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XUV Beam Line on Indus-1:

AIPES Meeting August 2013

1. Presentations(ZIP)

2. Publications(ZIP)

Dr. T. Shripathi /Dr. D. M. Phase/AvinashWadikar/Sadhan Chandra Das
UGC-DAE-CSR, Indore
Specifications
  • A toroidal grating monochromator TGM 2631 with three gratings of 200, 600 and 1800 lines/mm
  • Wavelength range 60 - 1600   (8 - 200 eV)
  • Pre - and Post - mirrors of toroidal type
  • Final spot size at sample < 1 mm2
  • Angle integrated photoelectron spectroscopy station
  • Average resolving power of 300









Beamline for photoelectron spectroscopy

Beamline Specifications:-

Optical configuration:
Energy/wavelength range:
Wave length rang
Gratings
Lines/mm Coating
Spectral resolution
l/Dl
measured with discharge source
540-1600 A°
200
Pt
650 at 584 A°
180-540 A°
600
Pt
950 at 304 A°
60-180 A°
1800
Pt

Experimental station:

UHV compatible angle integrated photoelectron spectrometer comprising of :

  1. Hemispherical analyser having mean radius of 95mm.

  1. Ion gun for sample cleaning.

  1. Sample manipulator with XYZ motion.

  1. Sample heating up to 900oC and cooling up to LN2 temperature.

  1. Sample preparation chamber with quick load lock and sample transfer system.

Experiments: Photoemission(angle integrated)
Photoelectron Spectrometer
Consist of:

1. Sample Preparation Chamber

2.Experimental Chamber

3.Electron Energy Analyser

4.Detection and Data acquisition system

1.Sample Preparation Chamber
  1. Magnetic transfer road with quick load lock system.
  2. Cleaning facility- Argon etching gun

2.Experiment Chamber
  1. Sample manipulating with x,y,z and rotation motions.
  2. Sample heating (9000C) and cooling (LN2) facility.
  3. Twine anode X-ray source with AL Ka and Mg Ka.

3. Electron Energy Analyser:

  1. Electronic lens (three electrodes)

Dimension of lens elements:-

First piece-      Length: 135mm; Diameter: 72mm;
                       Aperture:62mm with 10mm length bent at 60 degrees.

Second piece-  Length: 75mm; Diameter: 72mm.

Third  piece-     Length: 125mm; Diameter: 72mm;
                       Aperture:52mm with 20mm length bent at 60 degrees.

(ii)1800 Hemispherical Analyser: 

Dimensions of Analyser

Inner radius-65mm

Outer radius-125mm

Mean radius-95mm

Annular space-60mm

Entrance slit-  1-3mm(variable)

Exit slit-  1-3mm(variable)

4.Detection and Data acquisition system

(i) Detector-Channel Electron Multiplier

Input rasistence-8 x 108 M W

Input HV-2.5 KV to 4 KV maximum

Gain-108

(ii) Data acquisition system:

Spectrometer Control Unit:

  1. HV-Outer and Inner hemisphere

  1. PE variation

  1. K.E. variation

  1. Magnetic trimming

  1. Lens, Grid voltage supply

Pre amplifier?Amplifier?Rate meter                                 
                            PC

Resolution (DE)- (calculated)

For 20 ev pass energy

with slit width(w)-1mm, DE ~ 132mev

        slit width(w)-2mm, DE ~ 265mev

        slit width(w)-3mm, DE ~ 397mev

For 30 eV pass energy

with slit width(w)-1mm,   DE ~ 200mev

         slit width(w)-2mm,  DE ~ 400mev

         slit width(w)-3mm,  DE ~ 600mev

Vacuum =5 x 10-10 Torr
BEAMLINE DESCRIPTION

(i)   Optical design of the beamline : -
The optical components of the beamline consist of a pre-mirror to focus the incident radiation, a monochromator to select the wavelength of interest and a post-mirror to focus the monochromatic beam onto a sample. In the wavelength of interest the only useful optical elements with good imaging properties are reflecting mirrors and reflecting gratings. The reflectivity of these mirrors in this wavelength range is rather low and one has to use them in grazing incidence geometry, which has therefore been adopted in our design. The complete optical layout of the beamline and their locations from the tangent point is shown in Fig.1. Various physical constraints such as the length of front end, shielding wall thickness etc. restrict the placement of first mirror to 4 meters from the tangent point. This mirror which is a toroidal in shape accepts radiation over a horizontal acceptance angle of 10mrad and over a vertical acceptance angle of 2.5 mrad. The angle of incidence at the mirror is around 4.5o giving a deviation of 9o after reflection. The reflected beam is brought to focus at a distance of 2 meters from the center of the mirror where an entrance slit is located. The pre-mirror illuminates the entrance slit by 2:1 demagnification of the source. The toroidal grating monochromator (Jobin Yvon) has a grating size of 90mm x 30mm and it operates at constant vertical deviation of 162o. Three gratings having groove density 1800, 600 and 200  are used to cover wavelength from ranges 60-180 ?, 180-540 ? and 540-1600 ? respectively. Table-1 gives the details of the grating as well as the pre and post-mirror parameters. Different gratings can be brought into working position without breaking the vacuum so as to receive radiation originating from the source. The gratings can be scanned in the position around vertical axis passing through the grating.  The monochromatised light is focused on exit slit kept at 2600mm from grating center. Both the entrance and exit slit can be adjusted continuously in vertical direction and in steps in horizontal direction. The beam passing through the exit slit falls onto the post mirror, which is also a toroidal in shape. This mirror is kept at a distance of 990mm from the exit slit and at a same distance from post mirror a sample in experimental chamber is located. The deviation produced by post mirror is 9o so that the beam striking the sample is parallel to the beam emanating from the source. Post mirror produces a magnification of unity with the result that the spot size of the final image on the sample is a replica of the exit slit.

