the optical layouts based on detailed specifications of optical components have been prepared.

The mechanical layout of the beamline has been finalised on the basis of optical layout2. Various subassemblies of the beamline such as mirror chambers3, 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.   After mechanical integration of the beamline, the experimental station was connected to it. The experimental station consists of a ultra high vacuum chamber in which a spherical sector analyzer mounted on a goniometer for angle resolved studies, a LEED-Auger apparatus for sample characterization, an ion gun to sputter etch the sample etc. are housed. This chamber is coupled to the beamline through an adapter consisting of windowed gate valve, bellows, and beam pipe with pumping port. 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. 

3. Experimental Performance

In order to evaluate the overall performance of the beamline, photoemission measurements were performed on metal samples, namely, gold and stainless steel-304. A polycrystalline gold sample in the form of thin sheet of size 6mmx12mm was mounted on stainless steel sample holder which was coupled to sample manipulator through a ceramic spacer. That sample was cleaned in-situ by Ar+ gun and surface cleanliness was checked by Auger electron analysis. Barely detectable C and O Auger transition lines were present after the sputter etching the sample.  Measurements were performed over the photon energy range 123ev to 310eV using the 1800 grooves/mm toroidal grating housed in the TGM-1400 monochromator. An indigenously built 50 mm radius angle resolved hemispherical electron analyser with an angular acceptance of one degree was used for the photoelectron kinetic energy analysis. During the recording of photoelectron spectra the base pressure in the experimental chamber was maintained at 1x10-9 Torr or below up to beam current of 100mA in the storage ring. The recorded core level photoelectron spectrum of gold and iron matched with the published ones4,5. The recorded gold core level spectrum showed well-resolved two spin-orbit split peaks corresponding to 4f5/2 and 4f7/2 energy levels.  The spin-orbit splitting of 3.6eV matches well with the data reported in the literature which clearly demonstrates that the overall performance of the beam line and the electron analyser is quite reasonable.

4. References:
  1. N.C.Das, S.N.Jha and B.N.Raja Sekhar, “Optical design of angle resolved photoelectron spectroscopy beamline at INDUS-1 synchrotron radiation source”, Vol.31, 129 – 143 (2002) Journal of optics, India.

  2. S.V.N. Bhaskara Rao, S.N. Jha, N.C. Das, P.S. Murty, A.K. Sinha, K. Bandopadhyay, K.C. Ratnakala and A.S. Raja Rao, “ Design, Fabrication and Testing of Mirror Chambers for PES Beam Line at Indus-1”, Proceedings- International Conference on Vacuum Science and Technology and SRS Vacuum Science, CAT, Indore, India, Ed. N. Venkatramani, Vol. 1, pp 241-248(1995).

  3. N. C. Das, P. M. Raja Rao, S.V.N. Bhaskara Rao, B.N. Rajasekhar, P. Saraswaty, S.N. Jha, S.S. Bhattacharya and A.P. Roy, “Spectroscopic Beamlines at the Indus-1 Synchrotron Source Facility”, Proceedings-Fourth International Conference On Synchrotron Radiation Sources and Second Asian Forum on Synchrotron Radiation, (ICSRS-AFSR’95), Ed. Moohyun Yoon, et. Al., KYONGJU, KOREA, pp. 641 (1995).

  4. S.N.Jha, N.C.Das, S.V.N.Bhaskar Rao, B.N.Raja Sekhar, A.K.Sinha, K.Bandopadhyay, Jaganath, A.V.Korganokar ans K.G.Bhushan, “Development of photoelectron spectroscopy beamline at Indus-1 synchrotron radiation source”, -  Pg.585- 588. Solidstate Physics (India) 44, (2001).

  5. N.C.Das, S.N.Jha, S.V.N. Bhaskara Rao, B.N. Raja Sekhar, S.S.Bhattacharya, Jagannath, A.V.Korganokar, K.G.Bhushan, V.C.Sahani and S.K.Sikka, “Development of angle resolved photoemission spectroscopy beamline at Indus-1 synchrotron source”, Vol.32, Pg.27-32(2003), Journal of optics, India.

  6. S.N. Jha, N.C.Das, S.V.N. Bhaskara Rao, B.N. Raja Sekhar, Jagannath, A.V. Korganokar and V.C. Sahani “First result using Angle Resolved Photoelectron Spectroscopy Beamline at Indus-1” – Proceedings Indian particle accelerator conference (INPAC), held at Centre for Advanced Technology, Indore, during 3-6, February, 2003, Pg.28-29.
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