High resolution VUV Beamline at Indus-1
N C Das, S Padmanabhan, S Krishna Kumar
Spectroscopy Division
Bhabha Atomic Research Centre, Mumbai-400 085
1. Introduction
The high resolution VUV beamline will be used for recording high-resolution absorption spectra of atomic and molecular vapours. This beamline uses a combination of three cylindrical mirrors, which focuses the light originating from the synchrotron source onto
horizontal entrance slit of a spectrometer. The spectrometer uses a 6.65 meter concave grating having a grove density of 4800 gr/mm which is mounted in the off plane Eagle mounting. It will cover the spectral region of 400 Å to 2000 Å with an average spectral resolution of 0.005 Å. From the recorded photo-absorption spectra of atoms and molecules it has been decided to make the following studies.
Accurate determination of absorption cross-sections of atoms and molecules, which will in turn show the distribution of oscillator strength as a function of photo energy.
Determination of vibrational and rotational constants of molecules which will be used to construct potential curves and finding out the symmetry of excitation state, dissociation energy, etc.
Observation of long Rydberg series to high principal quantum numbers which will help in the determination of accurate ionization potential and quantum defects.
The study of the line profiles in the auto ionizing range which will give useful information on interchanged interactions.
2. Status
Based on the optical layout, the mechanical layout of the beamline has been finalised and the detailed design of various mechanical components has been completed. All the optical components like mirrors and gratings and UHV components like gate valves, bellows, beam pipes have been procured. The mirror chambers along with mounts have been fabricated. The support for the beamline has also been fabricated. A half-meter stainless steel absorption cell has been designed, fabricated and tested. A split furnace to heat the cell to temperature of 8000C has also been fabricated. The absorption cell is currently being used to carry out absorption experiments with an UV‑visible source. The optical components of the spectrometer such as, slits, gratings have been procured. The grating translation and rotation mechanisms have been tested for their smooth
performances. The spectrometer tank has been leak tested and evacuated to a pressure of 1x10-5 mbar. The scanning mechanism also has been fabricated and assembled. All the UHV components necessary for the spectrometer assembly have been procured. Preliminary optical alignment of the spectrometer with the laser light source has been completed.
3. Experimental Performance:
After completing the preliminary assembly work of the 6.65-meter off-plane Eagle spectrometer, the electronic control system for the grating drive and translation mechanism has been tested. It has been found that the grating translation and rotation mechanism work very smoothly through out the scanning region of the spectrum. The Hg-spectrum in the UV region has been recorded in the photographic mode
using 1200 gr/mm concave grating. The recorded spectral lines have been observed to be very sharp and the minimum resolution has been achieved as 0.3 Å. Further work is being carried out for electronic recording of spectrum and improvement of spectral resolution.
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