          To evaluate the imaging properties of the optical design of the beamline, detailed ray tracing studies were carried out using a general-purpose RAY program [2]. The optical configuration of the  beamline shown in Fig.1 is used as a starting point for the ray tracing studies. The various input parameters used for calculation are given in table-1. By varying different  parameters like source size, source divergence, tangent error, slit width and photon energy, the performance (resolution and throughput) of the beamline is determined. The details of the studies are reported elsewhere [3].


(II)     Mechanical design of the beamline: -
The corresponding mechanical design to accommodate the optical elements consist of various components such as laser alignment box, beam viewer at appropriate locations, four jaw aperture in front of gratings to cut the light horizontally and vertically, mirror chambers, bellows or flexibility, inter connecting pipes and a facility to introduce thin metal foils and photo diode to check the resolution of the beamline periodically. The overall performance of beamline depends on many factors such as alignment of the optical components with respect to synchrotron beam, the quality of optical elements (surface roughness, tangent error, coating etc.) and mechanical design. A good alignment scheme can effectively reduce the complications of mechanical design and also help to improve performance of the instrument. From the ray tracing studies it is cleared that the pre-mirror alignment with respect to SR beam and other components of the beamline is very important. To achieve better image quality not only pre-alignment of pre-mirror is needed but also fine-tuning has to be done online necessarily. The motions involved in alignment of pre-mirror are three transnational motions in X-Y-Z directions, two tilt motions in the parallel and perpendicular direction of SR beam and one rotational motion around the perpendicular to mirror surface.  The explicit values of tolerances required for these motions are estimated from ray tracing studies of complete beamline. The mechanical design of ultra high vacuum (UHV) compatible mirror holder along with precision motions consist of three parts viz. I) Mirror holder; II) X-Y-Z mount; and III) tilt and rotary motion mechanism, all assembled in 6-way cross. The entire assembly is conflatR flange mounted and can be directly inserted in UHV mirror chamber. The main mechanical features of this mirror mount system are: -a) All motions are independent and center of mirror remains fix during angular movement of mirror with respect to beam; b) In this design gears are avoided and spring loaded motion feedthroughs are employed to achieve backlash free precision motion. Identical mechanical design is also adopted for post mirror chamber and holder. The details of this mirror mount system and chamber are described elsewhere [4].  To avoid contamination of the optical components and to make beamline compatible with the storage ring the base pressure in the entire beamline is kept better than 1 x10-9 mbar. For this a combination of sputter ion pump and turbo pump backed with scroll pump at appropriate  locations along the length of beamline is used to maintain the required vacuum. The photograph of complete beamline installed on INDUS-1 is shown in fig.2.

The resolution of the beamline is tested using penning discharge type source (M/S McPherson, U.S.A.) capable of producing intense spectral lines of He, Ar and N2. In addition to this, source can be operated to produce spectral lines in soft x-ray range of 60 ? to 180 ? by employing a magnesium and aluminum electrode as a cathode material. The measured resolution keeping entrance and exit slit width of 200m for grating having 600 lines/mm is 1000 at l=304 ? and 600 for grating having 200 lines/mm at l= 584 ?. The measured resolution curves are shown in Fig.3

(III)      Photoelectron Spectrometer : -
The experimental station of this 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 an 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. 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 touching 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 enameled 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 analyser, a pulse amplifier to amplify the detected signal, a ratemeter to count the number of electrons per second. The total electronics system is interfaced to an IBM compatible personal computer. A windows based software program is then run, which scans the spectrometer and acquires the data and stores it in a file for further analysis.

The function of the analyser 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 analyser. The retardation of kinetic energy to pass energy is necessary to achieve the required resolution. Therefore, the electrons, which have been transmitted by the analyser 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 analyser, is the analyser constant given by the following relation:

H   =   -         ---(2)

The inner hemisphere is applied a positive potential with respect to the outer. The analyser is scanned by varying the retard voltage applied to the analyser plate, while holding the analyser 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:

  =      -----(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.4  while  Fig.5 is a photograph of the completely assembled spectrometer.

(Contact persons : Dr. T. Shripathi and Dr. D.M. Phase)
Acceptance:
10 mrad horizontal x 2.5 mrad vertical.
Mirror1:
toroidal mirror to focus the SR beam on the entrance slit, vertical deflection, Pt coated, demagnification 2:1.
Entrance slit:
horizontal: adjustable from 0.4 mm to 3mm in four discreet steps,
vertical : continuously adjustable from 0 to 1.8 mm
Monochroator:
TGM-2600 toroidal grating monochroator,total deflection 2q =162° ; three gratings interchangeable under vacuum.
Exit slit:
horizontal: adjustable from 0.4 mm to 3mm in four discreet steps,
vertical : continuously adjustable from 0 to 1.8 mm
Mirror 2:
toroidal mirror to refocus the monochromatic beam from the exit slit to a sample located
1meter from the mirror, vertical deflection, Pt coated demagnification 1:1.
Spot size:
typically 1mm horizontal x 1mm vertical.