Photophysics Beamline at Indus-1
N C Das, B N Raja Sekhar, S Padmanabhan, Aparna Shastri
Spectroscopy Division
Bhabha Atomic Research Centre, Mumbai-400 085
1. Introduction
The photophysics beamline is a medium resolution (1 Å) beamline using a one meter Seya-Namioka Monochromator and operating in the VUV region of 500 Å to 2000 Å. In this beamline a toroidal mirror focuses the light originating from the synchrotron source onto the horizontal entrance slit of the one meter Seya-Namioka Monochromator. The dispersed light emerging from the exit slit of the
monochromator is focused by a second toroidal mirror at the sample under study. Using this beamline a variety of atomic, molecular and condensed matter experiments will be carried out in the gas phase and solid state. In the gas phase, it is proposed to carry out photo absorption and photo fragmentation spectroscopy of atomic and molecular species. For this purpose an absorption cell will be used in the beamline. In addition to absorption, fluorescence from the excited species will also be studied. The initial experiments will be
confined to the wavelength region of 1040 Å to 2000 Å using LiF window. Later on this will be extended to 500 Å using differential pumping arrangements. To begin with spectra of some rare gases like He and Ar and molecules like N2 will be studied for calibrating the system. Subsequently, it is planned to study VUV absorption of ammonia, acetone, benzene, etc. In the solid state spectroscopy two types of experiments on optical reflectivity and fluorescence will be carried out. In both the experiments, time resolved studies will be carried out using the well defined time structure of synchrotron radiation source.
2. Status
At the initial stage of the project, the one meter
Seya-Namioka Monochromator was assembled and tested for its performance.
Wavelength scanning mechanism was tested for its reproducibility and spectrum
was record using UV and VUV sources. An average spectral resolution of 2.5 Å has
been achieved using a 1200 gr/mm grating. A wavelength repeatability of
±1Å has been obtained. The UHV compatibility of 2x10-8 mbar has been
achieved in the monochromator.
The optical layout of the
beamline using the above mentioned toroidal grating and Seya-Namioka
Monochromator has been prepared and its imaging properties have been evaluated
by actual ray tracing. Based on the optical layout, the mechanical layout of the
beam line consisting of mirror chambers, monochromator, experimental station and
beam pipes has been finalised.
The various subassemblies, such as, front-end,
mirror chambers, monochromator, experimental station, etc., have been positioned
at the photophysics beamline location of the INDUS-I hall after testing their
UHV compatibility. The beamline has been aligned using visible part of the
synchrotron radiation. The observed focused image at the sample position is of
the order of 1mm x 1mm, which suits the requirement of recording
photoluminescence spectra of various types of materials.
3. Experimental Performance
The experimental performance of the Seya-Namioka Monochromator has been tested by recording spectral lines of Hg source and Iron hollow cathode source. The Hg spectrum was scanned from 2500 Å to 6000 Å many times and the wavelength reproducibility was checked from the encoder reading. Having tested the monochromator for its wavelength reproducibility spectral resolution has also been tested. For this purpose, the spectrum of Iron hollow cathode source in the region of 2300 Å to 2700 Å has been recorded with a slit width of 100 mm. It has been found that the spectral lines separated by 2.5 Å corresponding to (2462.7 Å - 2465.2 Å) and (2574.4 Å – 2576.7 Å), are well resolved. After aligning the beamline, the visible part of the synchrotron radiation has been recorded using a photo detector based data acquisition system. It has been found that the resulting spectrum matches with the characteristics radiation of the INDUS-1 synchrotron
source.
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